Antibodies secreted by plasma cells may have several different effects – (1) They may interact with free (i.e., soluble) antigens causing precipitation; (2) They may interact with surface antigens of the pathogen (i.e., particulate antigens) causing agglutination; or (3) They may promote complement fixation.
1. Precipitation of Soluble Antigens:
Antigens may have one or more antigenic determinants. If antigens contain one determinant, they are called monodeterminants; if they contain two determinants, they are called bideterminants and so on.
Most antibodies are bivalents, meaning that they can simultaneously combine with two antigenic determinants. The products formed by interaction of antibody and antigen depend on the number of antigenic determinants that are present. For example, two monodeterminant antigens can be cross-linked by a single antibody, but the product is not usually insoluble unless the antigen itself is very large.
However, if two antigenic determinants are present, cross-linking by the antibody can produce chains of antigens that are insoluble and form precipitates. Multideterminant antigens react with antibody to produce cross-linked network or lattices that are insoluble.
Interaction between antibodies and free antigens can be considerably far more complex. For example, some antibodies may exist as dimers (e.g., IgA) or pentamers (e.g., IgM); these antibodies can simultaneously bind four or more antigenic determinants.
Moreover, antigens may possess more than one kind of antigenic determinant, each determinant capable of reacting with a different antibody. Finally, the predominant form of interaction that takes place between antibodies and antigens is influenced by the respective concentrations of the interacting molecules (= species).
Thus, small soluble complexes are favoured when there is an excess of antibody; chains of cross- linked antigens are favoured when there is an antigen excess; and cross-linked lattices are favoured by nearly equal amounts of antibody and antigen. Regardless of the nature of the products formed, antigen-antibody complexes are eventually eliminated by the phagocytic action of macrophages.
Antibodies that interact with antigens present on the surfaces of invading microorganisms or other foreign particles cause agglutination. During agglutination, the particles become cross-linked to form small masses, and the masses are limited by the phagocytic action of macrophages.
The plasma membranes of macrophages possess receptors that recognise and bind the C- terminal or FC regions of heavy chains of immunoglobulin. As a result, the macrophage receptors are called FC receptors.
Because, the FC regions of immunoglobulins include constant domains, macrophage FC receptors can bind a variety of different antibodies. Interaction between a macrophage and a mass of agglutinated cells is followed by phagocytosis.
Foreign cells that have attached antibodies can be destroyed by K (or Killer) cells. Killer cells bind the agglutinated mass by interacting with the FC regions of an antibody but do not internalise it. Instead, it is thought that there is the transfer of toxic substances from the K cells to the pathogen.
3. Complement Fixation (or the Complement System):
Complement, so called since it complements and amplifies the action of antibody is the principal means by which antibodies defend vertebrates against most bacterial infections. It consists of a system of serum proteins that can be activated by antibody-antigen complexes or microorganisms to undergo a cascade of proteolytic reactions whose end result is the assembly of membrane attack complexes (or lytic complex).
These complexes form holes in a microorganism and thereby destroy it. At the same time, proteolytic fragments released during the activation process promote the defense response by dilating blood vessels and attracting phagocytic cells to the sites of infection. Complement also amplifies the ability of phagocytic cells to bind, ingest and destroy the microorganisms being attacked.
Individuals with a deficiency in one of the central components of complement system (e.g., C3) are subject to repeated bacterial infections, just as are individuals deficient in antibodies themselves.
Complement-deficient individuals may also suffer from immune-complex diseases, in which antibody-antigen complexes precipitate in small blood vessels in skin, joints, kidney and brain, where they cause inflammation and destroy tissues; this suggests that complement normally helps to solubilise such complexes when they form during an immune response.
Mode of complement activation:
Complement consists of about 20 interacting proteins, of which reacting components are designated C1—C9, factor B and factor D— the rest comprising a variety of regulatory proteins. The complement components are all soluble proteins.
They are made mainly by the liver and circulate in the blood and extracellular fluid. Most are inactive unless they are triggered directly by an invading microorganisms or indirectly by an immune response.
The ultimate result of complement activation is the assembly of the late complement components such as C5, C6, C7, C8 and C9, into a large protein complex, the membrane attack complex, that mediates microbial cell lysis.
Because its function is to attack the plasma membrane of microbial cells, the activation of complement is focused on the microbial plasma membrane, where it is triggered either by antibody bound to the microorganism or by microbial envelope polysaccharides. Both of these activate the early complement components.
There are two sets of early complement component belonging to two distinct pathways of complement activation: C1, C2, and C4 belong to the classical pathway which is triggered by antibody binding; factor B and factor D belong to the alternative pathway, which is triggered by microbial polysaccharides.
The early complement components of both pathways ultimately act on C3, the most important complement component. The early complement components and C3 are proenzymes that are activated sequentially cleavage: as each proenzyme in the sequence is cleaved, it is activated to generate a serine protease, which cleaves the next proenzyme in the sequence and so on.
Many of these cleavages liberate a small peptide fragment and expose a membrane-binding site on the large fragment. The larger fragment binds tightly to the target cell membrane by its newly exposed membrane-binding site and helps to carry out the next reaction in the sequence.
In this way, complement activation is confined largely to the cell surface where it began. The smaller fragment often acts independently, as a diffusible signal that promotes an inflammatory response.
The activation of C3 by cleavage is the central reaction in the complement activation sequence, and it is here that the classical and alternative pathways converge. In both pathway, C3 is cleaved by an enzyme complex called a C3 convertase.
