Models of the Dynamics of Acquired Immunity to Helminth Infection in Man

Upon invading a host organism, parasitic species invariably trigger the defence mechanisms of the immune system. In vertebrate hosts, the system comprises of cells, antibodies, amplification factors and specialized organs. The immune system sometimes enables the host to regulate parasitic abundance and to build up a degree of acquired resistance to reinfection. However, in the case of most parasitic protozoa and helminths, the degree of acquired immunity illicited by infection is variable, and not so solid as that induced by many viruses or bacteria. In endemic areas of the world parasitic infections therefore tend to be persistent in character, where the human inhabitants are repeatedly exposed to reinfection and may harbour parasites for the majority of their lives. The major helminth infections of man (the intestinal nemotodes, the schistosome flukes and the filarial worms) are particularly remarkable in this sense, since man appears unable to develop fully protective immunity, despite repeated exposure to high levels of infection. In part, this is thought to be a consequence of the antigenic complexity of parasitic worms, and their often complex developmental cycles within the human host. Each developmental stage may express different surface or excretory antigens.