What is the function of organ labelled A?

Explanation

Site of T-lymphocyte differentiation.
The thymus is a lymphoid organ located in the mediastinal space. Thymus is the site of T-lymphocyte differentiation. The mature T-cells leave the thymus and migrate to secondary lymphoid organs like the spleen, lymph nodes, and other lymphoid tissues where they control cell-mediated immune responses. Thymus provides micro-environment for development and maturation of T-lymphocytes.