Immunodeficiencies in parasitic diseases.

Immunodeficiencies linked to parasitism are mainly seen in protozoan infections. The clinical expression varies from generalized unresponsiveness to failure to mount a protective immune response, non-sterilizing immunity being the common situation in most parasite infections. Besides becoming involved with general mechanisms, such as antigenic competition and induction of suppressor cells, parasites can interfere directly in the immune networks through the release of immunomodulatory factors. Antigenic mimicry, antigenic variation at the molecular level (i.e. change in antigen sequence) or topographical level (i.e., surface versus internal expression), and deviation of the immune response towards production of non-protective and even blocking effectors provide unique models of how an infective microorganism can survive and even take advantage of the immune response of its host to reach an equilibrium between host defence and parasite escape strategies responsible for the chronicity of the infection.