Elimination of the antigen
The foreign organism is eliminated by the immune system in conjunction with elements of the inflammatory process. The precise reaction varies according to the nature of the pathogen. If the whole cell is recognized as foreign, immunoglobulins will be directed against it and will bind to cell surface antigens. The binding of antibodies and antigens activates the complement system. Complement is a series of nine plasma proteins which become bound to antigen-antibody complexes in a specific sequence (cascade). The binding or fixation of complement results in either the lysis or engulfment by phagocytes of the antigen via the classical or alternative pathways respectively. In the classical pathway, if the binding is on the surface of a bacteria cell wall the final protein product, C9, is able to punch a hole in the membrane and lyse the cell. In the alternative pathway, complement has more important function in aiding the phagocytosis of material in a process of opsonization. Phagocytes, ( macrophages and granulocytes) , have receptors for the Fc component of immunoglobulin and the C3b component of complement. The presence of both of these proteins will render it liable to phagocytosis. In promoting the elimination of the pathogen, the T helper cells have a role in the B cell germinal centre reaction, and for stimulating monocytes to differentiate into macrophages. Cytotoxic T cells are able to kill other cells directly. When a cell becomes infected by a virus, components of the virus are expressed on the surface of the cell in association with the human leucocyte antigen (HLA). The receptors on the cytotoxic T cell recognizes the virus on the HLA molecule and is stimulated to secrete perforins, which punch holes in the membrane similar to those produced by complement C9. The helper or cytotoxic functions are determined by the CD4 or CD8 molecules expressed near the T cell receptor [1, 2, 4] .