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There are two types of immune systems: 1. The Humoral Immune System 2. The Cell Mediated Immune System.
1. The Humoral Immune System:
The humoral immune system involves the antibodies that get dissolved in extracellular fluid such as blood plasma, lymph and mucus secretion. These were formally known as humors.
The humoral immunity is conferred through B-cells that develop from stem cells of bone-marrow in adults and the liver in embryos. However, RBCs, neutrophils, macrophages and other types of WBCs are produced from the same stem cells.
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The specialized lymphocytes i.e. B-cells of this system responds when exposed to antigens. B-cells secrete antibodies in correspondence to antigens. This system responds mostly against bacteria (and their toxins) and viruses.
(i) The Antigens:
The antigen (Ag) or immunogen is a large organic molecule capable of stimulating the production of specific antibody with which it may chemically combine. Usually this response involves the formation of antibodies or highly specialized T-cells. The ability of the antigens to induce antibody formation is known as antigenicity.
The nature of antigens:
The majority of antigens are proteins, nucleoproteins (nucleic acid + proteins), lipoproteins (lipid + protein), glycoprotein (carbohydrate + protein) or large polysaccharides. The above compounds are also the components of invading microorganisms: the capsules, cell walls, flagella, pili, and toxins of bacteria, bacterial coat and cell surfaces of many organisms.
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The whole surface of antigen cannot be recognised by the antibodies because these can identify and counteract with specific regions of the antigen surface which is called antigen determinants. This counteraction depends on size and shape of antigenic determinant and the chemical nature of antibodies as well (Fig. 22.4 A).
There are several antigens containing different determinants attached to their surfaces. Therefore, different types of antibodies are required to recognise these determinants. Consequently, different types of antibodies are produced by our immune system. Size and shape of antigen determinants and the chemical nature of antibodies determines the nature of interactions.
There are many antigens which possess different types of determinants on their surface. The different determinants are identified by different antibodies; however, it is the immune system that may produce several antibodies against a single antigen. Most of antigens have molecular weight of 10,000 or more. The low molecular weight antigen is called hapten. It is too small to stimulate antibody formation.
The haptens are not functional unless attached to a carrier molecule, usually a serum protein (Fig. 22.4B). Both function as antigen and provoke the immune response. If an antibody is formed against the hapten, the later reacts with antibody independently even in the absence of a carrier.
For example, penicillin acts as α-hapten which is not antigen itself even then some people develop allergy against it. There are several antigens that contain more than one antigenic determinant site. There may be different kinds of sites on one antigen. At least two binding sites may be present on a single antibody.
(ii) The Antibodies:
An antibody is a glycoprotein which is produced in response to a specific antigen and binds to it via non-covalent interactions. The term ‘immunoglobulin’ is often used interchangeably with ‘antibody’.
The term immunoglobulin (Ig) is used to describe any antibody, regardless of specificity, and the ‘antibody’ to describe an antigen-specific ‘immunoglobulin’ . The Ig can identify and bind the antigens resulting in complete break down. A typical antibody is made up of four polypeptide chains, two light chains and two heavy chains (Fig. 22.5).
Individual antibody possesses at-least two sites known as antigen binding sites. These determines the antigens. Human antibodies have two binding sites therefore, they are bivalent. The bivalent antibody is also called as monomer because it is the simplest antibody. Two monomers are inter-connected by joining (J) chain. Similarly, in pentamer Ig molecules, five monomers are held in position by a J-chain (Fig. 22.6).
(a) Light and heavy chains:
The monomer has four protein chains, two similar light chains (kappa and lambda) and two similar heavy chains. The light chains 220 amino acid long and the heavy chains 440 amino acid long and two similar heavy chains. In humans 60% light chains are kappa and 40% are lambda, whereas in mice 95% of light chains are kappa and 5% are lambda. Each antibody contains one or the other type of light chains.
The heavy chains are of 5 types viz., mu, delta, gamma, epsilon and alpha. (Table 22.3). On the basis of presence of these heavy chains, nomenclature of antibodies has been done. The terra light and heavy refer to their relative molecular weights. These chains are linked to each other by the disulfide linkage.
