Figure 7. A model of immune response evolution.

Panels A and B show fitness as a function of immune response sensitivity ${\psi }_{F,I}$ under two parasite encounter rates, $a_I=0.5$ (blue optimum, ${\theta }_{\psi }$) and $a_I=0.9$ (red optimum, ${\theta }_{\psi }$), where costs of fibrosis exceed costs of infection (A) and where costs of fibrosis equal costs of infection (B). Panel C illustrates the optimum as a function of costs of fibrosis and costs of infection, holding $\gamma$ and $a_I$ constant; values where ${\beta }_F<{\beta }_I$ all exceed unity. When ${\beta }_F>{\beta }_I$, the optimum is determined by the mitigating effects ($\gamma$) of fibrosis on fitness costs of parasite infection, as well as parasite encounter rate $a_I$ (Panel D). When ${\beta }_F\le {\beta }_I$, these mitigating effects and encounter rates play little role in the position of the optimum (Panel E).