Central lymphoid organs Peripheral lymphoid organs Recirculation B lymphocytes Bone marrow stem cell B lymphocyte Bursa of Fabricius(birds) Blood Lymph nodes lineage Bone marrow(mammals) Spleen Mucosal and T lymphocyte Thymus Blood cutaneous neage lymph lymphoid tissues T lymphocytes Recirculation Figure 2-2 Maturation of lymphocytes Development of mature lymphocytes before antigen exposure occurs in the generative lymphoid organs, and immune responses to foraign antigens occur in the peripheral lymphoid tissues