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Fetal development of the immune system. The time-dependent ontogeny of the human (A) and mouse (B) immune systems is summarized for innate immune cells (dark blue), adaptive T (green) and B cells (red) and colonization of the lymph nodes and bone marrow (light blue). HSC, hematopoietic stem cells; NK, natural killer cells; TcR, T-cell receptor; LN, lymph nodes; BcR, B-cell receptor; AGM, aorta-gonad-mesonephros; GW, gestational weeks in the human; E, embryonic days in mice.
