Here is the detail of costimulation and how T cells help B cells. Undisclosed is how the Th anti-self is avoided. The story of how all this works can be found in a lengthy debate that was published on the web and which I have pilfered for your reading pleasure. You can find the links to the debate on the main page. The immune system is a defense against infectious diseases .
Pathogens can evolve far faster than their hosts.
An immune system that is allowed to undergo somatic evolution can evolve at a rate comparable to that of the pathogens. Somatic evolution comes as two parts. First , as the famous F1 test shows, there is somatic selection of the specificity elements that results in the elimination of any anti-S that is present. Each VLVH can be mutated in a way that chances specificity without inactivating the molecule. This means that the functional B cell repertoire has to be repeated as larger animals evolved.