The thymus is thought to have a role in the preparation of lymphocytes - specifically T-lymphocytes - for immune recognition in cell-mediated immunity. Lymphocytes from the fetal liver, and later from the bone marrow, migrate into the thymus where they are made immunocompetent, that is, they are instructed as to the sorts of antigens they should respond to. The local hormone thymosin that is produced by the epithelial cells of the thymus seems to have a role in this instruction.
The lymphocytes pass out into the blood to secondary lymph tissue e.g. the spleen and lymph nodes.
The role of the thymus is exemplified by its absence or deficiency - those afflicted suffer from defects in cell-mediated immunity.
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