Figure 3.

The order of effector function engagement, from lowest cost to highest cost. Defense mechanisms of the immune system have different costs to host fitness. The order of engagement of effector functions corresponds to their increasing costs (here, their potential to cause immunopathology). The low-cost effector mechanisms, such as antimicrobial peptides (AMPs), secretory IgA and circulatory IgM, are expressed constitutively. When they are insufficient to contain pathogens, the response of next lowest cost is induced (here, tissue-resident macrophages); when this response is insufficient, neutrophils are recruited, and so on. The examples of effector responses presented here and their costs are for illustration purposes only; they are neither comprehensive nor quantitative.