The process of multipotential lymphatic stem cell maturation and differentiation into immunocompetent T cells is accomplished by the expression and deletion of specific surface CD antigens. The CFU-Lstem cells enter the medulla of the thymus via a post capillary venule and then migrate to the periphery of the thymic lobule. The presence of CD2 and CD7 molecules on the cell surface indicates an early stage of differentiation. This is followed by expression of the CD1 molecule, indicating the midstage of Tcell differentiation. As maturation progresses, the cells express TCRs, CD3, CD4, and CD8 molecules. It seems that intensity of interreacion between TCR/co receptor molecule complex and self peptide/MHC complex determine the outcome of the thymocite selection process. If the lymphocyte recognizes self MHC and self or foreign antigen, it will survive the selection (positive selection); if not, death of the cell will occur. Cells that pass the positive selection test leave the cortex and enter the medulla. Here they undergo another selection process in which cells directed to selfantigen displayed by self MHC are eliminated (negative selection). Cells that survive that selection then become either cytotoxic CD8+ Tlymphocytes or helper CD4+ Tlymphocytes.

Dendritic cells (DCs) are antigen presenting cells which trace origin from bone marrow cells. These cells are discovered and described by Steinman i Cohn (1973) in peripheral lymphoid organs of mice. The cellular membrane of DCs are place of expression of plenty immunoregulatory molecules such as MHC class I and II, co stimulatory, and adhesion molecules, as well as many receptors for different cytokines. Armed on this way, DCs are one of the most effective antigen presenting and immunoregulatory cells. Moreover, role of DCs in development of immune reaction can be crucial due to they are one of the most important cellular “link” between native and adaptive immunity. DCs are engaged in mechanisms of antigen processing and presentation, induction of immune reaction, establishing immune tolerance and immune reaction regulation, balancing the immune reaction between autodestruction and protection of “self” cells. These functions makes that DCs play very important role in development of some pathological conditions and diseases such as autoimmunity, allergies and quality of anti-tumor and anti-microbe defense. Unbalanced immune reaction is hallmark of all cited diseases and immunopatological conditions, so that the function of DCs should be explored on the better way.