Polyclonal B cell response is a natural mode of immune response exhibited by theadaptive immune system ofmammals. It ensures that a singleantigen[citation needed] is recognized and attacked through its overlapping parts, calledepitopes, by multipleclones ofB cell.[1][2]
In the course of normal immune response, parts ofpathogens (e.g.bacteria) are recognized by the immune system as foreign (non-self), and eliminated or effectively neutralized to reduce their potential damage. Such a recognizable substance is called anantigen. The immune system may respond in multiple ways to an antigen; a key feature of this response is the production ofantibodies by B cells (or B lymphocytes) involving an arm of the immune system known ashumoral immunity. The antibodies are soluble and do not require direct cell-to-cell contact between the pathogen and the B-cell to function.
Antigens can be large and complex substances, and any single antibody can only bind to a small, specific area on the antigen. Consequently, an effective immune response often involves the production of many different antibodies by many different B cells against thesame antigen. Hence the term "polyclonal", which derives from the wordspoly, meaning many, andclones from Greekklōn, meaning sprout or twig;[3][4][5] a clone is a group of cells arising from a common "mother" cell. The antibodies thus produced in a polyclonal response are known aspolyclonal antibodies. Theheterogeneous polyclonal antibodies are distinct frommonoclonal antibody molecules, which are identical and react against a single epitope only, i.e., are more specific.
Although the polyclonal response confers advantages on the immune system, in particular, greater probability of reacting against pathogens, it also increases chances of developing certain autoimmune diseases resulting from the reaction of the immune system against native molecules produced within the host.
Diseases which can be transmitted from one organism to another are known asinfectious diseases, and the causative biological agent involved is known as apathogen. The process by which the pathogen is introduced into the body is known asinoculation,[note 1][6] and the organism it affects is known as abiological host. When the pathogen establishes itself in a step known ascolonization,[7] it can result in aninfection,[7] consequently harming the host directly or through the harmful substances calledtoxins it can produce.[7] This results in the varioussymptoms andsigns characteristic of an infectious disease likepneumonia ordiphtheria.
Countering the various infectious diseases is very important for the survival of thesusceptible organism, in particular, and the species, in general. This is achieved by the host by eliminating the pathogen and its toxins or rendering them nonfunctional. The collection of variouscells,tissues andorgans that specializes in protecting the body against infections is known as theimmune system. The immune system accomplishes this through direct contact of certainwhite blood cells with the invading pathogen involving an arm of the immune system known as thecell-mediated immunity, or by producing substances that move to sitesdistant from where they are produced, "seek" the disease-causing cells and toxins by specifically[note 2] binding with them, and neutralize them in the process–known as thehumoral arm of the immune system. Such substances are known as soluble antibodies and perform important functions in countering infections.[note 3][8]
Antibodies serve variousfunctions in protecting the host against the pathogen. Their soluble forms which carry out these functions are produced byplasma B cells, a type of white blood cell. This production is tightly regulated and requires the activation of B cells by activatedT cells (another type of white blood cell), which is a sequential procedure. The major steps involved are:[9]
Pathogens synthesizeproteins that can serve as"recognizable" antigens; they may express the molecules on their surface or release them into the surroundings (body fluids). What makes these substances recognizable is that they bind very specifically and somewhat strongly to certain host proteins calledantibodies. The same antibodies can be anchored to the surface of cells of the immune system, in which case they serve asreceptors, or they can be secreted in the blood, known as soluble antibodies. On a molecular scale, the proteins are relatively large, so they cannot be recognized as a whole; instead, their segments, calledepitopes, can be recognized.[1] An epitope comes in contact with a very small region (of 15–22 amino acids) of the antibody molecule; this region is known as theparatope.[1] In the immune system, membrane-bound antibodies are theB-cell receptor (BCR). Also, while the T-cell receptor is not biochemically classified as an antibody, it serves a similar function in that it specifically binds to epitopes complexed withmajor histocompatibility complex (MHC) molecules.[note 5][10] The binding between a paratope and its corresponding antigen is very specific, owing to its structure, and is guided by variousnoncovalent bonds, not unlike the pairing of other types ofligands (any atom, ion or molecule that binds with any receptor with at least some degree ofspecificity andstrength). The specificity of binding does not arise out of a rigidlock and key type of interaction, but rather requires both the paratope and the epitope to undergo slight conformational changes in each other's presence.[11]
In figure at left, the various segments that form the epitope have been shown to be continuously collinear, meaning that they have been shown as sequential; however, for the situation being discussed here (i.e., the antigen recognition by the B cell), this explanation is too simplistic. Such epitopes are known assequential orlinear epitopes, as all the amino acids on them are in the same sequence (line). This mode of recognition is possible only when the peptide is small (about six to eight amino acids long),[1] and is employed by the T cells (T lymphocytes).
