Protozoal infection results in tissue damage leading to disease. In chronic infections the tissue damage is often due to an immune response to the parasite and/or to host antigens as well as to changes in cytokine profiles.
Resistance is the ability of a host to defend itself against a pathogen. Resistance to protozoan parasites involves three interrelated mechanisms: nonspecific factors, cellular immunity, and humoral immunity.
Protozoal infection results in tissue damage leading to disease. In chronic infections the tissue damage is often due to an immune response to the parasite and/or to host antigens as well as to changes in cytokine profiles. Alternatively, it may be due to toxic protozoal products and/or to mechanical damage.
Escape mechanisms are strategies by which parasites avoid the killing effect of the immune system in an immunocompetent host. Escape mechanisms used by protozoal parasites include the following.
Antigenic Masking: Antigenic masking is the ability of a parasite to escape immune detection by covering itself with host antigens.
Blocking of Serum Factors: Some parasites acquire a coating of antigen-antibody complexes or noncytotoxic antibodies that sterically blocks the binding of specific antibody or lymphocytes to the parasite surface antigens.
Intracellular Location: The intracellular habitat of some protozoan parasites protects them from the direct effects of the host's immune response. By concealing the parasite antigens, this strategy also delays detection by the immune system.
Antigenic Variation: Some protozoan parasites change their surface antigens during the course of an infection. Parasites carrying the new antigens escape the immune response to the original antigens.
Immunosuppression: Parasitic protozoan infections generally produce some degree of host immunosuppression. This reduced immune response may delay detection of antigenic variants. It may also reduce the ability of the immune system to inhibit the growth of and/or to kill the parasites.
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