Development of The Immune System In The Foetal and Perinatal Period

During embryogenesis, the first haematopoietic cells develop outside the embryo, in the yolk sac. Then, in the 6th week of pregnancy, committed haematopoietic stem cells develop in the mesoderm of the fetus, the so-called aortagonad-mesonephros (AGM) region. Whether these haematopoietic stem cells are generated from endothelial cells within the aortic floor or originate from mesodermal cells (either within or below the aortic floor) remains a matter of debate [1]. Subsequently, haematopoietic stem cells migrate to the fetal liver and there initiate the erythropoiesis [2] In week 7 cells seed the developing thymus. Seeding into the bone marrow occurs much later (by week 20) [3], [4]. In the thymus, T lymphocytes develop that express either the αβ T cell receptor or the ρδ receptor. The processes involved are rearrangement of the T cell receptors, positive selection on MHC followed by negative selection for self-antigens. Note that the development of the T cell repertoire is antigen independent. Development of Natural Killer (NK) cells as well as various dendritic cells (DC) also takes place in the thymus. In bone marrow, B lymphocytes, granulocytes, monocytes and DC develop. The development of lymphoid cells and organs is a complex process that requires timely expression of growth factors (cytokines, chemokines), receptors as well as adhesion molecules. As already stated above, development of the immune system is, apart from maternal-fetal transfer, independent of antigen (either bacterial, viral or allergenic).