ABSTRACT
The most striking feature of the immune system in vertebrates is its ability to recognize a large diversity of pathogens by T and B lymphocytes that express on their surface specifi c receptors, the T cell receptor (TCR) and the B cell receptor (BCR), respectively. These cells derive from hematopoietic stem cells through discrete differentiation pathways that take place in the liver during embryonic and fetal life and in the bone marrow (BM) after birth. T cells mature in the thymus whereas B cells continue to differentiate in the BM. Lympho-stromal interactions in multiple microenvironments within the thymus and the BM have a crucial role in the regulation of lymphocyte development and selection. The genes encoding TCR and BCR antigen-binding domains are assembled during T and B cell differentiation by a series of gene rearrangements. Mechanisms of allelic exclusion prevent expression of multiple functional receptors on a given T or B cell.
