CD4+ T cells are helper T lymphocytes. They interact with class-II MHC molecules. T cells develop in the marrow from the myeloblastic stem cell. Thymic maturation of T cells is essential to their development and normal function. Athymic mice and humans have virtually not cellular immunity.
An important function of thymic maturation is that while there are many different T-cells that are capable of reacting to a whole variety of foreign peptides, all those that react with self are eliminated. This is achieved by positive and negative selection. Thymocytes are presented in the site of maturation with self-MHC antigens. If the T cell receptor on a T-cell fails to bind at all or if it binds too tightly the thymocyte dies by apoptosis. Failing to bind at all to a T cell receptor by the self-MHC expressed on the surface of the thymocyte means that the cell would be non-functional and of no use.