Thought to be antigen-free, the thymus is a primary lymphoid organ with two major functions: differentiation of T cells; and recognition of self from non-self. The thymus differs from lymph nodes and bone marrow in that no immune reactions occur, except in certain diseases. Pre-thymocytes arrive into the thymus via blood, move into subcapsular space and begin differentiating immediately. Differentiation markers expressed are CD3, TcR and CD4, CD8. TcR gene fragments are rearranged, as in B cells, and then the T cells begin migrating toward the medulla.