which lymphocytes lack specificity for antigenwhich lymphocytes lack specificity for antigen

The physician prescribes rehydration, supportive therapy, and antibiotics to stem the bacterial infection. The activating receptors on NK cells can recognize numerous ligands on target cells (eg, MHC class Irelated chain A [MICA] and chain B [MICB]); the inhibitory receptors on NK cells recognize MHC class I molecules. Although T cells can produce a variety of CD molecules, CD4 and CD8 are the two most important used for differentiation of the classes. contain purified, chemically denatured bacterial exotoxin. al. A. foreign to the immune system. Dendritic cells present antigen fragments to T cells in the lymph nodes. Antibodies are essential for fighting off certain types of bacterial and fungal infections. Copyright 2023 Merck & Co., Inc., Rahway, NJ, USA and its affiliates. This final step is sometimes referred to as central tolerance because it prevents self-reacting T cells from reaching the bloodstream and potentially causing autoimmune disease, which occurs when the immune system attacks healthy self cells. They differentiate from Th0 cells into one of the following: Th1 cells: In general, Th1 cells promote cell-mediated immunity via cytotoxic T cells and macrophages and are thus particularly involved in defense against intracellular pathogens (eg, viruses). 24. Once an infected cell is recognized, the TCR of the cytotoxic T cell binds to the epitope and releases perforin and granzymes that destroy the infected cell (Figure 18.18). Helper T cells help B cells in this process. A third subtype of helper T cells called TH17 cells was discovered through observations that immunity to some infections is not associated with TH1 or TH2 cells. 44. are not subject to the Creative Commons license and may not be reproduced without the prior and express written Cells that are not removed (ie, those that recognize nonself antigen) continue to develop into mature naive B cells, leave the marrow, and enter peripheral lymphoid organs, where they may encounter antigens. (1) = 1st signal; (2) = 2nd signal. Common symptoms include visual and oculomotor abnormalities, paresthesias, weakness read more ), and a Th2 response promotes IgE production and development of allergic disorders, as well as helps B cells produce autoantibodies in some autoimmune disorders (eg, Graves disease Hyperthyroidism Hyperthyroidism is characterized by hypermetabolism and elevated serum levels of free thyroid hormones. They recruit other defensive molecules in the bloodstream to the site, working toward killing the infection-causing organism. Helper T cells are arguably the most important cells in adaptive immunity, as they are required for almost all adaptive immune responses. Normally, IgM is present in the bloodstream but not in the tissues. More antibody is produced in this response (called the secondary immune response) than in the primary immune response. 28. (credit: modification of work by Microbiotic/YouTube). A precursor cell that, when appropriately stimulated, can differentiate into a Tc cell, An effector cell that has differentiated and can kill its appropriate target, A memory cell that is quiescent (no longer stimulated) but is ready to become an effector when restimulated by the original antigen-MHC combination. Symptoms include palpitations, fatigue, weight loss, heat intolerance, anxiety, and tremor read more , myasthenia gravis Myasthenia Gravis Myasthenia gravis is characterized by episodic muscle weakness and easy fatigability caused by autoantibody- and cell-mediated destruction of acetylcholine receptors. The T-cell receptor (TCR) is involved in the first step of pathogen epitope recognition during the activation process. It is learned. The TCR is structurally homologous to the B-cell receptor; the and (or gamma [] and delta []) chains have constant (C) and variable (V) regions. Tellier J, Shi W, Minnich M, Liao Y, Crawford S, Smyth GK, Kallies A, Busslinger M, Nutt S.L. Also, IgG is the most common class of antibody used in treatment. They settle down mostly in the spleen and lymph nodes to pump out antibodies. QUESTION 56 Which lymphocytes lack specificity for antigen and are cells that attack cancer cells and virus-infected cells? B. treatment with formalin, heat, or radiation. But this event alone does not activate the helper T cell. These diseases are essentially B-cell cancers. The maturation of thymocytes within the thymus can be divided into three critical steps of positive and negative selection, collectively referred to as thymic selection. After T and B cells are presented with the antigen, they become activated. 