In which organ does the differentiation of T cells primarily occur?

The thymus is the primary organ where T cell differentiation occurs. It plays a crucial role in the development of T cells from precursor cells derived from the bone marrow. In the thymus, these precursor cells undergo several stages of maturation, where they proliferate and differentiate into different subsets of T cells, including helper T cells and cytotoxic T cells.

During this process, T cells also undergo positive and negative selection. Positive selection ensures that only T cells capable of recognizing self-MHC molecules are allowed to survive, while negative selection eliminates T cells that strongly react to self-antigens, reducing the risk of autoimmunity.

The other options, while associated with aspects of the immune response, do not primarily function in the differentiation of T cells. The bone marrow is primarily responsible for the formation of blood cells, including lymphocytes, but T cell maturation specifically occurs in the thymus. Lymph nodes are critical for the activation and proliferation of T cells after they have matured, and the spleen serves to filter blood and facilitate immune responses but does not play a role in T cell differentiation.