Knowledge in Immune system
Immune system
Notes in form of slides for immune system.
Rounak Bhowal
COMPLEMENT ACTIVATION
Complement is normally present in the body in an inactive form but when its activity is inducedby antigen-antibody combination or other stimuli, components react in a specific sequence as a cascade.Basically, the C cascade is a series of reactions in which the preceding components act as enzymes on the succeeding components, cleaving them into dissimilar fragments. The larger fragments usually join the cascade. The smaller fragments which are releasedoften possess biological effects which contribute to defence mechanisms by amplifying the inflammatory process, increasing vascular permeability, inducingsmooth muscle contraction, causing chemotaxis of leucocytes, promoting virus neutralisation, detoxifying endotoxins and effecting the release of histamine frommast cells.The C cascade can be triggered off by two parallel but independent mechanisms or pathways which differ only in the initial steps. Once C3 activation occurs, the subsequent steps are common in both pathwayswhich have been called the classical C pathway and the alternative or properdin pathway.The classical pathway is so called because it was the first one identified. But actually it is a more recently evolved mechanism of specific active immunity, while the alternative pathway represents a more primitive system of nonspecific innate immunity.
Yamini Chandru
ANTIGENS
An antigen has been defined as any substancewhich, when introduced parenterally into the body stimulates the production of an antibody with which it reacts specifically and in an observable manner. This traditional description of an antigen is no longer comprehensive enough in the light of current concepts about the immune response. Some antigensmay not induce antibodies but may sensitise specific lymphocytes leading to cell mediated immunity or may cause immunological tolerance.The word 'parenteral' (meaning, outside the intestinal tract) is used in the definition because orally administered antigens are usually denatured by digestive enzymes and their antigenicity destroyed, so that no antibodyformation takes place. When given parenterally, antigens do not undergo any such inactivation and can induce antibody production. However, there are exceptions and some antigens can be immunogenic when given orally, such as oral vaccines. The word 'specifically in the definition is important as specificity is the hallmark of all immunological reactions.An antigen introduced into the body reacts only with those particular immunocytes (B or T lymphocytes) which carry the specific marker for that antigen and which produce an antibody cells complementary to that antigen only. The antibody so produced will react only with thatparticulate antigen and with no other though immunological cross-reaction may occur between closely related antigens.The two attributes of antigenicity are1. induction of an immune response (immunogenicity),and2. specific reaction with antibodies or sensitised cells (immunological reactivity).
HAPTENS, EPITOPES AND PARATOPES
Based on the ability to carry out these two functions, antigens may be classified into different types. A complete antigen is able to induce antibody formation and produce a specific and observable reaction withthe antibodies produced Haptens are substances that are incapable of inducing antibody formation by themselves but can react specifically with antibodies(The term hapten is derived from the Greek haptenwhich means to fasten'.) Haptens became immunogenic (capable of inducing antibodies) on combining with larger molecule carrier. Haptens may be complex or simple; while complex haptens can precipitate withspecific antibodies, simple haptens are nonprecipitating.They can inhibit precipitation of specific antibodies by the corresponding antigen or complex hapten. Complex and simple haptens have been described as polyvalent and univalent, respectively, since it is assumed thatprecipitation requires the antigen to have two or more antibody combining sites.The smallest unit of antigenicity is known as the antigenic determinant or epitope. The epitope is that small area on the antigen, usually consisting of four or five aminoacid or monosaccharide residues, possessing a specific chemical structure, electrical charge steric (spatial) configuration, capable of sensitising an immunocyte and of reacting with its complementary site on the specific antibody or T cell receptor.Epitopes may be present as a single linear segment of the primary sequence (sequential or linear epitope) or formed by bringing together on the surface residues from different sites of the peptide chain during its asfolding into the tertiary structure (conformational very epitope). T cells recognise sequential epitopes, while B cells identify the tertiary configuration of the conformational epitopes. The combining area on the are antibody molecule, corresponding to the epitope, is called the paratope. Epitopes and paratopes determine the specificity of immunological reactions. Antigens thesuch as bacteria or viruses carry many different types of epitopes, presenting an antigenic mosaic. The presence of the same or similar epitopes on different antigens accounts for one type of antigenic cross-reaction.
