Explaining Observed Infection and Antibody Age-Profiles in Populations with Urogenital Schistosomiasis
Figure 3
Schematic diagram of the memory model.
This model has state variables for worm burden (P), two populations of memory cells (M1 and M2) and two corresponding populations of plasma cells (A1 and A2). Worms accumulate at an age-dependent rate Λ and die at a per-capita rate μ. Egg output (E) is assumed to be directly proportional to current worm burden unless anti-fecundity responses are operating. The first memory response (with associated antibody) shown here (M1 and A1) receives its antigenic stimulus from the live worm population, and the second (M2 and A2) is stimulated by dying worms, but each response could be stimulated by any one of cercariae, live worms, dying worms or eggs (with the same antigen stimulating both memory cells and plasma cells for each response). Plasma cells are generated through activation of memory cells by antigen, at a rate governed by the parameter δj. Protection is mediated by the plasma cell population, as in the plasma cell-only models. Cross-regulation is shown using dashed lines - it is shown acting in one direction (reducing production of M2 and A2, with strength ρ2) for clarity, but may also act to reduce production of M1 and A1. Cross-regulation between the two responses affects both memory and plasma cell production. All parameters are defined in table 1, with parameter values given.