Immunology of Entamoeba histolytica in human and animal hosts.

Although Entamoeba histolytica induces humoral and cellular immune responses in both human and animal hosts, there is no indication of postinfection immunity in humans; in contrast, several other mammals are protected by prior infection or immunization. The exacerbation of the disease by immunosuppression suggests a protective function of still-unknown defense mechanisms. Specific local and circulating antibodies are produced regularly during invasive amebiasis. Although serum antibodies, together with complement, are lytic to the trophozoites in vitro, the poor correlation of these antibodies with resistance contradicts a protective capacity in vivo. The parasite may evade harm by shedding antigen-antibody complexes from its surface. Demonstration of immediate-type skin reactions, elevated IgE titers, and specific antiamebic IgE suggests that anaphylaxis occurs. The function of the anaphylactic reaction in pathology and resistance remains to be studied. Delayed hypersensitivity parallels healing or resistance and is retarded in human hepatic amebiasis. This observation is consistent with a protective role of cell-mediated immunity.