Das Immunsystem unter besonderer Berücksichtigung der Erkennungsstrukturen auf B- und T-Lymphozyten

Abstract

The essential function of the immune system is to defend against infection. Individuals with defects in central parts of this system continously suffer from infections and without medical care often die. The immune system consists of lymphocytes, macrophages and antibodies which are synthesized by the lymphocytes. Specific immune reactions are induced by antigens. Any substance can serve as antigen and induce immune responses in the host. Antigenic determinants are called epitopes. In case of proteins, an epitope is formed by about 10 aminoacids. Epitopes are recognized by antibodies or immunoglobulins. These are built up of 4 polypeptide chains, namely two identical heavy and two identical light chains. Within each chain one can distinguish a constant and a variable part. The variable parts of heavy and light chains form the antigenbinding site or paratope. Each paratope fits to an epitope as a key fits to its lock. As there exist millions of epitopes there must exist millions of paratopes formed by the variable regions of heavy and light chains. It is not yet fully understood how nature creates this enormous variability.
The immune response is initiated by the recognition of epitopes by receptors located on the surface of lymphocytes. Once the antigen is recognized, the lymphocytes get activated and start to produce antibodies which have the same specificity that means the same variable regions as the lymphocyte receptors. The production of specific antibodies is only one branch, namely the humoral response of the immune answer. The other branch includes the production of specific lymphocytes which represents the cellular immune response.
The lymphocytes can be divided into two major classes, namely B and T lymphocytes. Both have their stemcells in the bone marrow and B lymphocytes mature under the influence of a not yet fully characterized central lymphoid organ. In birds this organ is the Bursa of Fabricii. T lymphocytes mature in the thymus. B lymphocytes are responsible for the production of antibodies. Once activated, B cells differentiate either into plasmacells which secrete the immunoglobulins or into memory cells which wait within the body for a further contact with the same antigen against which the memory cells can very rapidly response in a so-called secondary immune response. In order to produce antibodies, B lymphocytes need the help of T lymphocytes. These T cells are called helper cells which represent one of the three subpopulation of T cells. The other two are killer cells and suppressor cells. The killer T cells can eliminate other cells and are e. g. activated in the course of a transplant rejection and responsible for it. Suppressor cells, once activated can specifically suppress an immune response.
A central problem of modern immunology is the question how lymphocytes can recognize epitopes. B cells do that by the use of immunoglobulin receptors. At the B cell level surface receptors and the effector molecules, the antibodies, are more or less the same. The nature of the T cell receptor is not yet known. There is ample evidence that T cells use in part the same variable regions as B cells in order to create the antigenbinding regions.