Immunodepression in Babesia microti infections.

Infection with the avirulent piroplasm Babesia microti in mice is accompanied by a marked depression in the ability of the mice to mount an immune response to sheep red blood cells. The period of immunodepression begins 3 days after peak parasitaemia and is maximal 4 days later. Thereafter, there is a slow return to normal immune responsiveness, correlated with the gradual disappearance of the parasites from the blood. Both IgM and IgG responses are depressed. Cell-mediated responses as determined by contact sensitivity to oxazolone and allograft survival are apparently unaffected. Phagocytic activity was measured by carbon clearance tests is increased, and is correlated with the parasitaemia.