DETERMINANTS OF ANTIGENCITY
A number of properties that make a substance antigenic have been identified but the exact basis of antigenicity is still not clear.1. Size: Antigenicity is related to molecular size. Very large molecules, such as hemocyanins (MW 6.75 million).are highly antigenic and particles with low molecular weight (less than 5000) are non antigenic or feebly so. Low molecular weight substances may be renderedantigenic by adsorbing them on large inert particles such as bentonite or kaolin. Some low molecular weight substances (such as picryl chloride, formaldehyde and penicillin) may be antigenic when applied on the skin, probably by combining with tissue proteins. They arehaptens of low immunogenicity, effective in some persons only and related to hypersensitivity.2.Chemical nature: Most naturally occurring antigens are proteins and polysaccharides. Lipids and nucleic acids are less antigenic. Their antigenicity is enhanced by combination with proteins. A certain degree of structural diversity is required for antigenicity. Thatprobably explains why proteins, which are composed of about 20 different amino acids, are better antigens than polysaccharides, which have only four or five monosaccharide units. However, not all proteins areantigenic. A well-known exception is gelatin, which is nonimmunogenic because of its structural instability3.Susceptibility to tissue enzymes: Only substances which are metabolised and are susceptible to the action of tissue enzymes behave as antigens. Antigens introduced into the body are degraded by the host into fragments of appropriate size containing the antigenic determinants. Phagocytosis and intracellular of enzymes appear to play an essential role in breaking or down antigens into immunogenic fragments.Substances not susceptible to tissue enzymes such as a polystyrene latex are not antigenic. Substancesvery rapidly broken down by tissue enzymes are also not antigenic Synthetic polypeptides, composed of D-amino acids which are not metabolised in the body, are not antigenic, while polypeptides consisting ofL-amino acids are antigenic.4.Foreignness: Only antigens which are 'foreign' to the individual (nonself) induce an immune response. The animal body contains numerous antigens that induce an immune response when introduced into another individual or species. An individual does not normally mount an immune response against his or her own normal constituent antigens. This was firstrecognised by Ehrlich who proposed the concept of 'horror autotoxicus' (which means fear of self poisoning). Tolerance of self-antigens is conditioned by contact with them during the development of the immune apparatus. Breakdown of this homeostaticmechanism results in auto immunization andautoimmune disease.
ANTIGENIC SPECIFICITY
The basis of antigenic specificity is stereochemical, as was first demonstrated byObermayer and Pick and confirmed by Landsteiner. Using haptens such as atoxyl coupled with protein, it was shown that antigenic specificity is determinedby single chemical groupings and even by a single acid radical. The importance of the position of the antigenic determinant group in the antigen molecule was evidenced by the differences in specificity in compounds with the group attached at the ortho, meta or para positions. The influence of spatial configuration of the determinant group was shown by differences in antigenic specificity of the dextro, levo and meso isomers of substances such as tartaric acid.Antigenic specificity is not absolute. Cross-reactions can occur between antigens that bear stereochemical similarities. In some instances, apparent cross-reactions may actually be due to the sharing of identical antigenic determinants by different antigens.The specificity of natural tissue antigens of animals may be considered under different categories as species, iso-, auto- and organ specificity.1.Species specificity: Tissues of all individuals in a species contain species-specific antigens. There existssome degree of cross-reaction between antigens from related species. This immunological relationship parallels phylogenetic relationship. It has been usedin tracing evolutionary relationships between species. It also has forensic applications in the identification of the species of blood and of seminal stains. Phylogenetic relationships are reflected in the extent of cross reaction between antigens from different species thatcause hypersensitivity. An individual sensitised to horse serum will react with serum from other equines but may not do so with bovine serum,2.Isospecificity: Antigen are antigens found insome but not all members of a species. A species may be grouped depending on the presence of different isoantigens in its members. The best examples of isoantigens are the human erythrocyte antigens basedon which individuals can be classified into different blood groups. These are genetically determined. They are of clinical importance in blood transfusion and in isoimmunisation during pregnancy. They were help in determining disputed paternity cases, but havebeen supplanted by the more discriminatory DNA fingerprinting tests. Blood groups