However, the B memory/naive cells recognize intact proteins present on the pathogen surface.[note 6] In this situation, the protein in itstertiary structure is so greatly folded that some loops of amino acids come to lie in the interior of the protein, and the segments that flank them may lie on the surface. The paratope on the B cell receptor comes in contact only with those amino acids that lie on thesurface of the protein. The surface amino acids may actually be discontinuous in the protein'sprimary structure, but getjuxtaposed owing to the complex protein folding patterns (as in the adjoining figure). Such epitopes are known asconformational epitopes and tend to be longer (15–22 amino acid residues) than the linear epitopes.[1] Likewise, the antibodies produced by the plasma cells belonging to the same clone would bind to the same conformational epitopes on the pathogen proteins.[12][13][14][15]
The binding of a specific antigen with corresponding BCR molecules results in increased production of the MHC-II molecules. This assumes significance as the same does not happen when the same antigen would be internalized by a relatively nonspecific process calledpinocytosis, in which the antigen with the surrounding fluid is "drunk" as a small vesicle by the B cell.[16] Hence, such an antigen is known as anonspecific antigen and does not lead to activation of the B cell, or subsequent production of antibodies against it.
Macrophages andrelated cells employ a different mechanism to recognize the pathogen. Their receptors recognize certainmotifs present on the invading pathogen that are veryunlikely to be present on a host cell. Such repeating motifs are recognized bypattern recognition receptors (PRRs) like thetoll-like receptors (TLRs) expressed by the macrophages.[1][17] Since the same receptor could bind to a given motif present on surfaces of widely disparatemicroorganisms, this mode of recognition is relatively nonspecific, and constitutes aninnate immune response.
After recognizing an antigen, anantigen-presenting cell such as themacrophage or B lymphocyte engulfs it completely by a process calledphagocytosis. The engulfed particle, along with some material surrounding it, forms the endocytic vesicle (thephagosome), which fuses withlysosomes. Within the lysosome, the antigen is broken down into smaller pieces calledpeptides byproteases (enzymes that degrade larger proteins). The individual peptides are then complexed with major histocompatibility complex class II (MHC class II) molecules located in the lysosome – this method of "handling" the antigen is known as theexogenous or endocytic pathway of antigen processing in contrast to theendogenous or cytosolic pathway,[17][18][19] which complexes theabnormal proteins produced within the cell (e.g. under the influence of aviral infection or in atumor cell) withMHC class I molecules.
