title: [Development of the immune system].
abstract: 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.
authors.affiliation: Laboratoire d'Immunologie du diabète, Faculté de Médecine, Nantes.
authors.forename: E
authors.initials: E
authors.lastname: Houssaint
authors.forename: P
authors.initials: P
authors.lastname: Sai
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology; D002060:Bursa of Fabricius / Q000196:embryology / Q000276:immunology*; D002454:Cell Differentiation; D006801:Humans; D007107:Immune System / Q000502:physiology*; D007109:Immunity; D008221:Lymphoid Tissue / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology; D013950:Thymus Gland / Q000196:embryology / Q000276:immunology*
vernacular_title: Le développement du système immunitaire.