Principles of innate and adaptive immunity – Immunobiology – NCBI Bookshelf

Principles of innate and adaptive immunity

The macrophages and neutrophils of the congenital immune system provide a beginning line of defense against many common microorganisms and are essential for the control of common bacterial infections. however, they can not always eliminate infectious organisms, and there are some pathogens that they can not recognize. The lymphocytes of the adaptive immune organization have evolved to provide a more versatile means of defense mechanism which, in addition, provides increase protection against subsequent reinfection with the same pathogen. The cells of the natural immune system, however, play a all-important function in the knowledgeability and subsequent guidance of adaptive immune responses, a well as participating in the removal of pathogens that have been targeted by an adaptive immune answer. furthermore, because there is a check of 4–7 days before the initial adaptive immune response takes effect, the unconditioned immune response has a critical function in controlling infections during this menstruation .

1-5. Most infectious agents induce inflammatory responses by activating innate
immunity

Microorganisms such as bacteria that penetrate the epithelial surfaces of the body for the first time are met immediately by cells and molecules that can mount an congenital immune response. phagocytic macrophages conduct the defense against bacteria by means of open receptors that are able to recognize and bind common constituents of many bacterial surfaces. bacterial molecules binding to these receptors trigger the macrophage to engulf the bacteria and besides induce the secretion of biologically active molecules. Activated macrophages secrete cytokines, which are defined as proteins released by cells that affect the demeanor of early cells that have a bun in the oven receptors for them. They besides release proteins known as chemokines that attract cells with chemokine receptors such as neutrophils and monocytes from the bloodstream ( ). The cytokines and chemokines released by macrophages in response to bacterial constituents initiate the work known as inflammation. local excitement and the phagocytosis of invading bacteria may besides be triggered as a leave of the energizing of complement on the bacterial cell surface. Complement is a system of plasma proteins that activates a shower of proteolytic reactions on microbial surfaces but not on horde cells, coating these surfaces with fragments that are recognized and bound by phagocytic receptors on macrophages. The cascade of reactions besides releases little peptides that contribute to excitement .

Figure 1.12

bacterial infection triggers an inflammatory reception. Macrophages encountering bacteria in the tissues are triggered to release cytokines that increase the permeability of blood vessels, allowing fluid and proteins to pass into the tissues. They besides produce ( more … )

inflammation is traditionally defined by the four Latin words calor, dolor, rubor, and tumor, meaning heat, pain, inflammation, and swelling, all of which reflect the effects of cytokines and other incendiary mediators on the local rake vessels. Dilation and increased permeability of the blood vessels during excitement run to increased local blood stream and the escape of fluid, and account for the estrus, inflammation, and swelling. Cytokines and complement fragments besides have significant effects on the adhesive properties of the endothelium, causing circulating leukocytes to stick to the endothelial cells of the lineage vessel wall and migrate between them to the web site of infection, to which they are attracted by chemokines. The migration of cells into the tissue and their local actions account for the pain. The independent cell types seen in an incendiary reply in its initial phases are neutrophils, which are recruited into the kindle, infect tissue in large numbers. Like macrophages, they have surface receptors for coarse bacterial constituents and complement, and they are the principal cells that steep and destroy the invade microorganism. The inflow of neutrophils is followed a short meter late by monocytes that quickly differentiate into macrophages. Macrophages and neutrophils are thus besides known as inflammatory cells. Inflammatory responses by and by in an infection besides involve lymphocytes, which have meanwhile been activated by antigen that has drained from the locate of infection via the sensory nerve lymphatics. The congenital immune response makes a crucial contribution to the activation of adaptive unsusceptibility. The incendiary response increases the menstruation of lymph containing antigen and antigen-bearing cells into lymphoid weave, while complement fragments on microbial surfaces and induced changes in cells that have taken up microorganism provide signals that synergize in activating lymphocytes whose receptors bind to specific microbial antigens. Macrophages that have phagocytosed bacteria and become activated can besides activate T lymphocytes. however, the cells that specialize in presenting antigen to T lymphocytes and initiating adaptive immunity are the dendritic cells .

