title: [Development of specific immunity in prenatal life].
abstract: The various defense mechanisms of specific immunity, which involves the T and B lymphocytes and the antigen presenting cells, are gradually developed during intra-uterine life. The first hematopoietic organ is the yolk sac which appears at the 4th week of development. Thereafter, the hematopoiesis takes place in the fetal liver (from the 6th week) followed by the bone-marrow during the 3rd trimester. The differentiation of the T lymphocytes begins around the 10th week. The thymic epithelial rudiments appear during the 7th week and the thymus migrates to its definitive place at ten weeks. It is then colonized by the T cell precursors, which there undergo their maturation process. From the 12th week of development, mature T cells are readily detectable in lymphoid organs and fetal blood. The maturation of B cells, which occurs firstly in fetal lever, and thereafter in bone marrow begins also early in fetal life (12th week). The antigen presenting cells, the precursors of which are detected in the yolk sac as soon as 4-6 weeks, are normally present and functional in secondary lymphoid organs as soon as 12 weeks. Thus, the specific immune response appears possible by the end of the 1st trimester. However, the naive nature of T and B lymphocytes is responsible for a delayed, slow and relatively ineffective primary response. This observation explains the particular susceptibility of neonates, especially premature neonates to bacterial and viral infections. The various antigenic stimulations and T/B cell cooperations allow a complete maturation of the immune system during the first years of life.
authors.affiliation: Inserm U429, hôpital Necker-Enfants-Malades, 149, rue de sèvres, 75015 Paris, France. durandy@necker.fr
authors.forename: A
authors.initials: A
authors.lastname: Durandy
mesh_terms: D001402:B-Lymphocytes / Q000276:immunology; D016207:Cytokines / Q000096:biosynthesis; D005333:Fetus / Q000276:immunology*; D006801:Humans; D013601:T-Lymphocytes / Q000276:immunology
chemical_list: D016207:Cytokines