2023 Dotdash Media, Inc. All rights reserved. The white blood cell increases are recorded at 28.5 K/L (normal range: 6.017.5 K/L). Constant part: This part can be one of five structures, which determines the antibodys class IgM, IgG, IgA, IgE, or IgD. B cells are produced in the bone marrow. Some helper T cells help B cells produce antibodies against foreign antigens. Different patterns of cytokine production identify other Th-cell functional phenotypes. When a B cell encounters an antigen, it is stimulated to mature into a plasma cell or a memory B cell. Elimination of these cells ensures that the immune system is less likely to recognize these antigens as foreign (immune tolerance). Follicular dendritic cells are a distinct lineage, do not express class II MHC molecules, and therefore do not present antigen to Th cells. As noted, proliferation and differentiation of cytotoxic T cells is also stimulated by cytokines secreted from TH1 cells activated by the same foreign epitope. The B-cell response to antigens has two stages: Primary immune response: When B cells first encounter an antigen, the antigen attaches to a receptor, stimulating the B cells. Killed or inactivated vaccines are prepared by. Consequently, these cells continue to mature and then exit the thymus, armed to inhibit the activation of self-reactive T cells. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites read more (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and other invaders. By secreting IFN-gamma, NK cells can influence the acquired immune system by promoting differentiation of type 1 helper T (Th1) cells and inhibiting that of type 2 (Th2) cells. T cells and B cells work together to destroy invaders. The first step in the activation process is TCR recognition of the specific foreign epitope presented within the MHC II antigen-binding cleft. Glucose levels in the CSF were registered at 30 mg/100 mL (normal range: 5080 mg/100 mL). C. Lymphocytes with specificity for self are destroyed. Which lymphocytes lack specificity for antigen and attack cancer cells and virus-infected cells? The wall around the microorganisms is called a granuloma. Keep in mind that sea ice reflects more incoming solar energy than water does. For example, the helper T cell produces and displays IL-2 receptors on its surface and also secretes IL-2 molecules, which bind to these receptors and stimulate the helper T cell to grow and divide. Which process involves antibodies covering surface receptors on a virus or toxin molecule, thereby disrupting their activity? If the first signal and one of the second signals are received, the helper T cell becomes activated to proliferate and to stimulate the appropriate immune cell. Instead they help activate cytotoxic T cells and macrophages to attack infected cells, or they stimulate B cells to secrete antibodies. 58. 20. During the development of the functional TCR in the thymus, genetic rearrangement in a T cell brings together one V segment and one J segment to code for the variable region of the chain. Tc cells play a role in organ transplant rejection. (b) The macrophage in this figure is presenting a superantigen that is not recognized by the TCR of the T cell, yet the superantigen still is able to bridge and bind the MHC II and TCR molecules. Both actions stimulate the T cell (1st signal) through the accessory CD3 chains. Now the fate of the immature lymphocyte will be determined by the specificity of its antigen receptor. Properties of effective antigens include all of the following except, 21. Regulatory T cells participate in peripheral tolerance by inhibiting the activation and function of self-reactive T cells and by secreting anti-inflammatory cytokines. An activated TH cell produces ___, which is a growth factor for T helper cells and cytotoxic T cells. The presence of MHC class I molecules (normally expressed on nucleated cells) on cells therefore prevents destruction of cells; their absence indicates that the cell is infected with certain viruses that inhibit MHC expression or has lost MHC expression because cancer has changed the cell. Most antigens are T-dependent. Brought to you by Merck & Co, Inc., Rahway, NJ, USA (known as MSD outside the US and Canada) dedicated to using leading-edge science to save and improve lives around the world. bacterial toxins that activate T cells at a 100 times greater rate than other antigens, The immunoglobulin class that has a dimer form found in mucus, saliva, colostrum, and other body secretions is, receptors located primarily on macrophages and B cells, Which of the following is not a property of B cells? then you must include on every digital page view the following attribution: Use the information below to generate a citation. Rather, cell-mediated immunity is the activation of phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen. All rights reserved. Monoclonal antibodies that target checkpoint molecules on either T cells or on tumor cells (termed checkpoint inhibitors, see table Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use ) are used to prevent downregulation of antitumor immune responses and effectively treat some heretofore resistant cancers and boost the antitumor response. Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. Although some antigens (Ags) can read more , although the 2 cell types have distinct lineages. Only T cells that can recognize nonself antigen complexed to self MHC molecules survive; they leave the thymus for peripheral blood and lymphoid tissues. In contrast, the primary role of regulatory T cells is to prevent undesirable and potentially damaging immune responses. _____ is the most abundant class of antibodies in serum. 42. (See also Overview of the Immune System Overview of the Immune System The immune system distinguishes self from nonself and eliminates potentially harmful nonself molecules and cells from the body. IgG protects against bacteria, viruses, fungi, and toxic substances. T Cell Production and Maturation. Nat Rev Immunol. Our mission is to improve educational access and learning for everyone. The neutrophil percentage was recorded as 60% (normal range: 2345%). 3. Helper T cells serve as the central orchestrators that help activate and direct functions of humoral and cellular immunity. Early symptoms are related to hyperglycemia and include polydipsia read more . Minnich M, Tagoh H, Bnelt P, et al. The alpha () and beta () chains of the T-cell receptor (TCR) bind to antigen (Ag)major histocompatibility complex (MHC) on an antigen-presenting cell (APC), and CD4 or CD8 interacts with the MHC. Learn more about the MSD Manuals and our commitment to Global Medical Knowledge. Regulatory T cells thus act to damp down the immune response and can sometimes predominate so as to suppress it completely. Our website is not intended to be a substitute for professional medical advice, diagnosis, or treatment. B cells can present antigen to T cells and release cytokines, but their primary function is to develop into plasma cells, which manufacture and secrete antibodies Antibodies The immune system consists of cellular components and molecular components that work together to destroy antigens (Ags). NK cells constitute 5 to 15% of peripheral blood mononuclear cells and have a round nucleus and granular cytoplasm. These blood-forming cells give rise to B-cells through a series of steps. IgG, the most prevalent class of antibody, is produced when a particular antigen is encountered again. T cells also play a more central role in orchestrating the overall adaptive immune response (humoral as well as cellular) along with the cellular defenses of innate immunity. Lymphocytes are round and are almost entirely composed of a nucleus (the core of a cell that contains its genetic material in the form of DNA ). natural . Viruses known to produce superantigens include Epstein-Barr virus (human herpesvirus 4), cytomegalovirus (human herpesvirus 5), and others. Cytotoxic T cells are powerful agents of cellular immunity. Manifestations include read more ) are especially susceptible to chronic bacterial and fungal infections. Certain bacterial and viral pathogens produce toxins known as superantigens (see Virulence Factors of Bacterial and Viral Pathogens) that can trigger such an unregulated response. These antibodies serve as flags or flares over a battle site. The link you have selected will take you to a third-party website. Blaschitz C., Raffatellu M. Th17 cytokines and the gut mucosal barrier. J Clin Immunol. Lymphocytosis refers to abnormally high levels of lymphocytes, which are a type of white blood cell. 31. The process, called clonal selection, is one of the . The genes segments associated with the chain of the TCR consist 70 or more different V segments and 61 different J segments. By releasing these mediators, mast cells play a key role in generating protective acute inflammatory responses; basophils and mast cells are the source of type I hypersensitivity reactions associated with atopic allergy Type I Allergic (including atopic) and other hypersensitivity disorders are inappropriate or exaggerated immune reactions to foreign antigens. Antitoxins contain antibodies to neutralize specific toxin. Some, however, are able to stimulate B cells without the help of T cells. Destruction of lymphocytes with self-specificity is called. Symptoms vary depending read more , in which the body attacks its own tissues. The WBC count in the CSF was 1,163/mm3 (normal range: 520/mm3). Most obviously, lymphocytes with strongly self-reactive receptors should be eliminated to prevent autoimmune reactions; this negative selection is one of the ways in which the immune system is made self-tolerant. T cells, like all other white blood cells involved in innate and adaptive immunity, are formed from multipotent hematopoietic stem cells (HSCs) in the bone marrow (see Figure 17.12). Which of the following is not true of the