A THEORETICAL FRAMEWORK FOR THE IMMUNOEPIDEMIOLOGY OF BLOCKING ANTIBODIES TO HELMINTH INFECTION

Epidemiological evidence is widely cited in support of the hypothesis that certain antibody responses to human helminth, especially schistos ome, infection exhibit blocking activity. This evidence includes posit ive correlations between antibody levels and the rate of re-establishm ent of infection following chemotherapy, antibody levels which peak in younger and more susceptible age classes, lower ratios of blocking an tibodies to others in older and less susceptible age classes. In this paper simple mathematical models are used to explore expected age-spec ific relationships between antibody levels, parasite burdens and re-es tablishment rates for different combinations of protective, neutral an d blocking immune response. In general, all the above cited pattens ma y be generated without invoking blocking activity, especially if the a bilities to produce different antibody responses have different immuno logical memories, i.e. persist for different lengths of time in the ab sence of continued exposure to antigen. None of these patterns, includ ing a positive correlation between antibody levels and rates of re-est ablishment following chemotherapy, offers unambiguous evidence for blo cking activity. Blocking activity is also predicted to affect the shap e of the age-intensity curve and the relationship between susceptibili ty to infection and age in ways which are not necessarily consistent w ith the epidemiological evidence. The importance of the blocking activ ity, which has been convincingly demonstrated in vitro, to population level immunoepidemiological patterns in the field therefore remains un certain.