Circumsporozoite (CS) proteins have been shown to act as the protective antigens capable of inducing sporozoite-specific immunity in several species of malaria. The gene coding for this antigen in Plasmodium falciparum has been isolated, cloned, and incorporated into E. coli. The recombinant protein induces antibodies in mice which react with sporozoites and blocks liver invasion in vitro. T cells are critical for malaria immunity, not only because they serve as helper cells for an antibody response, but because they also have an important role as effector cells. The level of the immune response to a given immunogen is determined by Immune Response (Ir) genes and tolerance. We have studied the interaction of the malaria parasite with the host immune system and we have found evidence that the parasite has lessened the level of the immune response against it by host mimicry (and resultant immunological tolerance) and by limitation and variation within T sites (i.e. through Ir gene control).