Figure 3.

Hypothesized scenario for the effect of a bacterial challenge on the immune space available to the trypanosome Crithidia bombi. The squares represent the immune space for (a,c) bacteria and (b,d) trypanosomes. Each dot (for bacteria) or letter (A–J for trypanosome strains) indicates the position a given strain of the parasite would assume in immune space. The grey areas represent the realized immune space where the parasite can infect whereas the remainder (white area) is the space that can be cleared by the host. A bacterial challenge (c,d) changes the immune space when compared with the naive host (a,b; dashed lines in panels c,d) such that the clearable space increases for bacterial infections (i.e. reducing the size of the grey area) but decreases for the trypanosomes (i.e. increasing the grey area) owing to the existence of trade-off within the immune system. The structure of the immune space varies among host individuals.