Optimal replacement type. A Relative fitness of replacement type τ1 = age-dependent against τ2 = random for the three different pathogen environments (standard (blue), positively correlated (red), random (green)) versus the fraction of memory cells replaced (nrep/nmp) with nmp = 50. The solid line denotes the average value for w over 15 simulations. The shaded area corresponds to the estimated pointwise 95%-confidence intervals. B Average frequency of individuals with replacement type τ1 in the total population at t = 20000.