find application in anthropology.Histocompatibility antigens are those cellulardeterminants specific to each individual of a species. They are recognised by genetically different individuals of the same species when attempts are made to transfer or transplant cellular material from one individual toanother.3. Autospecificity: Autologous or self-antigens are ordinarily non antigenic but there are exceptions Sequestered antigens that are not normally found free in circulation or tissue fluid (such as eye lens protein normally confined within its capsule) are not recognised as self-antigens. Similarly, antigens that are absent during embryonic life and develop later (such as sperm) are also not recognised as self-antigens.4. Organ specificity: Some organs, such as the brain, kidney and lens protein of different species, share the same antigen. Such antigens, characteristics of an organ or tissue and found in different species, arecalled organ-specific antigens. The neuroparalytic complications following anti rabies vaccination using sheep brain vaccines are a consequence of brain-specific antigens shared by sheep and human beings. The sheep brain antigens induce immunological response in the vaccinees, damaging their nervous tissue.5.Heterogenetic (heterophile) specificity: The same or closely related antigens may sometimes occur in different biological species, classes and kingdoms. These are known as heterogenetic or heterophileantigens, best exemplified by the Forssman antigen, which is a lipid carbohydrate complex widely distributed in many animals, birds, plants and bacteria. It is absent in rabbits, so anti-Forssman antibody can be prepared in these animals. Other heterophile antigens are responsible for some diagnostic serologicalreactions in which antigens unrelated to etiological agents are employed (heterophile reaction). The Weil Felix reaction in typhus fever, the Paul Bunnell test in infectious mononucleosis and the cold agglutinin testin primary atypical pneumonia are examples.
BIOLOGICAL CLASSES OF ANTIGENS
Depending on their ability to induce antibodyformation, antigens are classified as T cell dependent (TD) and T cell independent (TI) antigens. Antibody production is the property of B lymphocytes. For the full expression of this function, however, the cooperationof T lymphocytes is necessary. Some antigens can directly stimulate antibody production by B cells, without the apparent participation of T cells. Such antigens are called TI antigens. Others that require T cell participation to generate an immune responseare called TD antigens. Several important differences exist between TI andTD antigens.TI antigens•Antigens are structurally simple, being composed of a limited number of repeating epitopes, as in the case of the pneumococcal capsular polysaccharide, bacterial lipopolysaccharides andthe flagellar protein flagellin.•Their immune response is critically dosedependent. Too little is nonimmunogenic, whiletoo much results in immunological tolerancerather than immunity.•Their antibody response is usually limited to IgM and IgG3.•They do not show immunological memory.TI antigens do not appear to require preliminaryprocessing by macrophages.•They are metabolised very slowly and remain in the body for long periods.TD ANTIGENS•TD antigens, on the other hand, are structurallymore complex, such as erythrocytes, serum proteins and a variety of protein-hapten complexes.•They are immunogenic over a wide dose rangeand do not cause tolerance readily.•They induce the full gamut of immunoglobulinisotypes-IgM, IgG, IgA and IgE.•They show immunological memory.•They require preliminary processing.•They are rapidly metabolised in the body.
ANTIBODY- IMMUNOGLOBULINS
Towards the close of the nineteenth century, the humoral basis of immunity was established by the fact that following the introduction of an antigen into an animal, certain substances called antibodies appeared in the serum and tissue fluids, and reacted with the antigen specifically and in some observable manner.Depending on the observable reaction produced on mixing with antigens, the antibodies were designated variously as agglutinins, precipitins and so on. Serahaving high antibody levels following infection or immunisation were called immune sera.Fractionation of immune sera by half saturation with ammonium sulphate separated serum proteins into soluble albumin and insoluble globulins. Globulins could be separated into water soluble pseudo globulin and insoluble euglobulins. Most antibodies were foundto be euglobulins. Tiselius (1937) separated serum proteins into albumin, alpha, beta and gamma globulin based on their electrophoretic mobilities. Tiselius and Kabat (1938) showed that antibody activity was associated with the gamma globulin fraction. The termgammaglobulin thereafter became synonymous with 'antibody'. Later, many antibodies, such as equine antitoxins, were found to migrate as beta- or even alpha globulin.Sedimentation studies using the ultracentrifugealso disclosed the diversity of antibody molecules.Most antibody molecules sedimented at 7 S (mol. wt 150,000). Some were heavier-19 S globulins (mol. wt about 