1-6. Activation of specialized antigen-presenting cells is a necessary first step
for induction of adaptive immunity

The induction of an adaptive immune response begins when a pathogen is ingested by an immature dendritic cell in the infect weave. These specialize phagocytic cells are resident in most tissues and are relatively durable, turning over at a decelerate rate. They derive from the same cram marrow precursor as macrophages, and migrate from the bone marrow to their peripheral stations, where their character is to view the local environment for pathogens. finally, all tissue-resident dendritic cells migrate through the lymph to the regional lymph nodes where they interact with recirculating primitive lymphocytes. If the dendritic cells fail to be activated, they induce allowance to the antigens of self that they bear. The immature dendritic cell carries receptors on its surface that acknowledge common features of many pathogens, such as bacterial cell wall proteoglycans. As with macrophages and neutrophils, adhere of a bacteria to these receptors stimulates the dendritic cell to engulf the pathogen and degrade it intracellularly. Immature dendritic cells are besides continually taking up extracellular material, including any virus particles or bacteria that may be show, by the receptor-independent mechanism of macropinocytosis. The affair of dendritic cells, however, is not chiefly to destroy pathogens but to carry pathogen antigens to peripheral lymphoid organs and there present them to T lymphocytes. When a dendritic cell takes up a pathogen in infect weave, it becomes activated, and travels to a nearby lymph node. On energizing, the dendritic cell matures into a highly effective antigen-presenting cell ( APC ) and undergo changes that enable it to activate pathogen-specific lymphocytes that it encounters in the lymph node ( ). Activated dendritic cells secrete cytokines that influence both congenital and adaptive immune responses, making these cells necessity gatekeepers that determine whether and how the immune system responds to the presence of infectious agents. We shall consider the maturation of dendritic cells and their central role in presenting antigens to T lymphocytes in chapter 8 .

Figure 1.13

dendritic cells initiate adaptive immune responses. Immature dendritic cells nonmigratory in septic tissues take up pathogens and their antigens by macropinocytosis and receptor-mediated phagocytosis. They are stimulated by realization of the presence of ( more … )

1-7. Lymphocytes activated by antigen give rise to clones of antigen-specific
cells that mediate adaptive immunity

The defense systems of natural immunity are effective in combating many pathogens. They are constrained, however, by relying on germline-encoded receptors to recognize microorganisms that can evolve more quickly than the hosts they infect. This explains why they can only recognize microorganisms bearing open molecules that are common to many pathogens and that have been conserved over the course of evolution. not surprisingly, many infective bacteria have evolved a protective ejection seat that enables them to conceal these molecules and thereby avoid being recognized and phagocytosed. Viruses carry no invariant molecules similar to those of bacteria and are rarely recognized immediately by macrophages. Viruses and encapsulated bacteria can, however, distillery be taken up by dendritic cells through the nonreceptor-dependent serve of macropinocytosis. Molecules that reveal their infectious nature may then be unmasked, and the dendritic cellular telephone activated to present their antigens to lymphocytes. The recognition mechanism used by the lymphocytes of the adaptive immune response has evolved to overcome the constraints faced by the natural immune system, and enables recognition of an about space diversity of antigens, so that each different pathogen can be targeted specifically. rather of bearing respective different receptors, each recognizing a different surface feature shared by many pathogens, each uninstructed lymphocyte entering the bloodstream bears antigen receptors of a single specificity. The specificity of these receptors is determined by a alone genetic mechanism that operates during lymphocyte development in the bone marrow and thymus gland to generate millions of unlike variants of the genes encoding the sense organ molecules. therefore, although an individual lymphocyte carries receptors of entirely one specificity, the specificity of each lymphocyte is different. This ensures that the millions of lymphocytes in the body jointly carry millions of different antigen receptor specificities—the lymphocyte sense organ repertory of the individual. During a person ‘s life these lymphocytes undergo a process akin to natural choice ; alone those lymphocytes that encounter an antigen to which their receptor binds will be activated to proliferate and differentiate into effector cells. This selective mechanism was first proposed in the 1950s by Macfarlane
Burnet
to explain why antibodies, which can be induced in reception to about any antigen, are produced in each individual only to those antigens to which he or she is exposed. He postulated the preexistence in the body of many unlike potential antibody -producing cells, each having the ability to make antibody of a unlike specificity and displaying on its surface a membrane-bound version of the antibody that served as a receptor for antigen. On binding antigen, the cellular telephone is activated to divide and produce many identical offspring, known as a knockoff ; these cells can now secrete clonotypic antibodies with a specificity identical to that of the airfoil receptor that first triggered activation and clonal expansion ( ). Burnet called this the clonal survival theory .

