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I was recently at a Leukemia and Lymphoma Society conference where a particular oncologist lecturer claimed that all antibodies are created in the bone marrow (I won't mention his name, as he was a great lecturer, and I don't wish to sully his name if he happened to be wrong on this particular point).
Wikipedia seems to indicate that antigens are produced in "Secondary Lymphatic Organs" or occasionally in the bone marrow, but I've been misled by Wikipedia before. It seems to me that evolution would favor creating antibodies close to the infection, and not far away in the bone marrow or lymphatic organs (how would the antibodies "know" how to get back to the original infection? It would be very inefficient to send antibodies all over the body if the infection is localized.)
My questions is: Could someone "map out" the travel of a typical B-cell throughout the body from the time the B-cell detects an antigen up to the time it produces antibodies (and if it later stops producing anti-bodies, please include that)?
The oncologist seems to be incorrect.
Antibodies are created all over the body inside secondary lymphatic organs (lymphatic nodes) or any other mucosal associated lymphatic tissue (MALT).
B cells mature in the bone marrow and finish their maturity in the periphery, then they can live in lymph nodes, MALT, spleen, omentum or other structures and they can create antibodies in all of those structures.
In regard of your localized infection question, you are right. When a pathogen enters the body the B cells closer to it are in the lymph node that is closer to that site and thus there is where the antibody response will take place. That is why when you have a sore throat (throat infection) your tonsils are swollen (they are lymphatic organs too).
Memory B Cell
Memory B cells are generated during primary responses to T-dependent vaccines. They do not produce antibodies, i.e., do not protect, unless re-exposure to antigen drives their differentiation into antibody producing plasma cells. This reactivation is a rapid process, such that booster responses are characterized by the rapid increase to higher titers of antibodies that have a higher affinity for antigen than antibodies generated during primary responses ( Table 2-6 ).
With the possible exception of responses to live viral vaccines, vaccine antibody responses are deemed to wane and eventually decline below protective thresholds, unless repeat antigen exposure reactivates immune memory. Memory B cells are generated in response to T-dependent antigens, during the GC reaction, in parallel to plasma cells ( Fig. 2-5 ). At their exit of GCs, memory B cells acquire migration properties towards extrafollicular areas of the spleen and nodes. 72 This migration occurs through the blood, in which post-immunization memory B cells are transiently present on their way towards lymphoid organs.
It is essential to understand that memory B cells do not produce antibodies—i.e., they do not protect. Their participation to vaccine efficacy requires an antigen-driven proliferation and differentiation process. 72 This reactivation may occur in response to endemic or frequent pathogens, to colonizing or cross-reacting microorganisms (‘natural boosters’), or to booster immunization. The antigen-driven activation of memory B cells results in their rapid proliferation and differentiation into plasma cells that produce very large amounts of higher-affinity antibodies. 72 As the affinity of surface Ig from memory B cells is increased, their requirements for reactivation are lower than for naïve B cells: memory B cells may thus be recalled by lower amounts of antigen and without CD4 + T cell help. Antigen-specific memory cells generated by primary immunization are much more numerous (and better fit) than naïve B cells initially capable of antigen recognition. Thus, a first hallmark of memory responses ( Table 2-6 ) is to generate significantly higher antibody levels than primary immunization. Should this not be the case, the effective generation of memory B cells should be questioned. 72
The reactivation, proliferation and differentiation of memory B cells occur without requiring the induction and development of GC responses. This process is thus much more rapidly completed than that of primary responses. A window of 4–7 days after Haemophilus influenzae b PS immunization was reported as sufficient for high levels of PS-specific vaccine antibodies to appear in the blood of previously primed infants. 73 The rapidity with which antigen-specific antibodies appear in the serum is thus another hallmark of secondary responses ( Table 2-6 ). Slower kinetics suggests that memory B cell induction, persistence and/or reactivation may have been suboptimal.
Another hallmark of memory B cells is to display and secrete antibodies with a markedly higher affinity than those produced by primary plasma cells, as a result of somatic hypermutation and selection. 72 The affinity maturation process which is initiated within the GCs extends during several months after the end of the GC reaction. Consequently, vaccine antibodies with higher than baseline avidity (defined as the sum of epitope-specific affinities) for antigen are only induced when sufficient time has elapsed after priming. 62,74,75 A ‘classical’ prime-boost immunization schedule is thus to allow 4–6 months to elapse between priming and booster doses, hence the generic ‘0–1–6 months’ schedule. Secondary antigen exposure ( Table 2-6 ) thus results in the production of higher affinity antibodies than primary responses. 76 To note, this may not be the case when ‘natural’ priming, e.g., through cross-reactive bacteria, has taken place prior to immunization.