A different C3 convertase is produced by each pathway formed by the spontaneous assembly of two of the complement components activated earlier in the cascade. Both types of C3 convertase cleave C3 into two fragments.
The larger of these (C3b) binds covalently to the target-cell membrane and binds C5. Once bound, the C5 protein is cleaved by the C3 convertase (now acting as C5 convertase) to initiate the spontaneous assembly of the late components— (C5 through C9) — that creates the membrane attack complex.
Since each activated enzyme cleaves many molecules of the next protoenzyme in the chain, the activation of the early components consists of an amplifying proteolytic cascade: each molecule activated at the beginning of the sequence leads to the production of many membrane attack complexes.
The classical pathway is usually activated by IgG or IgM antibodies bound to antigens on the surface of a microorganism (or target cell). The binding of antigen by these antibodies enables their constant regions to bind in turn to the first component in the classical pathway, CI, which is a large complex composed of three subcomponents – C1q, C1r and C1s.
The molecule of C1q protein is large sized (-450,000 daltons) and made up of six identical subunits, each composed of three different polypeptide chains. The carboxyl-terminal halves of each of the three polypeptide chains in a subunit are folded into a globular structure; the amino-terminal halves have a typical collagen amino acid sequence and are bound together to form a collagen-like triple-stranded helix.
The six subunits are linked together by disulphide bonds between their triple-helical stems, forming a structure that resembles a bunch of tulips.
The binding of a globular head of C1q to an IgG or IgM antibody bound to antigen activates C1q to start the early proteolytic cascade of the classical pathway. Activation of the C1q activates C1r to become proteolytic, and C1r in turn cleaves and activates C1s.
Activated C1s then cleaves C4 into two fragments C4a and C4b. C4b immediately binds covalently to the membrane and then binds C2. Once bound, C2 is also cleaved by activated C1s.
There are some microbes which are ingested but not killed by macrophages. For example, Mycobacterium leprae, the causative agent of leprosy, and parasite of genus Leishmania (a flagellated protozoon causing leishmaniasis) actually grow only in the endocytic vesicles of macrophages, and for this reason they are extremely difficult to kill.
Clonal Selection Theory:
The fact that specific antibodies are produced on demand by the arrival of particular antigen, led to the development of the clonal selection theory.
This theory is based on the idea that an enormously wide range of B lymphocytes, each potentially capable of producing a specific kind of antibody, is present in the body before birth. When an antigen gets into the body after birth, it ‘selects’ a lymphocyte of the appropriate type, adheres to its surface and causes it to proliferate into a clone of cells, all of which proceed to manufacture the correct antibodies (To recall, a clone is a population of identical cells all derived from one original cell).
The lymphocyte is already programmed by its genes to make the right antibodies. The antigen simply triggers it into action, probably by switching on the appropriate part of its DNA.
The theory is based on the lymphocyte recognising its particular antigen. Burnet suggested that this is achieved by the antigen fitting into receptor site on the cell surface (antigen never gets inside the lymphocyte).
Now the question arises, why does each B lymphocyte make only one type of antibody? Mammalian cells are diploid; they carry two sets of genetic information coding for each of the antibody chains.
But only one productive genome rearrangement of light chain coding sequences and one productive genome rearrangement of heavy chain coding sequence occur in each B lymphocyte. This phenomenon is called allelic exclusion, because one of the “alleles” is excluded from being expressed. The exact mechanism involved in this phenomenon is not known.
However, it appears that there must be some type of a feedback mechanism that arrests the recombination processes) involved in these antibody gene rearrangement, once a productive rearrangement has occurred and the cell has started to synthesise a functional antibody. The simplest mechanism would involve inhibition of this process by the mature antibody itself.
A relationship has been observed between time and the appearance of antibodies in response to a first exposure to a given antigen. Following a short lag period, antibodies begin to appear in the blood, rising to maintain a plateau level for some time before falling again.
This characteristic response curve is called a primary immune response. As long as the antibody content of the blood remains at its plateau level, a condition of active immunity exists. The response to a second exposure to the same antigen—the secondary immune response—is much more dramatic.
In this case, the lag period is shorter, the response is more intense (i.e., greater quantities of antibody are produced) and the elevated antibody level is maintained for a longer period of time. The difference between the two responses indicates that the body has “remembered” its earlier exposure to the antigen. This is called immunological memory.
Immunological memory may be explained in the following way. The initial exposure to antigen causes differentiation of B lymphocytes into memory cells as well as plasma cells. Whereas the plasma cells have a relatively short life span in which they are actively engaged in antibody secretion, memory cells do not secrete antibody and continue to circulate in the blood and lymph for months or years.
These memory cells are able to respond more quickly to the reappearance of the same antigen than undifferentiated B lymphocytes. Memory cells are also produced by the multiplication and differentiation of T lymphocytes.
The immune system normally produces antibodies against foreign proteins but not against the native proteins of body, that is, the immune system can distinguish between “self” and “non-self.” However, in rare cases, individuals begin to produce antibodies against their own antigens.
These antibodies are called autoantibodies and the diseases resulting from their presence are the autoimmune diseases. Among these diseases are paroxysmal cold haemoglobinuria (antibodies against one’s own red blood cells), myasthenia gravis (antibodies against one’s own muscle cell acetylcholine receptors) and systemic lupus erythematosus (antibodies against one’s own nuclear DNA).