Figure 1.14

clonal selection. Each lymphocyte progenitor gives rise to many lymphocytes, each bearing a distinct antigen sense organ. Lymphocytes with receptors that bind omnipresent self antigens are eliminated before they become amply mature, ensuring allowance to ( more … )

1-8. Clonal selection of lymphocytes is the central principle of adaptive
immunity

unusually, at the time that Burnet formulated his theory, nothing was known of the antigen receptors of lymphocytes ; indeed the function of lymphocytes themselves was placid obscure. Lymphocytes did not take kernel stage until the early 1960s, when James Gowans discovered that removal of the small lymphocytes from rats resulted in the loss of all known adaptive immune responses. These immune responses were restored when the little lymphocytes were replaced. This led to the realization that lymphocytes must be the units of clonal excerpt, and their biology became the focus of the new discipline of cellular immunology. clonal choice of lymphocytes with divers receptors elegantly explained adaptive immunity but it raised one significant intellectual problem. If the antigen receptors of lymphocytes are generated randomly during the life of an individual, how are lymphocytes prevented from recognizing antigens on the tissues of the body and attacking them ? Ray Owen had shown in the late 1940s that genetically unlike duplicate calves with a common placenta were immunologically tolerant of one another ‘s tissues, that is, they did not make an immune reception against each early. Sir Peter
Medawar
then showed in 1953 that if exposed to foreign tissues during embryonic development, mouse become immunologically tolerant to these tissues. Burnet proposed that developing lymphocytes that are potentially self-reactive are removed before they can mature, a process known as clonal deletion. He has since been proved right in this excessively, although the mechanisms of tolerance are hush being worked out, as we shall see when we discuss the development of lymphocytes in chapter 7. clonal choice of lymphocytes is the single most crucial principle in adaptive immunity. Its four basic postulates are listed in. The last of the problems posed by the clonal choice theory —how the diversity of lymphocyte antigen receptors is generated—was solved in the 1970s when advances in molecular biota made it possible to clone the genes encoding antibody molecules .

Figure 1.15

The four basic principles of clonal choice .

1-9. The structure of the antibody molecule illustrates the central puzzle of
adaptive immunity

Antibodies, as discussed above, are the secrete shape of the B-cell antigen sense organ or BCR. Because they are produced in very large quantities in reception to antigen, they can be studied by traditional biochemical techniques ; indeed, their structure was sympathize retentive earlier recombinant DNA technology made it possible to study the membrane-bound antigen receptors of lymphocytes. The startle feature that emerged from the biochemical studies was that an antibody molecule is composed of two discrete regions. One is a constant region that can take one of only four or five biochemically distinct forms ; the other is a variable region that can take an obviously infinite variety of subtly different forms that allow it to bind specifically to an equally huge variety show of different antigens. This class is illustrated in the elementary schematic diagram in, where the antibody is depicted as a y-shaped atom, with the constant region shown in blue and the variable region in loss. The two variable star regions, which are identical in any one antibody molecule, determine the antigen -binding specificity of the antibody ; the changeless region determines how the antibody disposes of the pathogen once it is bound .

Figure 1.16

schematic structure of an antibody molecule. The two arms of the Y-shaped antibody molecule contain the variable star regions that form the two identical antigen-binding sites. The shank can take one of only a express number of forms and is known as the ceaseless ( more … ) Each antibody atom has a double bloc of isotropy and is composed of two identical heavy chains and two identical light chains ( ). Heavy and light chains both have variable star and constant regions ; the variable regions of a heavy and a alight chain aggregate to form an antigen-binding site, so that both chains contribute to the antigen-binding specificity of the antibody atom. The structure of antibody molecules will be described in detail in chapter 3, and the functional properties of antibodies conferred by their ceaseless regions will be considered in Chapters 4 and 9. For the clock time being we are concerned alone with the properties of immunoglobulin molecules as antigen receptors, and how the diversity of the variable regions is generated .