New insights into B cells and why humans can produce trillions of disease-fighting antibodies
No story in biology is more intriguing than the one involving B cells and the intricate processes that result in antibody production—it's a story about mathematics, a tale of genetics and a saga of how vast armies of tiny antibodies can control a diverse range of disease-causing agents.
The subject of B cells and antibody production was once the arcane topic of biologists and physicians. But in the grip of a global pandemic that has claimed the lives of more than 2 million people, concerns about antibodies and the immune response have become subjects of daily television news reports.
Antibody production is at the core of concerns about health and survival, such as the odds of recovering from COVID-19 and how well vaccines can prevent SARS-CoV-2 infection. Over a lifetime, humans are capable of generating 10 trillion different antibody molecules, a number so staggering that it raises the question of how it's even possible.
Antibodies are proteins, which means their production is encoded by genes. But the 10 trillion figure presents what seems a critical mathematical dilemma. How can the human genome, which is composed of 30,000 genes, produce 10 trillion different antibodies? It would seem impossible that a person could make more antibodies than genes existing in the genome, which would have to be orders of magnitude larger to accommodate the vast number of antibodies.
As it turns out, evolution has produced mechanisms to solve that mismatch. Humans can generate a seemingly endless supply of antibodies by joining together separate gene segments before they are transcribed. The process is called somatic hypermutation, which allows B cells to mutate the genes that they use to produce antibodies. This stunning process allows B cells to produce antibodies that keenly bind to SARS-CoV-2—or any other virus or bacterial species that invades the body.
These extraordinary biological events—customizing antibodies and B cells forming memories of the invaders—take place in the germinal centers of lymph nodes, a world unto themselves with "geographic" demarcations of dark and light zones. The germinal center is where B cells become activated and proliferate. It is also where various classes of immunoglobulins—antibodies—which are products of B cells, morph into the various immunoglobulin classes—IgA, IgD, IgE, IgG and IgM. In the germinal centers, immunoglobulins also increase their affinity for antigens, fragments from infiltrators that antibodies recognize as dangerous and seek to destroy.
Research is underway in London that is breaking new ground in the understanding of germinal center activity—how B cells become activated and how immunoglobulins reach their stunning breadth and diversity. B cells do not enter the light and dark zones of lymph nodes willy-nilly. Their entry and exit of these critical regions depend on a variety of factors, each aimed at producing specific types of B cells and floods of highly specific antibodies.
A B-cell can be a plasma cell whose role is to secrete large quantities of antibodies or, a B cell can be a memory B cell, which is formed inside germinal centers following primary infection. Memory B cells can survive for decades. Their role is to "remember" an infectious agent, the antigen.
Having a record of a previous cause of disease speeds the response the next time the same antigen is encountered. There are other B cells in the germinal centers, some in intermediary phases of development.
The London-based team at the Immunity and Cancer Laboratory of the Francis Crick Institute is studying a process known as affinity maturation inside the germinal centers of lymph nodes.
Affinity maturation is the process in which antibodies develop their keen affinities for antigens. An antigen can be a fragment of a virus or a snippet of a bacterium, for example, that are brought to the germinal centers by dendritic cells. Dendritics not only sound the alarm about danger, they present the evidence. T cells are also in the germinal centers and are kingpins in the overall immune response, even playing a role in helping B cells to mature. Germinal centers are beehives of activity.
"Affinity maturation depends on how efficiently germinal centers positively select B cells in the light zone, where dendritic cells deposit fragments of an infiltrator," wrote Dr. Rinako Nakagawa and a Crick Institute team. Their extensive analysis about activities in the germinal centers is published in the Proceedings of the National Academy of Sciences.
"Positively selected germinal center B cells recirculate between the light zone and dark zone and ultimately differentiate into plasmablasts and memory B cells," Nakagawa and the Crick Institute team report.
The research team underscored that "light zone B cells are selected in germinal centers in a cMyc-dependent manner, before dark zone migration." That means cMyc polices activities in germinal centers.
The oncogene cMyc also functions as a cell-cycle regulator. It is a multifunctional transcription factor driving a range of activities necessary for rapid cell division. It also inhibits the expression of genes with anti-proliferative functions. Because of its ability to induce apoptosis, cMyc's expression is tightly regulated.