900,000), designated M or macroglobulins.The indiscriminate use of various terms (for example, the term beta 2 globulin, 19 S gamma globulin, gamma M globulin) to refer to the same fractions led to confusion. This was resolved when, in 1964, endorsed by the WHO, the generic term immunoglobulin was internationally accepted for 'proteins of animalorigin endowed with known antibody activity and for certain other proteins related to them by chemical structure. The definition includes, besides antibody globulins, the abnormal proteins found in myeloma, macroglobulinemia, cryoglobulinemia and the naturally occurring subunits of immunoglobulins.Immunoglobulins are synthesised by plasma cells and to some extent by lymphocytes. Immunoglobulins provide a structural and chemical concept, while the term 'antibody' is a biological and functional concept. All antibodies are immunoglobulins, but allimmunoglobulins may not be antibodies.globulins Based on Immunoglobulin constitute 20-25 per cent of total serum proteins. Based on physicochemical and antigenic differences, five classes of immunoglobulins have been recognised-IgG, IgA, IgM, IgD and IgE. (Both Ig and y are accepted abbreviations for immunoglobulins)
IMMUNOGLOBIN STRUCTURE
Studies involving the cleavage of the immunoglobulin molecule, pioneered by Porter, Edelman, Nisonoff and their colleagues, have led to a detailed picture of its structure. Rabbit IgG antibody to egg albumin, digested by papain in the presence of cysteine, was split into two fractions an insoluble fraction which crystallised in the cold (called Fc for crystallized), and a soluble fragment which, while unable to precipitate with egg albumin, could still bind with it. This fragment is calledthe Fab (antigen binding) fragment. Each molecule of immunoglobulin is split by papain into three parts one Fc and two Fab pieces, having a sedimentation coefficient of 3.5 S. When treated with pepsin, a 5 S fragment is obtained, which is composed essentially of two Fab fragments held together in position. It isbivalent and precipitates with the antigen. This fragment is called F(ab')2. The Fc portion is digested by pepsin into smaller fragments Immunoglobulins are glycoproteins, each moleculeconsisting of two pairs of polypeptide chains of different sizes. The smaller chains are called 'light' (L) chains and the larger ones 'heavy' (H) chains.The Lichen has a molecular weight of approximately 25,000 and the H chain of 59,000. The L chain is attached to the H chain by a disulfide bond. The two H chains are joined together by 1-5 S-S bonds,depending on the class of immunoglobulins.The H chains are structurally and antigenicallydistinct for each class and are designated by the Greekletter corresponding to the immunoglobulin class.
IgG - IMMUNOGLOBULIN
IgG: This is the major serum immunoglobulin,constituting about 80 per cent of the total. It has a molecular weight of 1500007 S). IgG mayoccasionally exist in polymerized form. Itis distributed approximately equally between the intravascular and extravascular compartments. It contains less carbohydrate than other immunoglobulin ( has a half life of approximately 23 days The catabolism of IgG is unique in that it varies with its serum concentration. When its level is raised, as in chronic malaria, kala azar or myeloma, the IgG synthesised against a particular antigen will be metabolized rapidly and may result in the particular antibody deficiency.Conversely, in hypogammaglobulinemia, the IgG given for treatment will be catabolised slowly. The normal serum concentration of IgG is about 8-16 mg.per ml.IgG is the only maternal immunoglobulin that isnormally transported across the placenta and provides natural passive immunity in the newborn It is not synthesised by the fetus in any significant amount.IgG binds to microorganisms and enhances their phagocytosis. Extracellular killing of target cells coated with IgG antibody is mediated through recognitionof the surface Fe fragment by K cells bearing the appropriate receptors. Interaction of IgG complexes with platelet Fc receptors probably leads to aggregation and vasoactive amine release. IgG alone, among human immunoglobulin, can fix itself to guinea pigskin, but the significance of this property is not known. IgG participates in most immunological reactions such as complement fixation, precipitation and neutralisation of toxins and viruses. It may be considered a general purpose antibody, protectiveagainst infectious agents which are active in blood and tissues. Passively administered IgG suppresses homologous antibody synthesis by a feedback process.This property is utilised in the isoimmunisation of women by the administration of anti-Rh(D) IgG during delivery. With most antigen, IgG is a late antibody and makes its appearance after the initial immune response which is IgM in nature. Four subclasses of IgG have been recognised (IgG1, IgG2, IgG3, IgG4), each possessing a distinct type of gamma chain, identifiable with specific antisera.The four IgG subclasses are distributed in human serum in the approximate proportions of 65 per cent, 23 per cent, 8 per cent and 4 per cent, respectively.
