Immune responses in parasitic diseases. Part A: general concepts

Parasitic infections are incredibly varied and distinct in terms of interactions between hosts and pathogens as well as in complexity of life cycle, host range, vector or intermediary host requirements, forms of reproduction, and elicited response. A number of protozoan parasites are intracellular pathogens capable of surviving and multiplying within microbicidal cells such as macrophages. In contrast, nematodes generally do not multiply within the host, a trait that dramatically alters the epidemiologic, clinical, and immunologic consequences of infection. Parasites have acquired apparently effective mechanisms for escape from normal host defenses and clearance. These mechanisms may be classified as antigenic mimicry, antigenic depletion, antigenic variation, immunologic indifference, immunologic diversion, and immunologic subversion. A determination of the importance and relevance of these subterfuges to parasitic infection in humans and to therapeutic or prophylactic strategies is of the utmost urgency.