As Nakagawa and colleagues found, cMyc is also intimately involved in mechanisms of the germinal center, playing a role in the formation and maintenance of the centers in lymph nodes throughout the body. It is a 62-kilodalton protein composed of 439 amino acids and belongs to the helix-loop-helix class of zipper transcription factors.
"This study redefines germinal center B cell–positive selection as a dynamic process that ensures maintenance of a broad range of affinities in germinal centers," Nakagawa and colleagues wrote. "We found that affinity-dependent cell division occurred in the light zone and therefore that process is not restricted to the dark zone."
While the Crick Institute's research is redefining mechanisms in the light and dark zones, their work also builds on studies that date back decades to elucidate how B cells and antibodies mature into key forces of the immune defense.
The story behind the human ability to produce trillions of antibodies is one of the most amazing in nature, and highlights why the mammalian immune system is one of the most complex networks of surveillance and response in the known universe.
Indeed, the immune systems of humans and other animals have evolved genetic mechanisms that allow them to generate a staggeringly high number of antibodies. By joining separate gene segments together before they are transcribed, an abundance of antibodies can be produced. Not all mammals use the same strategies, but the end result is a molecular army of disease-fighting immunoglobulins that the body tailors to fight infiltrators.
B cells immunophenotyping
B cells are mediators of the humoral response, or antibody-mediated immunity. By studying this particular cell group we learn more about the inner workings of the immune system, which consequently increases our awareness of the possible causes behind a variety of autoimmune disorders and cancers. Broad immunological research unlocks valuable insight of what future steps might be taken to treat these pathologies.
Development from stem cell to B cell
Generation of the B cell begins in the bone marrow where stem cells give rise to lymphoid cells. Throughout each stage of development the antibody locus— a site where an antigen interacts with the cell— undergoes genetic recombination. This recombination is specific to the developmental stage of the B cell. Development starts with the pro-B cell, which expresses Igα and Igβ. The cell matures further into the pre-B cell that expresses the pre-B cell receptor (Igμ) on its surface. Maturation in the bone marrow ends with the naïve B cell that expresses the B cell receptor (containing IgM and IgD) capable of recognizing an antigen. These cells then leave the bone marrow and enter the periphery (Cambier JC, et al. Nat Rev Immunol. 2007).
Subtypes of conventional B cells
Conventional B cells, also referred to as B-2 cells, terminally differentiate into one of two common subtypes upon activation:
- Plasma B cells: a plasma cell is the sentry of the immune system. The naïve B cell circulates throughout the body. When it encounters a unique antigen, the plasma cell takes in the antigen through receptor-mediated endocytosis. Antigenic particles are transferred to the cell surface, loaded onto MHC II molecules and presented to a helper T cell. The binding of the helper T cell to the MHC II-antigen complex activates the B cell. The activated B cell goes through a period of rapid proliferation and somatic hypermutation. Selection occurs for those cells that produce antibodies with a high affinity for that particular antigen. Once terminally differentiated, the plasma B cell only secretes antibodies specific for that antigen and can no longer generate antibodies to other antigens.
- Memory B cells: memory cells are held in reserve, in the germinal centers of the lymphatic system, for when the immune system re-encounters a specific antigen. During any repeat exposure the follicular helper T cell causes the memory cell to differentiate into a plasma B cell that has a greater sensitivity to that specific antigen. This jump-starts the immune system to mount a quicker, more powerful response than was possible previously.
Other B cell subtypes include:
- B-1 cells: a minor subtype, only about 5% in humans, of self-renewing fetal B cells that act in a similar fashion to plasma cells. B-1 cells are primarily present during fetal and neonatal life.
- Marginal zone (MZ) B cells: mature memory B cells that are found only in the marginal zone of the spleen. These cells can be activated through toll-like receptor-ligation and not necessarily through the B cell receptor.
- Follicular (FO) B cells: these are mature, but inactive, B cells. This subset of B cells is primarily found in the follicles of the spleen and lymph nodes. Activation of these cells requires the aid of T cells. FO B cells can differentiate into either plasma or memory B cells.
