Lymphocytes form the specific immune system, capable of recognizing and responding to any foreign antigen, while remaining indifferent to self components. Throughout human life, lymphocytes are continuously generated from pluripotent hematopoietic stem cells. These hematopoietic stem cells are already detectable in the yolk sac and in the fetal liver from the second month of gestation onwards. After birth, the hematopoietic stem cells are mainly found in the bone marrow (BM). Two types of lymphocytes exist: B-lymphocytes and T-lymphocytes. Progenitor Bcells differentiate into mature B-lymphocytes in the BM, while progenitor T-cells differentiate into mature T-lymphocytes in the thymus. Mature B- and T-lymphocytes recognize foreign antigens via smface receptor molecules, the so-called antigen specific receptors. The antigen specific receptors of B-and T-lymphocytes are called B-cell receptor (BCR) or immunoglobulin (Ig) molecules and T-cell receptor (TCR) molecules, respectively. Differentiation of progenitor B-cells into mature B-lymphocytes is regulated via interaction with stromal cells in the BM. In the earliest stages, progenitor B-cells must be in direct contact with the stromal cells, and fmiher differentiation is dependent on growth factors, such as cytokines and hematopoietic growth factors, which are secreted by the stroma. These growth factors and cytokines induce proliferation, differentiation, and maturation in an organized way.

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J.J.M. van Dongen (Jacques) , R. Benner (Robbert)
Erasmus University Rotterdam
Erasmus MC: University Medical Center Rotterdam

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