Adaptive Immune System - Immunological Diversity

Immunological Diversity

Most large molecules, including virtually all proteins and many polysaccharides, can serve as antigens. The parts of an antigen that interact with an antibody molecule or a lymphocyte receptor, are called epitopes. Most antigens contain a variety of epitopes and can stimulate the production of antibodies, specific T cell responses, or both.

A very small proportion (less than 0.01%) of the total lymphocytes are able to bind to a particular antigen, which suggests that only a few cells will respond to each antigen.

For the adaptive response to "remember" and eliminate a large number of pathogens the immune system must be able to distinguish between many different antigens, and the receptors that recognize antigens must be produced in a huge variety of configurations, essentially one receptor (at least) for each different pathogen that might ever be encountered. Even in the absence of antigen stimulation, a human is capable of producing more than 1 trillion different antibody molecules. Millions of genes would be required to store the genetic information used to produce these receptors, but, the entire human genome contains fewer than 25,000 genes.

This myriad of receptors are produced through a process known as clonal selection. According to the clonal selection theory, at birth, an animal will randomly generate a vast diversity of lymphocytes (each bearing a unique antigen receptor) from information encoded in a small family of genes. In order to generate each unique antigen receptor, these genes will have undergone a process called V(D)J recombination, or combinatorial diversification, in which one gene segment recombines with other gene segments to form a single unique gene. It is this assembly process that generates the enormous diversity of receptors and antibodies, before the body ever encounters antigens, and enables the immune system to respond to an almost unlimited diversity of antigens. Throughout the lifetime of an animal, those lymphocytes that can react against the antigens an animal actually encounters, will be selected for action, directed against anything that expresses that antigen.

It is important to note that the innate and adaptive portions of the immune system work together and not in spite of each other. The adaptive arm, B and T cells, would be unable to function without the input of the innate system. T cells are useless without antigen-presenting cells to activate them, and B cells are crippled without T-cell help. On the other hand, the innate system would likely be overrun with pathogens without the specialized action of the adaptive immune response.