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Antibody-Mediated Immunity

B-cells are responsible for antibody-mediated or humoral immunity. When each individual B cell matures, it begins to express on its surface a single type of antibody, a protein molecule composed of four polypeptide chains. The amazing aspect of this phenomenon is that every B cell displays a different antibody; more than a million (up to 100 million) unique B cells are thus present at any time in an individual. Molecules on the surface of pathogens, known as antigens, bind specifically with existing antibodies. With such a vast diversity of B cells and antibodies in the system, it is overwhelmingly likely that there will be an antibody present to react with any antigen that enters the body. When this antigen/antibody binding takes place, the B cell displaying the antibody is said to be stimulated or activated, and a series of changes rapidly ensues. The B cell quickly proliferates by mitosis, making more and more B cells, each able to express the same antibody; since each of these cells is a genetic clone of the original, this entire process is often referred to as clonal selection and expansion. Members of the new population of identical B cells soon begin to differentiate into two types of cells with different functions: plasma B cells and memory B cells.

Structure of an antibody molecule


Plasma cells are specialized to manufacture and release huge quantities of the antibody that initially responded to the antigen. These antibodies can now bind with virtually all of the stimulating antigen, and, by doing so, they rid the body of the antigen-containing invader in two ways. The binding of the antibody to the antigen may disable the intruder directly. More commonly, the antibody/antigen complex is phagocytized or targeted by a system of plasma proteins known as the complement system. Complement proteins make holes in the membranes of targeted cells, causing them to lyse and die.
Memory cells, which also have the ability to produce antibodies, persist for long periods of time, sometimes the entire lifetime of an individual. If the same antigen is encountered again in the future, the immediate large scale production of antibodies disables the pathogen before it can cause significant effects. This phenomenon accounts for the ability of the body to acquire active immunity against a specific disease and is the reason why, after an individual contracts a disease once, he or she is often immune to future infection. It is also the rationale behind the effectiveness of vaccination (immunization).
An antibody protein is composed of four polypeptides, each of which has a constant region and a variable region. It is the variable region that differs from antibody to antibody, providing the specificity of the B cell response. Since antibodies are proteins, the amazing diversity of antibodies that exists in any person presents an apparent mystery. Every polypeptide manufactured by a cell must have a corresponding gene to supply the instructions, but there are only about 100,000 genes in the entire human genome. How can over a million different antibodies be produced when it seems there are not enough genes to encode them? It has been established that a unique process called somatic recombination provides the answer. The segments of DNA that encode the variable regions of each chain are broken up into several hundred “modules”. In any particular B cell, several of the modules are selected and ordered randomly as DNA is cut and rejoined together; this unique combination of modules is then transcribed and translated into a unique antibody. This “shuffling” of DNA segments accounts for the vast diversity of observed antibodies and the paradox of their existence.

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