- Regulatory B (Breg) cells: Breg cells negatively regulate the strength of the immune response and inflammation by secreting chemical messages called cytokines, such as IL-10. Although these cells make up a small portion of the B cell population (
Immunophenotyping of B cells through flow cytometry
Immature B cells express CD19, CD 20, CD34, CD38, and CD45R, but not IgM. For most mature B cells the key markers include IgM and CD19, a protein receptor for antigens (Kaminski DA. Front Immunol. 2012). Activated B cells express CD30, a regulator of apoptosis. Plasma B cells lose CD19 expression, but gain CD78, which is used to quantify these cells. Memory B cells can be immunophenotyped using CD20 and CD40 expression. The cells can be further categorized using CD80 and PDL-2 regardless of the type of immunoglobulin present on the cell surface (Zuccarino-Catania GV et al. Nat Immunol. 2014.). Globally, cytokines (such as interlukein-10) and chemokines involved with chemokine receptor 3 play an important role in transmitting the biological messages to drive the immune response.
A table of common B cell subtypes with some cell markers which can be useful for flow cytometry:
B cells produce antibodies 'when danger calls, but not when it whispers,' scientists report
The immune system&rsquos B cells protect us from disease by producing antibodies, or "smart bullets," that specifically target invaders such as pathogens and viruses while leaving harmless molecules alone. But how do B cells determine whether a threat is real and whether to start producing these weapons?
An international team of life scientists shows in the May 16 issue of the journal Science how and why these cells respond only to true threats.
"It is critical for B cells to respond either fully or not at all. Anything in between causes disease," said the study&rsquos senior author, Alexander Hoffmann, a professor of microbiology, immunology and molecular genetics in the UCLA College of Letters and Science. "If B cells respond wimpily when there is a real pathogen, you have immune deficiency, and if they respond inappropriately to something that is not a true pathogen, then you have autoimmune disease."
The antibodies produced by B cells attack antigens &mdash molecules associated with pathogens, microbes and viruses. A sensor on the cell&rsquos surface is meant to recognize a specific antigen, and when the sensor encounters that antigen, it sends a signal that enables the body&rsquos army of B cells to respond rapidly. However, there may be similar molecules nearby that are harmless. The B cells should ignore their signals &mdash something they fail to do in autoimmune diseases.
So how do the B cells decide whether to start producing antibodies?
"These immune cells are somewhat hard of hearing, which is appropriate because the powerful and potentially destructive immune responses should jump into action only when danger calls, not when it whispers," said Hoffmann.
The B cells make their response only when a rather high threshold is reached, Hoffmann and his colleagues report. A small or moderate signal &mdash from a harmless molecule, for instance &mdash gets no response, which reduces the risk of false alarms.
"It&rsquos like your car&rsquos airbag, which won&rsquot be deployed unless you really need it," Hoffmann said. "You can imagine that if the airbag were poorly designed and if you brake very hard or have a slight accident, it could deploy slowly and be useless. You want it to deploy fully or not at all. That is what the B cell does when deciding whether it confronts something that is truly pathogenic or harmless. No B cell responds partially."
We have billions of B cells, and each one creates this threshold through a molecular circuit involving two molecules. One of these molecules, known as CARMA1, activates the other, IKKb, which further activates the first one.
"Positive feedback between the two causes infinite growth, and once you trigger it, there is no way to turn it off until the smart bullets are shot," said Hoffmann, whose research aims to understand and decode the language of cells. "But a second feature of positive feedback is that it can create a threshold only above which this runaway activation occurs."
He and his colleagues developed mathematical equations based on the molecular circuit and were then able to simulate, virtually, B cell responses. The team&rsquos resulting predictions were tested experimentally by their collaborators at the Laboratory for Integrated Cellular Systems at Japan&rsquos RIKEN Center for Integrative Medical Sciences. In one part of the study, the researchers made specific mutations in IKKb so that it could not signal back to CARMA1. They also made mutations in CARMA1 to prevent it from receiving the signal from IKKb. In both cases, the B cells responded partially, some of the time, like a weakly inflating airbag.
"It became a gray-zone response rather than a black-and-white response," said Hoffmann, who constructs mathematical models of biology.
(The primary image accompanying this release illustrates this. The threshold is 0, and under normal circumstances the B cells, shown in blue, either clearly exceeded it or failed to reach it. But when the circuit was altered, many of the B cells, in green, fell just slightly above or below the threshold.)
The research could lead to better diagnosis of disease if patients with an autoimmune disorder, such as lupus, have a defect in this molecular circuit.