An alternate pathway of endocytic processing had also been demonstrated wherein certain proteins likefibrinogen andmyoglobin can bind as a whole to MHC-II molecules after they aredenatured and theirdisulfide bonds arereduced (breaking the bond by addinghydrogenatoms across it). The proteases then degrade the exposed regions of the protein-MHC II-complex.[19]
After the processed antigen (peptide) is complexed to the MHC molecule, they both migrate together to thecell membrane, where they are exhibited (elaborated) as a complex that can be recognized by theCD 4+ (T helper cell) – a type of white blood cell.[note 7][20] This is known asantigen presentation. However, the epitopes (conformational epitopes) that are recognized by the B cell prior to their digestion may not be the same as that presented to the T helper cell. Additionally, a B cell may present different peptides complexed to different MHC-II molecules.[16]
The CD 4+ cells through their T cell receptor-CD3 complex recognize the epitope-bound MHC II molecules on the surface of the antigen presenting cells, and get'activated'. Upon this activation, these T cells proliferate and differentiate intoTh1 or Th2 cells.[16][21] This makes them produce soluble chemical signals that promote their own survival. However, another important function that they carry out is the stimulation of B cell by establishingdirect physical contact with them.[10]
Complete stimulation of T helper cells requires theB7 molecule present on the antigen presenting cell to bind withCD28 molecule present on the T cell surface (in close proximity with the T cell receptor).[10] Likewise, a second interaction between the CD40 ligand or CD154 (CD40L) present on T cell surface andCD40 present on B cell surface, is also necessary.[21] The same interactions that stimulate the T helper cell also stimulate the B cell, hence the termcostimulation. The entire mechanism ensures that an activated T cell only stimulates a B cell that recognizes the antigen containing thesame epitope as recognized by the T cell receptor of the "costimulating" T helper cell. The B cell gets stimulated, apart from the direct costimulation, by certain growth factors, viz.,interleukins2,4,5, and6 in aparacrine fashion. These factors are usually produced by the newly activated T helper cell.[22] However, this activation occurs only after the B cell receptor present on amemory or anaive B cell itself would have bound to the corresponding epitope, without which the initiating steps of phagocytosis and antigen processing would not have occurred.
A naive (orinexperienced) B cell is one which belongs to a clone which has never encountered the epitope to which it is specific. In contrast, a memory B cell is one which derives from an activated naive or memory B cell. The activation of a naive or a memory B cell is followed by a manifold proliferation of that particular B cell, most of the progeny of which terminally differentiate intoplasma B cells;[note 8] the rest survive as memory B cells. So, when the naive cells belonging to a particular clone encounter their specific antigen to give rise to the plasma cells, and also leave a few memory cells, this is known as theprimary immune response. In the course of proliferation of this clone, the B cell receptorgenes can undergo frequent (one in everytwo cell divisions)[8]mutations in the genes coding for paratopes of antibodies. These frequent mutations are termedsomatic hypermutation. Each such mutation alters the epitope-binding ability of the paratope slightly, creating new clones of B cells in the process. Some of the newly created paratopes bindmore strongly to the same epitope (leading to theselection of the clones possessing them), which is known asaffinity maturation.[note 9][8][21] Other paratopes bind better to epitopes that areslightly different from the original epitope that had stimulated proliferation. Variations in the epitope structure are also usually produced by mutations in the genes of pathogen coding for their antigen. Somatic hypermutation, thus, makes the B cell receptors and the soluble antibodies in subsequent encounters with antigens, more inclusive in their antigen recognition potential ofaltered epitopes, apart from bestowing greater specificity for the antigen that induced proliferation in the first place. When the memory cells get stimulated by the antigen to produce plasma cells (just like in the clone's primary response), and leave even more memory cells in the process, this is known as asecondary immune response,[21] which translates into greater numbers of plasma cells and faster rate of antibody production lasting for longer periods. The memory B cells produced as a part of secondary response recognize the corresponding antigen faster and bind more strongly with it (i.e., greater affinity of binding) owing to affinity maturation. The soluble antibodies produced by the clone show a similar enhancement in antigen binding.[21]
Responses are polyclonal in nature as each clone somewhat specializes in producing antibodies against a given epitope, and because, each antigen contains multiple epitopes, each of which in turn can be recognized by more than one clone of B cells. To be able to react to innumerable antigens, as well as multiple constituent epitopes, the immune system requires the ability to recognize a very great number of epitopes in all, i.e., there should be a great diversity of B cell clones.
