[Development of the immune system].

Immune protection in vertebrates is provided by a dual system: the cellular immune response, mediated by T lymphocytes and the humoral immune response, mediated by B lymphocytes. These lymphocytes develop immunocompetence in the primary lymphoid organs: thymus, for the T lymphocytes and bursa of Fabricius (in birds), on its equivalent (in mammals), for B lymphocytes. The crucial period during which thymus and bursa influence the immunological development is the embryonic and early postnatal life. Hemopoietic stem cells home to these primary lymphoid organs during well-defined periods of colonization. Under the influence of thymic and bursal microenvironments, they become oriented respectively toward the T or B cell differentiation pathway. They acquire various membrane antigens and an antigen receptor. T lymphocytes also learn to recognize self-antigens (antigens encoded by the Major Histocompatibility Complex). T and B lymphocytes then colonize respectively T-dependent and B-dependent areas of secondary lymphoid organs where they become functional. Some immune deficiencies result from a defect in development which can affect selectively T or B lymphocytes or both systems in case of severe combined immunodeficiency disorders.