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Due to linkage of light and heavy chains with disulfide and other bonds the monomer looks a flexible and Y-shaped structure. Heavy and light chains are folded into domains, each containing 110 amino acid residues and intra-chain disulfide bond that form a loop. It attains the three dimensional structure. The domains form discrete structural regions.
(b) Variable regions:
The terminal ends (100-110 amino acids) of both the heavy and light chains i.e. Y arms are of variable (V) in nature because of changes in amino acid sequence at the ends. Therefore, these ends are called V-regions. Thus, two V-regions (one of heavy chain and the other of light chain) form one antigen binding site.
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On each antibody two such sites are located. Hence a single antibody molecule contains only one type of light chain and one type of heavy chain in-spite of being made up of more than one monomer. This shows that one antibody possesses only one type of antigen binding site.
However, more than one antibody can combine with an antigen. The antigens can be aggregated into clumps, if two antigen binding sites on an antibody combines with antigenic determinants on two different antigens. These clumps can be an important factor in diagnosis of a few diseases.
(c) Constant region:
The lower part of Y-shaped monomer antibody is called constant (C) region. The term C refers to relatively invariable feature of amino acid sequence of both the heavy and light chains. The stem of Y-shaped monomer is known as FC (crystallizable fragment) region. An antibody molecule attaches to its host at FC region.
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Based on sequence of C regions, five different types of sequences of heavy chain and two sequences of light chain have been categorized. A different class of immunoglobulin is determined by each heavy chain sequence. The length of C region is about 330 amino acids for α, Ƴ and δ, and 440 amino acids for µ and e (epsilon) (Fig. 22.7).
(iii) Immunoglobulin (Ig) Isotypes:
There are five classes of Ig molecules such as IgG (gamma), IgM (mu), IgA (alpha), IgD (delta) and IgE (epsilon) (Fig. 22.6). The heavy chains have been named with Greek notations. Each class of Ig molecules play a different immuno response. Special features of different Ig molecules are given in Table 22.2 and briefly described herewith.
(a) IgG:
The antibodies account for 50-80% of total antibodies present in serum. The maternal molecule can pass the placenta and provide passive immunity to the foetus. These can also pass the walls of blood vessels and enter in tissue fluid. These can bind to bacteria and viruses, and also can neutralize the toxins secreted by them. After binding with antigens, these enhance the effectiveness of phagocytic cells to engulf and ingest them.
(b) IgM:
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About 5-10% IgM molecules are found in serum. It has a pentamer structure of the antibody (Fig. 22.6). When exposed to antigens, it is the IgM molecule which appears first. In the beginning the concentrations of IgM molecules in blood declines and that of Ig increases. After a second exposure to antigens the concentration of IgG molecules is increased in blood serum.
IgM molecules are especially effective at cross linking particulate antigens and causing their aggregation because of its numerous antigen-binding sites. It can increase the digestion of target cells by the phagocytic cells as IgG. IgM molecules predominate and is involved in ABO blood group antigens on the surface of RBCs.
After infection of the pathogens, IgM appears first. Due to short life it is valuable in disease diagnosis. The high concentration of IgM against a pathogen in blood of a sick person denotes that the disease is really caused by the pathogen.
(c) IgA:
The concentration of IgA in blood serum remains about 15% of total antibodies. They are found in body secretions e.g. saliva, sweat and secretion from gastrointestinal tract and colostrum as well.
During their transport from blood to secretary tissues, Ig A gets attached to a protein caused secretory component, which protects IgA from enzymatic degradation, and facilitates its entry into secretory tissues. It checks the attachment of pathogens to mucosal surfaces, and protects the gastrointestinal tracts of infants from infection.
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(d) IgD:
IgD accounts for only 0.2% of total antibodies of serum. It resembles with IgG. These are present on the upper surface of B-cells. Like others, they also cannot pass across the placenta and help the new baby to initiate the immune response as their population remains very high on the surface of B-cells.
