The importance of the primary lymphoid organs is demonstrated by its involvement in autoimmune disease. Two autoimmune diseases, DiGeorge syndrome and Nezelof disease, result in the failure of the thymus to develop and in the subsequent reduction in T cell numbers, and removal of the bursa from chickens results in a decrease in B cell counts. The destruction of bone marrow also has devastating effects on the immune system, not only because of its role as the site of B cell development but also because it is the source of the stem cells that are the precursors for lymphocyte differentiation.