S antigens are soluble heat-stable proteins present in the sera of some infected individuals. Localizes to the cell periphery in early schizonts and then to the parasitophorous vacuole in late schizonts (PubMed:2406433). Following schizont rupture, the S-antigen is released in host sera (PubMed:2406433). Expressed during the parasite blood stage, specifically in schizonts (at protein level) (PubMed:2406433). Not expressed in free merozoites (at protein level) (PubMed:2406433). Diversity in S-antigen is mainly due to polymorphism in the repetitive regions.