(e) IgE:
The concentration of IgE molecules remains around 0.002% of the total antibodies. They are a little larger than IgG. It binds very tightly to the receptors (mast cells and basophils) with the help of FC region. The mast cells and basophils are the specialized cells that take part in allergy reactions. In a highly allergic person, abnormally a high concentration of IgE is found.
(iv) Antibody Kinetics:
Fig. 22.10 shows the production of antibody in response to antigenic substances. An animal was injected with Antigen A at day 0 which invokes a primary response after 4 days as indicated by a rise in the specific antibody titer i.e. measure of the amount of antibody in the animal’s serum per unit volume. In the beginning, this antibody is mostly IgM (and some IgG). After 7 to 10 days of a peak titer, the response decreases rapidly.
If the animal is then re-injected with antigen A at 28th day, production of antibody begins almost immediately and reaches a level 1000-fold greater than that was measured in the primary response. This is known as the secondary response and the principal antibody produced is IgG. If a second antigen (antigen B) is also injected at the same time as the re-injection of antigen A, only a primary response to antigen B is observed.
These results show that:
(i) The immune response is specific, and
(ii) The immune response has memory.
Clonal Selection Hypothesis (Jerne and Burnet Hypothesis):
The clonal selection hypothesis attempts to explain the findings described above by suggesting the following:
(i) Animals contain numerous cells which are called lymphocytes,
(ii) Each lymphocyte is responsive to a particular antigen due to the presence of specific surface receptor molecules,
(iii) After making contact to its appropriate antigen, the lymphocyte is stimulated to proliferate (clonal expansion) and differentiate, and
(iv) The expanded clone is responsible for the secondary response (more cells to respond) while the differentiated (‘effector’) cells secrete antibody.
(v) Regulation of Humoral Response:
The immune response is regulation possibly in several ways. First, a specific group of T-cells (i.e. suppressor T-cells) are involved in turning down the immune response. Like helper T-cells the suppressor T-cells are stimulated by antigen.
But suppressor T-cells release factors that suppress the B-cell response, instead of releasing lymphokines that activate B-cells (and other cells). It is more complicated than the activation pathway, and possibly involves the additional cells.
(a) Antigen Blocking and Receptor Cross-Linking:
The other ways of regulation involves interactions between antibody and B-cells (Fig. 22.11 A-B). When high doses of antibody interact with all of the antigen’s epitopes, the ‘antigen-blocking’ mechanism occurs, which inhibits interactions with B-cell receptors.
A second mechanism, ‘receptor cross-linking’ results when antibody binds to a B-cell via its Fc receptor, and the B-cell receptor combines with antigen. This ‘cross-linking’ inhibits the B-cell from producing further antibody.
(b) Idiotypic Networking:
The idiotypic network hypothesis suggests that the idiotypic determinants of antibody molecules are very unique and act to the immune system like foreign molecules; therefore, they are antigenic in nature.
Hence, antibody production in response to antigen leads to the production of anti-antibody in response, and anti-anti-antibody and so on. Eventually, the level of [anti]n-antibody is not sufficient to induce another round and the cascade ends (Fig. 22.12).
(vi) Monoclonal Antibodies:
So far the production of a large number of antibodies in response to antigens has been described. All the antibodies are mixed in serum which are the product of a large number of clones of B-cells. Therefore, they are termed as polyclonal antibodies.
When a single clone of B-cells produces antibodies, all the antibodies are alike and, therefore, called monoclonal antibodies. Monoclonal antibodies are produced by B-cells. But the B-cells do not proliferate on artificial medium.
For the first time Kohler and Milstein (1975) got success in producing monoclonal antibodies from the hybrid B-cells. The hybrid B-cells were obtained after fusing the myeloma cells (cancerous cells) with B-cells of spleen obtained after injecting the antigen in mouse (Fig. 22.13). Myeloma cells have the feature of cell division and B-cells have the characters of antibody production.
Therefore, the hybrids could successfully proliferate on the artificial media, and produce antibodies. After a successive sub-culturing of hybrids and clone selection, pure line hybrids was obtained. This new technology of Kohler and Milstein (1975) for production of hybrid cell line is known as hyhridoma technology.