Figure 1.17

Antibodies are made up of four protein chains. There are two types of chain in an antibody atom : a larger chain called the heavy chain ( green ), and a smaller one called the light up chain ( jaundiced ). Each chain has both a variable and a constant area, ( more … )

1-10. Each developing lymphocyte generates a unique antigen receptor by rearranging
its receptor genes

How are antigen receptors with an about space range of specificities encoded by a finite count of genes ? This question was answered in 1976, when Susumu Tonegawa discovered that the genes for immunoglobulin variable regions are inherited as sets of gene segments, each encoding a part of the variable region of one of the immunoglobulin polypeptide chains ( ). During B-cell development in the bone marrow, these gene segments are irreversibly joined by DNA recombination to form a stretch of DNA encoding a complete variable region. Because there are many different gene segments in each set, and unlike gene segments are joined together in unlike cells, each cell generates singular genes for the varying regions of the heavy and light chains of the immunoglobulin molecule. Once these recombination events have succeeded in producing a functional sense organ, foster rearrangement is prohibited. thus each lymphocyte expresses only one receptor specificity .

Figure 1.18

The diverseness of lymphocyte antigen receptors is generated by bodily gene rearrangements. different parts of the variable regions of antigen receptors are encoded by sets of gene segments. During a lymphocyte ‘s development, one member of each set of ( more … ) This mechanism has three crucial consequences. First, it enables a limited number of gene segments to generate a huge number of different proteins. second, because each cell assembles a unlike set of gene segments, each cell expresses a alone sense organ specificity. Third, because gene rearrangement involves an irreversible change in a cellular telephone ‘s DNA, all the offspring of that cell will inherit genes encoding the same sense organ specificity. This general system was late besides confirmed for the genes encoding the antigen sense organ on T lymphocytes. The main distinctions between B- and T-lymphocyte receptors are that the immunoglobulin that serves as the B-cell antigen receptor has two identical antigen-recognition sites and can besides be secreted, whereas the T-cell antigen sense organ has a unmarried antigen-recognition site and is constantly a cell-surface atom. We shall see late that these receptors besides recognize antigen in very different ways. The potential diversity of lymphocyte receptors generated in this means is enormous. Just a few hundred different gene segments can combine in different ways to generate thousands of unlike receptor chains. The diverseness of lymphocyte receptors is far amplified by junctional diverseness, created by adding or subtracting nucleotides in the process of joining the gene segments, and by the fact that each sense organ is made by pairing two unlike variable star chains, each encoded in distinct sets of gene segments. A thousand unlike chains of each type could therefore generate 106 distinct antigen receptors through this combinatorial diversity. Thus a little amount of genic fabric can encode a sincerely astonishing diverseness of receptors. alone a subset of these randomly generated sense organ specificities survive the selective processes that shape the peripheral lymphocyte repertoire ; however, there are lymphocytes of at least 108 different specificities in an individual at any one time. These provide the sensitive corporeal on which clonal excerpt acts .

1-11. Lymphocyte development and survival are determined by signals received
through their antigen receptors

evenly amaze as the coevals of millions of specificities of lymphocyte antigen receptors is the shape of this repertoire during lymphocyte exploitation and the homeostatic alimony of such an extensive repertoire in the periphery. How are the most useful sense organ specificities selected, and how are the numbers of peripheral lymphocytes, and the percentages of B cells and T cells kept relatively changeless ? The suffice seems to be that lymphocyte maturation and survival are regulated by signals received through their antigen receptors. potent signals received through the antigen sense organ by an immature lymphocyte induce it to die or undergo farther receptor rearrangement, and in this way self-reactive sense organ specificities are deleted from the repertory. however, a arrant absence of signals from the antigen receptor can besides lead to cell death. It seems that in order to survive, lymphocytes must sporadically receive certain signals from their environment via their antigen receptors. In this way, the body can ensure that each sense organ is functional and regulate the number and character of lymphocytes in the population at any given time. These survival signals appear to be delivered by other cells in the lymphoid organs and must derive, at least in separate, from the body ‘s own molecules, the self antigens, as altering the self environment alters the life-span of lymphocytes in that environment. Developing B cells in the cram kernel interact with stromal cells, while their final maturation and proceed recirculation appears to depend on survival signals received from the B-cell follicles of peripheral lymphoid tissue. T lymphocytes meet survival signals from self molecules on specialize epithelial cells in the thymus gland during development, and from the lapp molecules expressed by dendritic cells in the lymphoid tissues in the periphery. The self ligands that interact with the T-cell receptor to deliver these signals are partially defined, being composed of known cell-surface molecules complexed with undefined peptides from early self proteins in the cell. These same cell-surface molecules function to present alien intracellular antigens to T cells, as we shall explain in Section 1-16, and in Chapter 5. They select only a subset of T-cell receptors for survival, but these are the receptors most likely to be utilitarian in responding to foreign antigens, as we shall see in chapter 7. Lymphocytes that fail to receive survival signals, and those that are clonally deleted because they are self-reactive, undergo a human body of cell suicide called apoptosis or programmed cell death. Apoptosis, derived from a greek word meaning the falling of leaves from the trees, occurs in all tissues, at a relatively constant rate in each weave, and is a mean of regulating the issue of cells in the body. It is responsible, for exemplar, for the death and shedding of clamber cells, the employee turnover of liver cells, and the death of the oldest intestinal epithelial cells that are constantly replaced by newly cells. frankincense, it should come as no surprise that immune system cells are regulated through the lapp mechanism. Each day the bone marrow produces many millions of fresh neutrophils, monocytes, red blood cells, and lymphocytes, and this production must be balanced by an adequate loss of these cells. Regulated loss of all these blood cells occurs by apoptosis, and the dying cells are ultimately phagocytosed by specialize macrophages in the liver and irascibility. Lymphocytes are a extra casing, because the loss of an individual primitive lymphocyte means the passing of a sense organ specificity from the repertory, while each newly matured cell that survives will contribute a different specificity. The survival signals received through the antigen receptors appear to regulate this summons by inhibiting the apoptosis of individual lymphocytes, thus regulating the care and typography of the lymphocyte repertory. We shall return to the question of which ligands deliver these signals, and how they contribute to shaping and maintaining the sense organ repertoire, in chapter 7 .