antigen-independent period of lymphocyte development? Lymphocytes constitute around 20% and 40% of all white blood cells. Creative Commons Attribution License D. low numbers circulating in the blood B. molecular complexity. The white blood cells responsible for acquired immunity are, Other participants in acquired immunity are, Dendritic cells Dendritic Cells One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and read more, Cytokines Cytokines One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and read more, The complement system Complement System One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and read more (which enhances the effectiveness of antibodies). Anergy refers to a state of nonresponsiveness to antigen stimulation. Activation is carried out through a cell-to-cell interaction that occurs between a protein called the CD40 ligand, which appears on the surface of the activated helper T cells, and the CD40 protein on the B-cell surface. In addition, helper T cells enhance the pathogen-killing functions of macrophages and NK cells of innate immunity. Luckily, there are millions of them in our body so we can fight many different types of infection. When these cells encounter certain antigens, the bivalent IgE molecules bound to the receptors become cross-linked, triggering cell degranulation with release of preformed inflammatory mediators (eg, histamine, platelet-activating factor) and generation of newly synthesized mediators (eg, leukotrienes, prostaglandins, thromboxanes). Which lymphocytes lack specificity for antigen and attack cancer cells and virus-infected cells? Usually, to be activated, T cells require the help of another immune cell, which breaks antigens into fragments (called antigen processing Recognition ) and then presents antigen from the infected or abnormal cell to the T cell. Secondary (anamnestic or booster) immune response: When memory B and Th cells are reexposed to the antigen, the memory B cells rapidly proliferate, differentiate into mature plasma cells, and promptly produce large amounts of antibody (chiefly IgG because of a T cellinduced isotype switch). David N. Shier, Jackie L. Butler, Ricki Lewis, John David Jackson, Patricia Meglich, Robert Mathis, Sean Valentine. Effector B cells are called plasma cells and secrete antibodies, and activated T cells include cytotoxic T cells and helper T cells, which carry out cell-mediated responses. The phagocyte can then recognize the bacteria. The molecular fragment on an antigen molecule that a lymphocyte recognizes and responds to is called a/an A. epitope. (See also Overview of the Immune System Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Cellular immunity, on the other hand, targets and eliminates intracellular pathogens through the actions of T lymphocytes, or T cells (Figure 18.13). About 5 to 15% of lymphocytes in the blood are B cells; they are also present in the bone marrow, spleen, lymph nodes, and mucosa-associated lymphoid tissues. B. disulfide bonds between polypeptide chains. (See also Overview of the Immune System.) Once activated, cytotoxic T cells serve as the effector cells of cellular immunity, recognizing and kill cells infected with intracellular pathogens through a mechanism very similar to that of NK cells. Patients with functional mutations in Foxp3 develop the autoimmune disorder IPEX syndrome IPEX Syndrome IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) is an X-linked recessive syndrome involving aggressive autoimmunity. They remember the antigen they are specific for and are ready to respond quickly if they see it again. A. two identical heavy polypeptide chains. When T cell activation is controlled and regulated, the result is a protective response that is effective in combating infections. What process generates many B cells and T cells that are activated against specific antigens? Acute phase reactants are plasma read more that work together to destroy antigens. So, for example, if the antigen is microbial and an individual is reinfected by the microbe, the memory cells trigger a rapid rise in the level of protective antibodies and thus prevent the associated illness from taking hold. So, for example, cytotoxic T cells can attack target cells that express antigens made by viruses or bacteria growing within them. Which T cells can be activated by antigens presented by cells other than APCs? When you get immunized, the vaccine contains antigens that stimulate the B-cells to produce antibodies that will then attack the virus, bacteria, or toxin you are being immunized against. This mimics what is happening in your body when you are infected with that germ,but without the same risks of the disease caused by the germ or toxin. The co-stimulation that comes from these TH1 cells is provided by secreted cytokines. $$ When examined under a microscope, lymphocytes will be bigger than red blood cells but fewer in number. D. binding of the T cell to a class II MHC receptor on a macrophage. They can also promote the production of some antibody classes. Syngeneic: Generated in response to self (autologous) cells modified by viral infection or other foreign proteins, Allogeneic: Generated in response to cells that express foreign MHC products (eg, in organ transplantation when the donors MHC molecules differ from the recipients). Each T lymphocyte has a T cell receptor (TCR) which is specific to a particular antigen. 25. Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. In its lifetime a lymphocyte may or may not come into contact with the antigen it is capable of recognizing, but if it does it can be activated to multiply into a large number of identical cells, called a clone. Cell-mediated immunity or cellular immunity is an immune response that does not involve antibodies. In most cases, however, B-cell activation is dependent on a second factor mentioned abovestimulation by an activated helper T cell. Helper T cells become activated by interacting with antigen-presenting cells, such as macrophages. Tc cells can secrete cytokines and, like Th cells, have been divided into types Tc1 and Tc2 based on their patterns of cytokine production. The counterpart to humoral immunity is cell-mediated immunity. Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells. Enter search terms to find related medical topics, multimedia and more. Each plasma B-cell makes antibodies to only one antigen. Perforin is a protein that creates pores in the target cell, and granzymes are proteases that enter the pores and induce apoptosis. The antigen attaches to the variable part. Follicular dendritic cells help B cells respond to an antigen. Once a helper T cell has been activated by an antigen, it becomes capable of activating a B cell that has already encountered the same antigen. and you must attribute OpenStax. IgM. Helper T cells help other immune cells. Thus, it can always recognize the specific antigen that it was formed to attach to. Usually, allergies cause sneezing, watery and itchy eyes, a read more . B-cells become "activated" when they encounter foreign antigens, as in foreign markers on the outside of bacteria cells during an infection. citation tool such as, Authors: Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster. Various cytokines produced by TH2 cells orchestrate antibody class switching, which allows B cells to switch between the production of IgM, IgG, IgA, and IgE as needed to carry out specific antibody functions and to provide pathogen-specific humoral immune responses. The cerebrospinal fluid (CSF) obtained appears cloudy and is further evaluated by Gram stain assessment and culturing for potential bacterial pathogens. It helps these cells mature. The immunoglobulin/s found on the surface of B cells is/are, 40. Memory cells, especially those with the most effective receptors, multiply extensively, but they do not secrete antibody. 1. Chemicals with incidental protective effects, Proteins from naturally occurring bacteria, Basic structure of the immunoglobulin molecule, Transfer of antibodies from mother to offspring, The development of immunity in major animal groups, 17 Questions About Health and Wellness Answered. Intracellular antigens (eg, viruses) can be processed and presented to CD8 cytotoxic T cells by any nucleated cell because all nucleated cells express class I MHC molecules. Dendritic cells reside in the skin, lymph nodes, and tissues throughout the body. Regulatory (suppressor) T (Treg) cells mediate suppression of immune responses and usually express the Foxp3 transcription factor. DNA and histones are also released, and they, with granule contents such as elastase, generate fibrous structures called neutrophil extracellular traps (NETs) in the surrounding tissues; these structures facilitate killing by trapping bacteria and focusing enzyme activity. Then, the components of acquired immunity learn the best way to attack each antigen and begin to develop a memory for that antigen. **Predict** how the energy absorbed by the Arctic Ocean would change if the amount of the sea ice covering the ocean is reduced. 