Immunity to Parasitic and Fungal Infections

Infections with protozoa and helminths are typically chronic (often lasting for the remainder of the lifetime of the host), with the onset of disease symptoms in many instances developing years after the initial infection. Cutaneous leishmaniasis (also called Oriental sore) is probably the only major human parasitic infection for which there appears to be immunity to reinfection. As with many other intracellular infections, cell-mediated immunity seems to be the most important mechanism of resolving the infection, with little if any role for antibodies. T cells are critical in the control of all parasitic infections. Despite an apparent absence of protective immunity that protects against reinfection, the cell-mediated arm of the immune system is required to control parasitic infections. The most serious fungal infections are the systemic mycoses, including histoplasmosis, cryptococcosis, and coccidioidomycosis, which usually begin as lung infections and are acquired by inhaling the spores of free-living fungi. If macrophages are not activated, they are less potent at killing fungi than are neutrophils. Thus, the role of macrophages in resistance to fungal infection is more important in acquired immunity, where they become an effective tool of the specific immune response following activation by TH1-cell-derived cytokines.