Human evolution known to be a process in which species adapt to certain conditions of life, and in the battle of life and death, they are forced to become stronger in order to survive in this world. Diseases have always been present during the evolution, acting themselves as a natural selectors - the weaker organisms get sick and die. Not just humans, but all species have to establish a great protecting mechanism in order to survive. That is how our bodies are built, to be able to withstand and survive in the conditions we live in, and that is why our immune system has to be taken care of.
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We can expect to find dividing and maturing B cells, aside from the lymph nodes, is in the spleen. The spleen is one of the major lymphoid tissues (i.e. lymph nodes, thymus, mucosa associated lymphoid tissue, etc) where lymphoid cells proliferate. The major cell from the lymphoid lineage that proliferates in the spleen are the B cells.
I believe the answer is genetic variability
