Host Defenses to Protozoa

The innate and adaptive immune systems respond to the diverse protozoan pathogens with a wide array of weapons. Neutrophils, macrophages, and natural killer (NK) cells mediate the innate response against extracellular protozoan parasites. NK cell– and cytokine-activated macrophages are central to the innate response to intracellular parasites. Innate cytokine and dendritic cell (DC) responses also play a critical role in the induction of adaptive immunity. For the intracellular pathogens (e.g., Plasmodium spp., Leishmania spp., T. cruzi, T. gondii), the early production of interleukin-12 (IL-12) and interferon-γIFN- drives the differentiation of CD4 T cells to a protective T helper 1 (Th1) phenotype. CD8 T cells can also play a critical role through cytokine production (e.g., Plasmodium spp., T. cruzi, T. gondii) or direct cytotoxic activity (e.g., Cryptosporidium). For the parasites that have an extracellular stage (e.g., Plasmodium spp., Trypanosoma spp., Giardia, and Trichomonas), specific antibodies play a role in the acquired immune response. Protozoan parasites utilize diverse mechanisms to evade the host immune response and establish chronic infection.