The manner in which an antigen is dealt with in the body depends on factors such as the physical and chemical nature of the antigen, its dose and route of entry, and whether the antigenic stimulus is primary or secondary.
Antigens introduced intravenously are rapidly localized in the spleen, liver, bone marrow, kidneys and lungs. They are broken down by reticuloendothelial cells and excreted in the urine, about 70-80 per cent being thus eliminated within one or two days.
In contrast, antigens introduced subcutaneously are mainly localized in the draining lymph nodes, only small amounts bring found in the spleen.
Particulate antigens are removed from circulation in two phases. The first is the non immune phase during which the antigen is engulfed by the phagocytic cells, broken down and eliminated.
With the appearance of the specific antibody, the phase of immune elimination begins, during which antigen – antibody complexes are formed and are rapidly phagocytosed, resulting in the accelerated disappearance of the antigen from circulation.
With soluble antigens, three phases can be recognized – equilibration, metabolism and immune elimination.
The phase of equilibration consist of diffusion of the antigen to the extra vascular space. During metabolic phase, the level of the antigen falls due to catabolic decay. During the phase of immune elimination, there is rapid elimination of the antigen with the formation of antigen – antibody complexes.
Such complexes can cause tissue damage and may be responsible for the ‘immune complex diseases’ such as serum sickness.
The speed of elimination of an antigen is related to the speed at which it is metabolized. Protein antigens are generally eliminated within days or weeks, whereas polysaccharides which are metabolized slowly, persist for months or years. Pneumococcal polysaccharide, for instance, may persist up to 20 years in human beings, following a single injection.