Red bone marrow
= Site of lymphocyte origin
= Site of development of immunocompetence as B or T cells; primary lymphoid organs = Site of antigen challenge,
Circulation in blood
activation, and final diff erentiation of B and T cells
1 Lymphocytes destined to become T cells migrate to the thymus and develop immunocompetence there. B cells develop immunocompetence in red bone marrow.
2 After leaving the thymus or bone marrow as naïve immunocompetent cells, lymphocytes “seed” the lymph nodes, spleen, and other lymphoid tissues where the antigen challenge occurs.
3 Antigen-activated immunocompetent lymphocytes circulate continuously in the bloodstream and lymph and throughout the lymphoid organs of the body.
Activated Immunocompetent B and T cells recirculate in blood and lymph
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Immunocompetent, but still naive, lymphocyte migrates via blood
Lymph nodes, spleen, and other lymphoid tissues