THE FATE OF ANTIGENS ON THE PRIMARY AND SECONDARY RESPONSES
Following intravenous injection of a soluble antigen, its concentration in serum tends to decrease in three phases (Fig. 12.3):
1. Equilibration phase. This phase is characterized by a sharp decrease of brief duration corresponding to the equilibration of the antigen between intra- and extravascular spaces.
2. Metabolic decay. During this phase the antigen slowly decays due to its catabolic processing by the host.
3. Immune elimination. When antibodies start to be formed, there will be a phase of rapid immune elimination in which soluble antigen-antibody complexes will be formed and taken up by macrophages. The onset of this phase of immune elimination is shorter in the secondary immune response and virtually immediate if circulating antibody exists previously to the introduction of the antigen.
A similar sequence of events, with less distinct equilibration and metabolic decay phases, occurs in the case of particulate antigens. If the antigen is a live, multiplying organism, there might be an initial increase in the number of circulating or tissue-colonizing organisms, until the immune response promotes the elimination of the antigen by a variety of mechanisms .