1-12. Lymphocytes proliferate in response to antigen in peripheral lymphoid organs,
generating effector cells and immunological memory

The large diversity of lymphocyte receptors means that there will normally be at least a few that can bind to any given foreign antigen. however, because each lymphocyte has a unlike receptor, the numbers of lymphocytes that can bind and respond to any given antigen is very small. To generate sufficient antigen-specific effecter lymphocytes to fight an contagion, a lymphocyte with an allow sense organ specificity must be activated to proliferate before its offspring last differentiate into effector cells. This clonal expansion is a feature of speech coarse to all adaptive immune responses. As we have seen, lymphocyte activation and proliferation is initiated in the drain lymphoid tissues, where primitive lymphocytes and activated antigen -presenting cells can come together. Antigens are therefore presented to the naive recirculating lymphocytes as they migrate through the lymphoid tissue before returning to the bloodstream via the efferent lymph. On recognizing its specific antigen, a modest lymphocyte stops migrating and enlarges. The chromatin in its nucleus becomes less dense, nucleolus appear, the volume of both the nucleus and the cytoplasm increases, and new RNAs and proteins are synthesized. Within a few hours, the cell looks completely different and is known as a lymphoblast ( ) .

Figure 1.19

Transmission electron micrographs of lymphocytes at assorted stages of activation to effector routine. little resting lymphocytes ( acme control panel ) have not even encountered antigen. Note the pantie cytoplasm, the absence of rocky endoplasmic reticulum, and the ( more … ) The lymphoblasts now begin to divide, normally duplicating themselves two to four times every 24 hours for 3 to 5 days, so that one naive lymphocyte gives rise to a ringer of around 1000 daughter cells of identical specificity. These then differentiate into effecter cells ( see ). In the case of B cells, the differentiate effecter cells, the plasma cells, secrete antibody ; in the casing of T cells, the effecter cells are able to destroy infect cells or activate early cells of the immune system. These changes besides affect the recirculation of antigen -specific lymphocytes. Changes in the cell-adhesion molecules they express on their open allow effector lymphocytes to migrate into sites of infection or stay in the lymphoid organs to activate B cells. After a uninstructed lymphocyte has been activated, it takes 4 to 5 days before clonal expansion is complete and the lymphocytes have differentiated into effector cells. That is why adaptive immune responses occur lone after a delay of respective days. Effector cells have only a limit life-span and, once antigen is removed, most of the antigen-specific cells generated by the clonal expansion of small lymphocytes undergo apoptosis. however, some persevere after the antigen has been eliminated. These cells are known as memory cells and form the basis of immunological memory, which ensures a more rapid and effective response on a moment meet with a pathogen and thereby provides lasting protective immunity. The characteristics of immunological memory are readily observed by comparing the antibody answer of an individual to a first or primary
immunization
with the answer elicited in the same individual by a secondary or supporter immunization with the lapp antigen. As shown in, the secondary antibody reply occurs after a shorter lag phase, achieves a markedly higher charge, and produces antibodies of higher affinity, or potency of oblige, for the antigen. We shall describe the mechanism of these noteworthy changes in Chapters 9 and 10. The cellular basis of immunological memory is the clonal expansion and clonal specialization of cells specific for the elicit antigen, and it is therefore wholly antigen specific .