Co-authors of the study included Mariko Okada-Hatakeyama, a professor at Japan&rsquos RIKEN Center, and Marcelo Behar, a postdoctoral scholar in Hoffmann&rsquos laboratory who has now accepted a position as an assistant professor at the University of Texas, Austin.
Funding sources for the research included federal grants to Hoffmann by the National Cancer Institute and National Institute of Allergy and Infectious Diseases (grants R01CA141722, R01AI083453), both part of the National Institutes of Health, and funding to Okada-Hatakeyama from the Cell Innovation Program of Japan&rsquos Ministry of Education, Culture, Sports, Science and Technology, and the Japan Society for the Promotion of Science.
Hoffmann&rsquos research: Correcting cellular miscommunication
Many diseases are related to miscommunication in cells, Hoffmann said. In other research, he and colleagues showed for the first time that it is possible to correct a certain type of cellular miscommunication &mdash one involving the connection of receptors to genes controlled during inflammation &mdash without severe side effects. That research, federally funded by the NIH, was published in the journal Cell on Oct. 10, 2013.
Hoffmann and his colleagues may be able to develop therapeutic strategies that do not simply inhibit or shut down faulty communication lines in diseased cells but actually correct the misunderstanding. (They have already accomplished this with cells in a Petri dish. Their next step is to see if this can be done in an animal, and then in a human.)
Pathogens are organisms which cause disease. We’re all adapted to prevent these from getting into our bodies in the first place. If a pathogen does manage to sneak it’s way in, our immune system kicks into action, activating various types of white blood cells to manufacture antibodies and kill the pathogen.
Barriers to prevent entry of pathogens
Our bodies have several defensive barriers to prevent us becoming infected by pathogens. For example:
Our body cavities (e.g. eyes, nose, mouth, genitals) are lined with a mucus membrane which contain an enzyme called lysozyme. Lysozyme kills bacteria by damaging their cell walls, causing them to burst open.
Our skin acts as a physical barrier to stop pathogens from getting inside of us. If our skin is cut or wounded, our blood quickly clots to minimise the entry of pathogens.
The trachea (windpipe) contains goblet cells which secrete mucus. Pathogens that we inhale become trapped in the mucus, which is swept towards the stomach by the action of ciliated epithelial cells.
Our stomach contains gastric juices which are highly acidic - these will denature proteins and kill any pathogens that have been ingested in our food and drinks.
The insides of our intestines and the surface of our skin are covered in harmless bacteria which will compete with any pathogenic organisms and reduce their ability to grow.
Barriers against the entry of pathogens into the body.
Non-Specific Immune Response
The non-specific immune response is our immediate response to infection and is carried out in exactly the same way regardless of the pathogen (i.e. it is not specific to a particular pathogen). The non-specific immune response involves inflammation, the production of interferons and phagocytosis.
Inflammation - the proteins which are found on the surface of a pathogen (antigens) are detected by our immune system. Immune cells release molecules to stimulate vasodilation (the widening of blood vessels) and to make the blood vessels more permeable. This means that more immune cells can arrive at the site of infection by moving out of the bloodstream and into the infected tissue. The increased blood flow is why an inflamed part of your body looks red and swollen.
Production of interferons - if the pathogen which has infected you is a virus, your body cells that have been invaded by the virus will start to manufacture anti-viral proteins called interferons. They slow down viral replication in three different ways:
Stimulate inflammation to bring more immune cells to the site of infection
Inhibit the translation of viral proteins to reduce viral replication
Activate T killer cells to destroy infected cells
Phagocytes are a type of white blood cell which can destroy pathogens - types of phagocyte include macrophages, monocytes and neutrophils. They first detect the presence of the pathogen when receptors on its cell surface bind to antigens on the pathogen. The phagocyte then wraps its cytoplasm around the pathogen and engulfs it. The pathogen is contained within a type of vesicle called a phagosome. Another type of vesicle, called a lysosome, which contains digestive enzymes (lysozymes) will fuse with the phagosome to form a phagolysosome. Lysozymes digest the pathogen and destroy it. The digested pathogen will be removed from the phagocyte by exocytosis but they will keep some antigen molecules to present on the surface of their cells - this serves to alert other cells of the immune system to the presence of a foreign antigen. The phagocyte is now referred to as an antigen-presenting cell (APC).
B Cell Differentiation and Activation
B cells differentiate in the bone marrow. During the process of maturation, up to 100 trillion different clones of B cells are generated, which is similar to the diversity of antigen receptors seen in T cells.
