[Organization and function of the immune system (I)].

An immune reaction is a complex process. Nonspecific resistance is always induced besides specific humoral and cellular defense reactions. After binding to an antigen (antigen-antibody reaction) antibodies can activate complement. If the entire complement cascade occurs, for example at the surface of cells (bacteria), this leads to complement-induced lysis and to destruction of the antigen. Complement cleavage products which are formed during the activation of complement have biological activities. They can attract granulocytes, macrophages and lymphocytes chemotactically or induce and accelerate processes of phagocytosis. In consequence of the activation of T-lymphocytes sensitized T-cells are formed. T-helper-cells intensify both the humoral cellular immune response mediated by B-cells and the cellular immune response mediated by cytotoxic T-cells. They effect this by secretion of substances resembling hormones, the so-called lymphokines. Cytotoxic T-cells (killer cells) can kill their target cells directly. Immune reactions are regulated in multifarious ways. T-suppressor-cells slow down the immune reactions of B- and T-cells. The close interaction of the T-helper and T-suppressor-cells designated as "regulatory T-cells" ensures the functional equilibrium within the immune system.