FIG. 4.

Hypothetical basis of age-dependent inversion of susceptibility to disease with acute versus chronic exposure in children and adults. Consider Th1- and Th2-type immune responses as surrogates for any immune response that changes with age independent of exposure and plays a critical role in infection outcomes. Th1-driven effectors may dominate the immune response of children (hollow arrows), whereas Th2-driven effectors may dominate the adult immune response (solid arrows). These distinct, age- and exposure-dependent responses cause harm or benefit to the host. (Reproduced from reference 11 with permission of the Liverpool School of Tropical Medicine.)