 However, the genes that function in the reaction of the immune system to antigens have two peculiarities. First, they incorporate mechanisms designed to produce very high frequencies of mutation; second, none of the genes involved in the functioning of the immune system are germ-line. For instance, the genes that give rise to B-cells are designed to mutate extensively until one of these cells identifies an invader as foreign. It then proliferates extensively as it attacks the invader. As an organism matures, it accumulates more and more of the B-cells that have been successful in its past. More than this, as the process of proliferation continues, the strength of the affinities to binding sites increases. Initially, the primary antibodies almost always exhibit a weak affinity for their targets; however, as the reaction to the antigen continues, these affinities become stronger.