The letter T in T-lymphocyte refers to:
Thalamus
Tonsil
Thymus
Thyroid
The letter T in T cells refers to the thymus. T cells or T cells are a type of lymphocytes that play a central role in cell-mediated immunity. They are distinguished from other lymphocytes such as B lymphocytes and natural killer cells by the presence of T lymphocyte receptors on the cell surface. They are called T cells because they mature in the thymus. Hence, option 3 is correct.