Memory and naïve B cells normally exist in relatively small numbers. As the body needs to be able to respond to a large number of potential pathogens, it maintains a pool of B cells with a wide range of specificities.[17] Consequently, while there is almost always at least one B (naive or memory) cell capable of responding to any given epitope (of all that the immune system can react against), there are very few exact duplicates. However, when a single B cell encounters an antigen to which it can bind, it can proliferate very rapidly.[21] Such a group of cells with identical specificity towards the epitope is known as aclone, and is derived from a common "mother" cell. All the "daughter" B cells match the original "mother" cell in their epitope specificity, and they secrete antibodies with identical paratopes. These antibodies aremonoclonal antibodies, since they derive from clones of the same parent cell. A polyclonal response is one in which clones of multiple B cells react to the same antigen.
A single antigen can be thought of as a sequence of multiple overlapping epitopes. Many unique B cell clones may be able to bind to the individual epitopes. This imparts even greater multiplicity to the overall response.[3] All of these B cells can become activated and produce large colonies of plasma cell clones, each of which can secrete up to 1000 antibody molecules against each epitope per second.[21]
In addition to different B cells reacting todifferent epitopes on the same antigen, B cells belonging to different clones may also be able to react to thesame epitope. An epitope that can be attacked by many different B cells is said to be highlyimmunogenic. In these cases, thebinding affinities for respective epitope-paratope pairs vary, with some B cell clones producing antibodies that bind strongly to the epitope, and others producing antibodies that bind weakly.[1]
The clones that bind to a particular epitope with greater strength are more likely to beselected for further proliferation in the germinal centers of the follicles in various lymphoid tissues like thelymph nodes. This is not unlikenatural selection: clones are selected for their fitness to attack the epitopes (strength of binding) on the encountered pathogen.[23]What makes the analogy even stronger is that the B lymphocytes have to compete with each other for signals that promote their survival in the germinal centers.
Although there are many diverse pathogens, many of which are constantly mutating, it is a surprise that a majority of individuals remain free of infections. Thus, maintenance of health requires the body to recognize all pathogens (antigens they present or produce) likely to exist. This is achieved by maintaining a pool of immensely large (about 109) clones of B cells, each of which reacts against a specific epitope by recognizing and producing antibodies against it. However, at any given time very few clones actually remain receptive to their specific epitope. Thus, approximately 107 different epitopes can be recognized by all the B cell clones combined.[21] Moreover, in a lifetime, an individual usually requires the generation of antibodies against very few antigens in comparison with the number that the body can recognize and respond against.[21]
If an antigen can be recognized by more than one component of its structure, it is less likely to be "missed" by the immune system.[note 10] Mutation of pathogenic organisms can result in modification of antigen—and, hence, epitope—structure. If the immune system "remembers" what the other epitopes look like, the antigen, and the organism, will still be recognized and subjected to the body's immune response. Thus, the polyclonal response widens the range of pathogens that can be recognized.[24]
Many viruses undergo frequentmutations that result in changes in amino acid composition of their important proteins. Epitopes located on the protein may also undergo alterations in the process. Such an altered epitope binds less strongly with the antibodies specific to the unaltered epitope that would have stimulated the immune system. This is unfortunate because somatic hypermutation does give rise to clones capable of producing soluble antibodies that would have bound the altered epitope avidly enough to neutralize it. But these clones would consist of naive cells which are not allowed to proliferate by the weakly binding antibodies produced by the priorly stimulated clone. This doctrine is known as theoriginal antigenic sin.[21] This phenomenon comes into play particularly in immune responses againstinfluenza,dengue andHIV viruses.[25] This limitation, however, is not imposedby the phenomenon of polyclonal response, but rather,against it by an immune response that is biased in favor of experienced memory cells against the "novice" naive cells.