After B cells are activated by their binding to antigen, they differentiate into plasma cells. Plasma cells often leave the secondary lymphoid organs, where the response is generated, and migrate back to the bone marrow, where the whole differentiation process started. After secreting antibodies for a specific period, they die, as most of their energy is devoted to making antibodies and not to maintaining themselves. Thus, plasma cells are said to be terminally differentiated.
Memory B cells function in a way similar to memory T cells. They lead to a stronger and faster secondary response when compared to the primary response, as illustrated below.
MCAT Biology : Antibodies and Antigens
Hypersensitivity reactions occur when body tissues are affected by an abnormal immune reaction. The result is damage to normal tissues and clinical illness. A peanut allergy is an example of a hypersensitivity reaction, but there are three additional broad classes.
One class involves the abnormal production or deposition of antibodies. Antibodies are B-cell derived molecules that normally adhere to pathogens, rendering them unable to continue an infection. When antibodies are produced against normal tissues, however, disease can result. Figure 1 depicts a schematic structure of an antibody.
Antibodies can be divided into two peptide chains: heavy and light. Heavy chains form the backbone of the antibody, and are attached to light chains via covalent bonding. Each heavy and light chain is then further divided into constant and variable regions. Variable regions exhibit molecular variety, generating a unique chemical identity for each antibody. These unique patterns help guarantee that the body can produce antibodies to recognize many possible molecular patterns on invading pathogens.
Unlike B-cells, T-cells do not make antibodies. T-cells are important in the execution of cytotoxic immunity, such as neutralizing virus-infected cells. A scientist is studying the T-cell response in a mammal, and finds that his CD8 + T-cells are interacting with a surface protein found on many different types of cells in his model organism. This protein is most likely __________ .
Q&A: B cells and antibodies in COVID-19: what does 'good' look like?
On Thursday 13 August, our 'Connecting on Coronavirus' webinar was presented by BSI Trustee Professor Deborah Dunn-Walters on the topic of 'B cells and antibodies – what does 'good' look like?' If you missed the live webinar, you can catch up and watch again here.
Professor Deborah Dunn-Walters is Professor of Immunology, Head of Immunology Section and Lead for Lifelong Health Research Theme at the University of Surrey. We hear a lot about the production of antibodies against SARS-CoV-2, but not all B cells go on to produce antibodies, and of those that do, some produce a different type of antibodies than others. Making good antibodies without making potentially harmful ones is the goal of vaccines, but do we know how to tell the difference easily? In this webinar, Professor Dunn-Walters provided us with a summary of research to date and posed some key questions for areas to research in the future.
Unfortunately, we lost the internet connection during the Q&A at the end of the webinar. However, the audience had sent a substantial number of questions in and Professor Dunn-Walters very kindly took the time answer some of these for this blog.
Given what we currently know about the production of antibodies and COVID-19, what implications does this have for understanding the results of antibody tests against the disease? If I have antibodies, does that mean I’m immune to future infection?Graphic from BSI resources on COVID-19 testing
Deborah Dunn-Walters: There is a lot of variability in antibody response between people and in the results depending on which method is used to measure the antibody. Antibodies may not be seen early in the disease in SARS-CoV-2 positive patients but are likely to be seen by 20 days after initial infection. IgM and IgA antibodies would not be detected a couple of months after recovery, but IgG would be there for longer. You are more likely to develop antibodies that can be picked up by a standard antibody test if you had a more severe disease than someone who was asymptomatic or only had mild disease.
A positive test using a method that is 100% specific, means that you have had the disease. If a test were, for example, 99.7% specific then a positive result would mean that you are highly likely to have had the disease, but 3 in every thousand results might be a false positive.
A negative test could mean that either you haven’t had the virus at all, or that you did have the virus but it was such a mild/asymptomatic infection you didn’t make antibodies or that any antibodies made are in such low quantity that an antibody test is not sensitive enough to detect them. A recent study of 341 volunteers with a previous positive PCR test showed that even after correcting for the lack of sensitivity of the test there were 4% negative results. 1
Either way, because we do not yet have an answer as to whether the presence of antibodies indicates immune protection against infection we can only use antibody tests to estimate disease prevalence, not to assess our level of immunity.
Do you think that any of the antibodies produced in response to COVID-19 might cross-react with antibodies produced for the other coronaviruses that infect humans (and which cause common colds)?