55. She is not diabetic and does not currently have an infection of any kind to her knowledge. A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected by intracellular . Enter search terms to find related medical topics, multimedia and more. The response triggered by the first encounter with an antigen is the primary immune response. In this video, you can see a cytotoxic T cell inducing apoptosis in a target cell. It is learned. https://openstax.org/books/microbiology/pages/1-introduction, https://openstax.org/books/microbiology/pages/18-3-t-lymphocytes-and-cellular-immunity, Creative Commons Attribution 4.0 International License, APCs presenting antigens associated with MHC II, Orchestrate humoral and cellular immunity, Involved in the activation of macrophages and NK cells, Involved in peripheral tolerance and prevention of autoimmune responses, APCs or infected nucleated cells presenting antigens associated with MHC I, Destroy cells infected with intracellular pathogens, Stimulate cytotoxic T cells and produce memory cytotoxic T cells, Stimulate macrophages and neutrophils (PMNs) for more effective intracellular killing of pathogens, Stimulate NK cells to kill more effectively, Stimulate B cell activation and differentiation into plasma cells and memory B cells, Direct antibody class switching in B cells, Stimulate immunity to specific infections such as chronic mucocutaneous infections, Remember a specific pathogen and mount a strong, rapid secondary response upon re-exposure, Describe the process of T-cell maturation and thymic selection, Explain the genetic events that lead to diversity of T-cell receptors, Compare and contrast the various classes and subtypes of T cells in terms of activation and function, Explain the mechanism by which superantigens effect unregulated T-cell activation. She is on no other medications and is not abusing any drugs or alcohol. differentiation. Mast cells are tissue-based and functionally similar to basophils circulating in the blood. The most significant cells in graft rejection are. For example, a Th1 response dominates in tuberculoid leprosy Symptoms and Signs , and a Th2 response dominates in lepromatous leprosy. C. contain select antigenic components of a pathogen rather than whole cells or viruses. Known bacterial superantigens include toxic shock syndrome toxin (TSST), staphylococcal enterotoxins, streptococcal pyrogenic toxins, streptococcal superantigen, and the streptococcal mitogenic exotoxin. Some accessory molecules (eg, CTLA-4 [cytotoxic T-lymphocyte antigen 4] on the T cell, which also interacts with CD80 and CD86 on the antigen-presenting cell, PD-1 [programmed cell death protein 1] on the T cell, which interacts with PD-L1 [programmed cell death protein ligand 1] on the antigen-presenting cell) inhibit previously activated T cells and thus dampen the immune response. After the first encounter with an antigen, production of enough of the specific antibody takes several days. Use to remove results with certain terms The wall is formed when phagocytes, particularly macrophages, adhere to each other. C. antigen binding sites. Another type of dendritic cell, the follicular dendritic cell, is present in lymph nodes and presents unprocessed (intact) antigen that has been linked with antibody (antibody-antigen complex) to B cells. B. haptens. At this point, any cells that interact with self antigen (autoimmune cells) are removed from the immature B cell population via inactivation (anergy) or apoptosis. How does a superantigen activate a helper T cell? Some bacteria thus imprisoned may survive in the body indefinitely. The first steps of differentiation occur in the red marrow of bones (Figure 18.14), after which immature T lymphocytes enter the bloodstream and travel to the thymus for the final steps of maturation (Figure 18.15). For adults, normal lymphocyte count is between 1,000 and 4,800 lymphocytes per microliter of blood. The trusted provider of medical information since 1899, Molecular Components of the Immune System, Last review/revision Sep 2021 | Modified Sep 2022. About 5 to 15% of lymphocytes in the blood are B cells; they are also present in the bone marrow, spleen, lymph nodes, and mucosa-associated lymphoid tissues. Antigen-presenting cells ingest a microbe, partially degrade it, and export fragments of the microbei.e., antigensto the cell surface, where they are presented in association with class II MHC molecules. Cytotoxic T cells, in turn, can attack and kill other cells that express the foreign antigen in association with class I MHC molecules, whichas explained aboveare present on almost all cells. Then the B cells are stimulated by various cytokines to develop into the antibody-producing cells called plasma cells. D. large molecules with a minimum molecular weight of 1,000. Genes that encode the TCR, like Ig genes, are rearranged, resulting in defined specificity and affinity for antigen. $$ Which of the following is not a property of B cells? CD molecules function in cell adhesion, cell signaling, as receptors for the Fc region of immunoglobulins, as receptors for components of the complement system, and others. TCRs are epitope-specific, and it has been estimated that 25 million T cells with unique epitope-binding TCRs are required to protect an individual against a wide range of microbial pathogens. The main purpose of B cells is to produce antibodies, which tag an antigen for attack or directly neutralize it.

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