IgA - IMMUNOGLOBULIN
IgA is the second most abundant class,constituting about 10-13 per cent of serumimmunoglobulins. The normal serum level is0.6-4.2 mg per ml. It has a half life of 6-8 days. It is the major immunoglobulin in the colostrum, saliva and tears. IgA occurs in two forms. Serum IgA is principally a monomeric 7 S molecule (MW about 160,000). IgA found on mucosal surfaces and in secretions is a dimerformed by two monomer units joined together at their carboxy terminal by a glycopeptide termed the J chain (J for joining). This is called the secretory IgA (SIgA) Dimeric SlgA is synthesised by plasma cells situatednear the mucosal or glandular epithelium. The J chain is also produced in the same cells. J chains are also present in other polymeric immunoglobulin such as IgM.SlgA contains another glycine-rich polypeptidecalled the secretory component or secretory piece. This is not produced by lymphoid cells but by mucosal or glandular epithelial cells. Dimeric IgA binds to a receptor on the surface of the epithelial cells and is endocytosed and transported across the cells to the luminal surface. During this process, a part of thereceptor remains attached to the IgA dimer. This part is known as the secretory component. The secretory piece is believed to protect IgA from denaturation by bacterial proteases in sites such as the intestinal mucosa which have rich and varied bacterial flora.SlgA is a much larger molecule than serum IgA (11 S MW about 400,000).Slag is selectively concentrated in secretions and on mucus surfaces forming an antibody paste and is believed to play an important role in local immunity against respiratory and intestinal pathogens. Secretary IgA is relatively resistant to the digestive enzymes and reducing agents. IgA antibodies may functionby inhibiting the adherence of microorganisms to the surface of mucosal cells by covering the organisms and thereby preventing their entry into body tissues. IgA does not fix complement but can activate the alternative complement pathway. It promotes phagocytosis and intracellular killing of microorganisms.Two IgA subclasses have been described, IgA, and IgA. IgA, lakhs interchain disulphide bonds between the heavy and light chains. Though IgA, is a minor component of serum IgA, it is the dominant form in the secretions.
IgM - IMMUNOGLOBULIN
IgM constitutes 5-8 per cent of serumimmunoglobulins, with a normal level of 0.5-2 mg per ml. It has a half life of about five days. It is a heavy molecule (19 S; MW 900,000 to 1000000 hence called 'the millionaire molecule). IgM molecules are polymers of five four-peptide subunits, each bearing an extra CH domain (Fig. 12.5). As with IgA, polymerisation of the subunits depends upon the presence of the chain. Though the theoretical valency is ten, this is observed only with small haptens.With larger antigens, the effective valency falls to five, probably due to steric hindrance. Most IgM (80 per cent) is intravascular in distribution. Phylogenetically IgM is the oldest immunoglobulin class. It is alsothe earliest immunoglobulin to be synthesised by the fetus, beginning by about 20 weeks of age. As it is not transported across the placenta, the presence of IgM in the fetus or newborn indicates intrauterine infectionand its detection is useful in the diagnosis of congenital infections such as syphilis, rubella, HIV infection and toxoplasmosis. igM antibodies are relatively short lived, disappearing earlier than IgG. Hence, theirdemonstration in serum indicates recent infection.Treatment of serum with 0.12 M 2-mercaptoethanol selectively destroys IgM without affecting IgG antibodies. This is a simple method for the differential estimation of IgG and IgM antibodies. The isohemagglutinins (anti-A, anti-B) and many other natural antibodies to microorganisms are usually IgM, as also antibodies to typhoid 'O' antigen(endotoxin) and reagin antibodies in syphilis.The unique structural features of IgM appearparticularly suited to the biological role of providing protection against microorganisms and other large antigens that have repeating antigenic determinants on their surface. A single molecule of IgM can bring aboutimmune hemolysis, whereas 1000 IgG molecules are required for the same effect. IgM is also 5001000 times more effective than IgG in opsonisation, 100 times more effective in bactericidal action and about 20 times in bacterial agglutination. In the neutralisationof toxins and viruses, however, it is less active than IgG. Being largely confined to the intravascular space, IgM is believed to be responsible for protection againstblood invasion by microorganisms. IgM deficiency is often associated with septicemia.Monomeric IgM is the major antibody receptor on the surface of B lymphocytes for antigen recognition.