DDW: Yes. The systems serology paper by Butler et al. presented in the webinar shows that there are antibodies produced in COVID-19 patients, in both serum and nasal washes, that react with other common coronaviruses. 2
Do you have any further thoughts about the use of convalescent serum containing high levels of antibodies for severe COVID-19 patients. Is there any evidence that it is helping recovery?
DDW: If my hypothesis is correct, that later antibodies are more matured than the antibodies made early in the disease, then it follows that antibodies from convalescent patients will be useful. For example, they may be of higher affinity and have different levels of sugars on them. There has been no evidence of harm from convalescent serum and randomised controlled trials are ongoing. Indications are that transfusion at an early stage with convalescent plasma containing high levels of IgG would be of use. 3
What determines low or high antibody titers in COVID-19? For example, the amount of virus, or how many B-cells recognise the antigen, or both?
DDW: Firstly, I am very keen to stress the distinction between correlation of two factors versus “determination” or “causation” between one and the other. The observations we make about COVID-19 now are correlations. Plus, I don’t yet know of any quantitative studies of antigen-specific B cells during the disease, so this is difficult to answer. In cases with high virus titres and severe disease you will see high levels of antibody.
Some reports suggest that there is T cell depletion in COVID-19. Will this affect T-dependent B cell response in COVID-19 and subsequent antibody formation?
DDW: I certainly think this is an issue. Helper T cells are needed to help both Killer T cells and B cells. A B cell in the germinal centre needs T cell help to survive when it is affinity maturing its antibody. I should also mention that B cells are excellent antigen presenting cells to help activate T cells. You need both for the best responses.
Do you think that the generation of appropriate memory B cells is more important than having long-lived antibodies in building immunity to COVID-19?
DDW: I would like to have both. Having long-lived neutralising antibody circulating in the blood (or present in the nose) is good because the antibody can block infection in the first place. Having memory B cells is also good because the cells can reactivate quickly in response to challenge and replenish the long-lived antibody producing cells. It may be possible to improve the antibody even further if a memory B cell enters a new germinal centre reaction the second time round. This might be an opportunity to slightly adjust the antibody in case the virus had changed a little in the meantime.
What implications does the half-life of COVID-19 antibodies have for vaccine development and for the type of approach that might be successful?
DDW: My personal hope is that the response to vaccines would be different than the response to disease. If in the disease the B and T cells decrease and this hinders the type of response that makes high affinity antibodies, perhaps in a vaccine the response is more controlled – more T-dependent response and less extrafollicular response. So better longer-lived antibodies and memory B cells will be made from the beginning. This is my conjecture and hope not fact.
We’d like to thank Professor Deborah Dunn-Walters for her time and expertise in both presenting and answering these questions. You can watch the full webinar here.
How are Antibodies Produced?
How are Antibodies Produced?
Although detailed mechanics of the immune response are beyond the scope of this site, it is useful, in the context of developing a custom antibody, to have an overview of how antibodies are produced by the immune system.
When an organism’s immune system encounters a foreign molecule (typically a protein) for the first time, specialized cells such as macrophages and dendritic cells capture the molecule and begin breaking it down so that it can present these antigens to B cell lymphocytes.
Once Antigen Presentation to the B cell lymphocytes has occurred, a process known as Somatic Hypermutation allows the B cell to begin coding for a new antibody that will contain a unique Antigen Binding Site in the variable region that is capable of binding specifically to an epitope from the antigen.
Each B cell lymphocyte produces one unique antibody against one unique epitope.
Once antibodies with sufficient specificity to the epitope can be encoded, the B cell begins to release antibodies into the bloodstream. These antibodies then bind specifically with the foreign molecule and allow the immune system to eliminate the molecule from the system.
In some cases, these antibodies can disable pathogens such as viruses directly due to the binding action. In other cases, such as with bacterial pathogens, these antibodies bind to surface proteins on the bacterium’s surface, thereby signaling to the rest of the immune system that the pathogen should be destroyed.
After the foreign molecule has been eliminated, B cells remain in the bloodstream ready to produce antibodies if the antigen is encountered again.
From the perspective of developing a custom antibody against a protein antigen, the immune system captures the protein, breaks it down into individual epitopes and presents these epitopes to the B cells so that development of antibodies specific to those epitopes can begin. These antibodies can then be collected directly in the serum or by isolating the individual B cells that produce antibody against the epitope of interest. With a full-length protein antigen, there will typically be multiple B cells generating antibodies against multiple epitopes from different regions of the protein.