Differentiation in the direction of the T-lymphocyte starts with the pre-T-cell of the bone marrow. By entering the thymus cell, under the influence of the microenvironment of thymus and its hormones, proliferates maturing from the stage of an early thymocyte, through the thymocyte to the stage of a matured thymocyte. This process occurs predominantly in the thymus cortex. Thymocytes enter the circulation as T-lymphocytes.

In peripheral lymphoid organs, T-lymphocytes can be divided in several subgroups:

• helper-inducer T-lymphocytes (Th) - cells inducing immunological reaction,  
• suppressor T-lymphocytes (Ts) - cells suppressing immunological reaction,
• cytotoxic T-lymphocyte (Tc) - cells killing other cells in the foreign antigens,
• delayed hypersensitivity effector cell (Tdh),
• memory cell (Tm).

T-lymphocytes are the most significant mediators of the cellular immunity. They compose a major part of lymphocytes in the bloodstream (69-82%).
T-lymphocytes play an important part in directing and activating the stem cells differentiation. They can activate stem cells and accelerate the production functionally matured blood cells.

Thymocytes
Thymocytes are lymphocytes, 10-16 µm in size, located in the cortical part of the thymus lobes. Their active immunity is rather weak, so few of them are capable of migrating from blood to lymph. The cytoplasm is distinctly basophilic with a high nucleocytoplasmatic ratio. It does not contain granules. Nucleoli are not visible.