In recent years, through the hybridoma technology antibodies of desired properties can be produced. The monoclonal antibodies are very useful because (i) they are uniform, (ii) they are highly specific, and (iii) they can be quickly produced in adequate amount. For a detailed account readers may consult the book Biotechnology by R.C. Dubey (2006).
2. The Cell Mediated Immune System:
The cell mediated immune system directly involves the specialized lymphocytes known as T- cells. After differentiation, the T-cells migrate to lymphoid organs. They do not secrete antibodies but they contain antibodies like molecules called antigen receptors which is attached to their surfaces. Several kinds of T-cells are found.
This system of immunity is most effective against bacteria or viruses when present within the phagocyte or even infected host cells, or in infected host cells against protozoa, fungi and helminths, transplanted tissues and cancer.
The receptors help the T-cells to interact with a variety of antigens. After recognizing the antigens, T-cells differentiate in a variety of effector T-cells. Only the effector cells recognise the antigen and regulate the immune system.
(i) Types of Effector T-Cells:
The effector T-cells are of three main types such as:
(i) Helper T-cells,
(ii) Suppressor T-cells, and
(iii) Cytotoxic T-cells.
(a) The Helper T-Cells:
The helper T-cells play a variety of important roles. They contain a surface antigen which provide a vigorous immune response. Some helper T cells provide T dependent antigens to B-cells, and some others (e.g. delayed hypersensitivity T-cells) are associated with certain allergic reactions and rejection of transplanted tissues. When the antigen interacts, the delayed hypersensitivity T-cells secretes lymphokines that recruit defense cells like macrophages. The delayed hypersensitivity T-cells also defends the body against the development of cancer.
(b) Suppressor T-Cells:
Not much information is available on suppressor T-cells but the general concept is that they check the conversion of B-cells into plasma cells as well as the T-cells. Hence, they check the immune response, and help the body to develop tolerance. Therefore, both the helper cells and suppressor T-cells are known as regulatory T-cells as they regulate the immune response of the body against the antigens.
(c) Cytotoxic T-Cells:
As the term denotes, cytotoxic T-cells destroy the target ceils e.g. transplanted tissue, cancer cells, viral and bacterial infections, etc. Viruses and some of the bacteria multiply within the host cells and, therefore, they rescue from the attack of antibodies.
Hence, the cytotoxic T-cells recognise their antigens on the surface of host cells that produce viruses and destroy them. Cytotoxic T-cells come in the contact of host cell; release a protein (perforin) that makes a pore in the target cell which finally is destroyed.
(d) The Killer Cells and Lymphokines:
Some of the effector cells pose cytotoxic effect and therefore, called the killer cells. The killer cells are not very specific. They can invade any such cell that are located with antibodies. The killer cells contain the receptors which combine with FC region of antibodies.
It has earlier been mentioned that the delayed hypersensitive T-cells when stimulated by antigen, secrete proteins which are known as lymphokines. It attracts the macrophages to the infection site, checks the movement away from the infection site and activates them to destroy the cellular antigens.
(ii) Mechanism of Cell Mediated Immunity:
The T cells are very specific (like B-cells) to only a specific antigen. But unlike B-cells, the T-cells do not respond to antigens. It responds to the antigens present on cell surfaces. However, before showing the responses, the antigens on cell surfaces need to be processed by the antigen presenting cell (APC) (Fig. 22.14) as described earlier. Thus, the APC possesses antigens on its surfaces. The T-cells also respond against the MHC.
The T-cells display the associative recognition i.e. the T-cells recognise the antigen only when it is in close association of an MHC antigen. An APC cell secretes inter-leukine-1 when stimulated by an antigen. Interleukin-1 is a monokine, a biologically active substance secreted by macrophages that activates the T-cells.
The activated T-cells in turn secrete interleukin-2 and surface receptors for interleukin-2 which binds the former. After the surface receptor binds to the interleukin-2, the T-cells start proliferating and differentiating into different types of effector cells, cytotoxic T-cells, killer cells, natural killer cells and activated macrophages (Fig. 22.14).
