Frank, S. A. 2000. Specific and non-specific defense against parasitic attack. Journal of Theoretical Biology 202:283-304.

Specific defense protects against some parasite genotypes but not others, whereas nonspecific defense is effective against all genotypes of a parasite. Some empirical studies observe hosts with variability only in nonspecific defense, other studies and only specific defense. I analyze a model with combined specific and nonspecific defense to determine the conditions that favor detectable variation in each form of defense. High variation in nonspecific defense is often maintained when resistance increases in an accelerating way with investment, whereas low variation tends to occur when resistance increases at a decelerating rate with investment. Variation in specific defense rises as the parasite pays a higher cost to attack a broad host range (high cost of virulence), as the number of alternative specificities declines, and as the average level of nonspecific defense increases. The last condition occurs because greater nonspecific protection tends to stabilize the gene frequency dynamics of specific defense. Selection favors a negative association between costly components of specific and nonspecific defense: hosts defended by one component are favored if they have reduced allocation to other costly components. A negative association confounds the measurement of costs of resistance. Individuals with specific defense may have reduced investment in costly nonspecific defense. This leads to an apparent advantage of specifically defended hosts in the absence of parasites and a measured cost of resistance that is negative.