Figure 1.20

The path of a typical antibody response. beginning meet with an antigen produces a elementary reception. Antigen A introduced at time zero encounters little specific antibody in the serum. After a lag phase, antibody against antigen A ( blue ) appears ; its ( more … ) It is immunological memory that enables successful inoculation and prevents reinfection with pathogens that have been repelled successfully by an adaptive immune reply. immunological memory is the most significant biological consequence of the exploitation of adaptive unsusceptibility, although its cellular and molecular basis is still not in full understand, as we shall see in chapter 10 .

1-13. Interaction with other cells as well as with antigen is necessary for
lymphocyte activation

Peripheral lymphoid tissues are specialized not only to trap phagocytic cells that have ingested antigen ( see Sections 1-3 and 1-6 ) but besides to promote their interactions with lymphocytes that are needed to initiate an adaptive immune reception. The spleen and lymph nodes in detail are highly organized for the latter function. All lymphocyte responses to antigen command not entirely the sign that results from antigen binding to their receptors, but besides a second signal, which is delivered by another cell. primitive T cells are by and large activated by activate dendritic cells (, left gore ) but for B cells (, right panel ), the second signal is delivered by an arm effector T cell. Because of their ability to deliver activating signals, these three cell types are known as professional antigen-presenting cells, or often barely antigen-presenting cells. They are illustrated in. dendritic cells are the most authoritative antigenpresenting cell of the three, with a central function in the knowledgeability of adaptive immune responses ( see Section 1-6 ). Macrophages can besides mediate unconditioned immune responses directly and make a all-important contribution to the effecter phase of the adaptive immune response. B cells contribute to adaptive immunity by presenting peptides from antigens they have ingested and by secreting antibody .

Figure 1.21

Two signals are required for lymphocyte activation. a well as receiving a bespeak through their antigen sense organ, mature uninitiate lymphocytes must besides receive a irregular signal to become activated. For T cells ( impart panel ) it is delivered by a professional ( more … )

Figure 1.22

The professional antigen-presenting cells. The three types of professional antigen-presenting cell are shown in the form in which they will be depicted throughout this book ( top rowing ), as they appear in the unhorse microscope ( second row ; the relevant cell ( more … ) therefore, the final examination contend of adaptive unsusceptibility is that it occurs on a cell that besides presents the antigen. This appears to be an absolute convention in
vivo
, although exceptions have been observed in in
vitro
systems. however, what we are attempting to define is what does happen, not what can happen .

Summary

The early congenital systems of defense, which depend on invariant receptors recognizing common features of pathogens, are crucially important, but they are evaded or overcome by many pathogens and do not lead to immunological memory. The abilities to recognize all pathogens specifically and to provide enhanced protective covering against reinfection are the unique features of adaptive unsusceptibility, which is based on clonal selection of lymphocytes bearing antigen -specific receptors. The clonal choice of lymphocytes provides a theoretical framework for understanding all the key features of adaptive immunity. Each lymphocyte carries cell-surface receptors of a unmarried specificity, generated by the random recombination of variable receptor gene segments and the copulate of different variable star chains. This produces lymphocytes, each bearing a distinct sense organ, so that the sum repertoire of receptors can recognize virtually any antigen. If the receptor on a lymphocyte is specific for a omnipresent self antigen, the cell is eliminated by encountering the antigen early on in its development, while survival signals received through the antigen sense organ choice and maintain a functional lymphocyte repertory. adaptive exemption is initiated when an natural immune reaction fails to eliminate a new infection, and antigen and activated antigen-presenting cells are delivered to the draining lymphoid tissues. When a recirculating lymphocyte encounters its specific extraneous antigen in peripheral lymphoid tissues, it is induced to proliferate and its offspring then differentiate into effecter cells that can eliminate the infectious agent. A subset of these proliferating lymphocytes differentiate into memory cells, quick to respond quickly to the like pathogen if it is encountered again. The details of these processes of recognition, development, and specialization form the independent material of the center three parts of this record .

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