Which of the following organs is not involved in the generation of immune response?

The origin, maturation, and proliferation of lymphocytes take place in lymphoid organs.
Immature lymphocytes differentiate into antigen-sensitive lymphocytes in the bone marrow and thymus, which are the primary lymphoid organs.
The lymphocytes migrate to secondary lymphoid organs such as the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, and the appendix after maturation. The secondary lymphoid organs serve as locations for lymphocytes to engage with antigens before proliferating to become effector cells.

Hence, the spleen, thymus, and lymph nodes all play roles in the generation of immune response.

The brain does not participate in the generation of immunological responses, but it does regulate the overall function of the body.