Dispersal, vaccination and regression of immune defence organs

The thymus in vertebrates and the bursa of Fabricius in birds regress befor e reproduction, while the immunological information of these organs is main tained as cell memory. Regression at a certain age presupposes that individ uals have achieved exposure to a large fraction of parasites in the environ ment. Here we present a new scenario for regression of immune defence organ s, based on optimality reasoning. This scenario links early involution of i mmune defence organs with (1) effects of exposure to parasites on adaptive immune responses to these parasites, (2) exposure to local parasite communi ties during natal dispersal and migration as a means of "vaccination" again st local parasites, and (3) the function of visits to future breeding sites by juveniles as a means of exposure to local parasites. This scenario prov ides explanations for why natal dispersal is longer than breeding dispersal , for sex differences in dispersal, and for why the bursa of Fabricius regr esses relatively early in life among bird species with delayed start of rep roduction.