To determine whether significant changes could be detected in the phenotype of circulating lymphocytes following infection, the frequencies of T cells and B cells expressing the activation markers CD69 or CD38 were measured prior to and seven days post-infection with P. falciparum. In all volunteers, infection led to a significant increase in the frequency and absolute numbers of CD69+ T cells and B cells (Fig 1A and S1 Fig). Here, CD38 is linked to infection.