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] .