LYMPHOID TISSUES
Lymphoid tissue is the site, where lymphocytes are formed, and where they develop. Two types of lymphoid tissues are distinguished, central and peripherical. Central lymphoid tissues include thymus (thymus gland) and bone marrow. Peripherical lymphoid tissues include adenoids, tonsils, lymph glands, spleen, Peyer's follicles in small intestine, and appendix.
Lymphocytes are developed in central tissues; birds also have bursa of Fabricius, and mammals have blood-forming tissue. Lymphocytes are also partially formed there. That is why one can add these tissues to central lymphoid tissues too.
Lymphocytes migrate with the blood current from central lymphoid tissues to peripherical ones, where they become T-lymphocytes and B-lymphocytes, and where they react with antigens.