The protective antigen of malarial sporozoites (Plasmodium berghei) is a differentiation antigen.

Pb44, the protective antigen of rodent malaria sporozoite (Plasmodium berghei) covers the entire surface of mature, salivary gland sporozoites. This antigen is undetectable in approximately 50% of the immature, i.e., oocyst sporozoites. On the surface of the remaining oocyst sporozoites, Pb44 is found in patches. Pb44 is present in early exoerythrocytic liver stages of P. berghei but is present in early exoerythrocytic liver stages of P. berghei but becomes undetectable after 30 hr of intrahepatocytic development. It seems likely that Pb44 is a differentiation antigen associated with an unique function of the sporozoites, perhaps penetration in the target host cells.