The site where immature lymphocytes differentiate into antigen-sensitive lymphocytes are :

primary lymphoid organs

The process of lymphocyte development and maturation takes place in specialized organs called primary lymphoid organs. These organs are responsible for producing and educating immature lymphocytes, shaping their antigen-specific receptors, and promoting their differentiation into functional antigen-sensitive lymphocytes.

One of the primary lymphoid organs involved in lymphocyte development is the bone marrow. Within the bone marrow, hematopoietic stem cells give rise to various types of blood cells, including lymphocytes. In the case of B-cells, they undergo maturation and differentiation within the bone marrow. During this process, they undergo gene rearrangement to generate a diverse repertoire of antigen receptors on their cell surface. B-cells that successfully generate functional antigen receptors are then released into the bloodstream and migrate to secondary lymphoid organs.

The thymus is another important primary lymphoid organ involved in lymphocyte development. It is responsible for the maturation and selection of T-cells. Immature T-cells, known as thymocytes, migrate from the bone marrow to the thymus.

Once lymphocytes have matured and become antigen-sensitive, they populate the secondary lymphoid organs, including lymph nodes, spleen, and tonsils. These secondary lymphoid organs serve as sites for the encounter of lymphocytes with antigens. When antigens enter the body through infection or vaccination, they are captured and presented to lymphocytes within the secondary lymphoid organs. This interaction triggers an immune response, leading to the proliferation and activation of antigen-specific lymphocytes.

In summary, primary lymphoid organs, such as the bone marrow and thymus, are involved in the differentiation and maturation of lymphocytes. They provide the foundation for the development of a diverse and functional immune system. Secondary lymphoid organs then serve as sites for immune responses, where mature lymphocytes can encounter antigens and mount an effective immune response.