Inautoimmunity the immune system wrongly recognizes certain native molecules in the body as foreign (self-antigen), and mounts an immune response against them. Since these native molecules, as normal parts of the body, will naturally always exist in the body, the attacks against them can get stronger over time (akin to secondary immune response). Moreover, many organisms exhibitmolecular mimicry, which involves showing those antigens on their surface that are antigenically similar to the host proteins. This has two possible consequences: first, either the organism will be spared as a self antigen; or secondly, that the antibodies produced against it will also bind to the mimicked native proteins. The antibodies will attack the self-antigens and the tissues harboring them by activating various mechanisms like the complement activation andantibody-dependent cell-mediated cytotoxicity. Hence, wider the range of antibody-specificities, greater the chance that one or the other will react against self-antigens (native molecules of the body).[26][27]
Monoclonal antibodies are structurally identical immunoglobulin molecules with identical epitope-specificity (all of them bind with the same epitope with same affinity) as against their polyclonal counterparts which have varying affinities for the same epitope.They are usually not produced in a natural immune response, but only in diseased states likemultiple myeloma, or through specialized laboratory techniques. Because of their specificity, monoclonal antibodies are used in certain applications to quantify or detect the presence of substances (which act as antigen for the monoclonal antibodies), and for targeting individual cells (e.g. cancer cells). Monoclonal antibodies find use in various diagnostic modalities (see:western blot andimmunofluorescence) andtherapies—particularly of cancer and diseases with autoimmune component. But, since virtually all responses in nature are polyclonal, it makes production of immensely useful monoclonal antibodiesless straightforward.[8]
The first evidence of presence of a neutralizing substance in the blood that could counter infections came whenEmil von Behring along withKitasato Shibasaburō in 1890 developed effectiveserum against diphtheria. This they did by transferring serum produced from animals immunized against diphtheria to animals suffering from it. Transferring the serum thus could cure the infected animals. Behring was awarded theNobel Prize for this work in 1901.[28]
At this time though the chemical nature of what exactly in the blood conferred this protection was not known. In a few decades to follow, it was shown that the protective serum could neutralize and precipitate toxins, and clump bacteria. All these functions were attributed to different substances in the serum, and named accordingly asantitoxin,precipitin andagglutinin.[17] That all the three substances were one entity (gamma globulins) was demonstrated byElvin A. Kabat in 1939. In the preceding year Kabat had demonstrated the heterogeneity of antibodies throughultracentrifugation studies of horses' sera.[29]
Until this time, cell-mediated immunity and humoral immunity were considered to be contending theories to explain effective immune response, but the former lagged behind owing to lack of advanced techniques.[17] Cell-mediated immunity got an impetus in its recognition and study when in 1942,Merrill Chase successfully transferred immunity againsttuberculosis between pigs by transferring white blood cells.[17][30]
It was later shown in 1948 by Astrid Fagraeus in her doctoral thesis that the plasma B cells are specifically involved in antibody production.[31] The role of lymphocytes in mediating both cell-mediated and humoral responses was demonstrated by James Gowans in 1959.[30]
In order to account for the wide range of antigens the immune system can recognize,Paul Ehrlich in 1900 had hypothesized that preexisting"side chain receptors" bind a given pathogen, and that this interaction induces the cell exhibiting the receptor to multiply and produce more copies of the same receptor. This theory, calledthe selective theory was not proven for next five decades, and had been challenged by severalinstructional theories which were based on the notion that an antibody would assume its effective structure by folding around the antigen.[17] In the late 1950s however, the works of three scientists—Jerne,Talmage andBurnet (who largely modified the theory)—gave rise to theclonal selection theory, which proved all the elements of Ehrlich's hypothesis except that the specific receptors that could neutralize the agent were soluble and not membrane-bound.[17][30]
The clonal selection theory was proved correct when SirGustav Nossal showed that each B cell always produces only one antibody.[32]
In 1974, the role of MHC in antigen presentation was demonstrated byRolf Zinkernagel andPeter C. Doherty.[30]
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