Two distinct populations of lymphocytes have been identified: T lymphocytes, which are thymus-dependent, and B cells, first observed in the Bursa Fabricus of birds. Mammals do not have an equivalent structure, and there are varying opinions as to the similarity of these cells between species. In humans, current theories are that B lymphocytes differentiate in the fetal liver and in the bone marrow of adults. Human T and B cells are most easily obtained either from peripheral blood or from biopsy of lymphoid tissues (lymph nodes, spleen, Peyer’s patches from gut, tonsils, and adenoids).