This thesis reports two immuno-epidemiological studies of cellular immune responses to Schistosoma haematobium infection in humans. The first study was a cross sectional infection study. The study cohort consisted of 59 Gambians made up of two distinct age groups: children (12-16 years old) and adults (25-88 years old). The study examined three hypotheses: 1) protection against infection is associated with a Th2-type immune response, 2) Th1 and Th2 responses are dichotomous options in individuals and 3) cytokine production is affected by cross-reactive antigen. The second, a re-infection study, was based in Zimbabwe. The study cohort consisted of 83 Zimbabwean children (6 to 15 years) recruited from two separate villages. One site had significantly lower prevalence of infection than the other, conferring an opportunity to examine the effects of transmission dynamics on the development of a protective immune response. The study addressed two major hypotheses: 1) an appropriate protective type of immune response develops faster in the high prevalence area compared to the low prevalence area and 2) individuals produce either IL-4 or IL-5 but not both.