The question asks which organ is not involved in eliciting an immune response in humans. Let`s go through each option to determine the correct answer.

Option 1: Thymus. The thymus is a gland located in the upper chest region and plays a crucial role in the development and maturation of T-cells, which are important for immune responses. Therefore, the thymus is involved in the immune response.

Option 2: Brain. The brain is not directly involved in the immune response. Its primary function is to control and coordinate various bodily functions, including the immune system. While the brain does indirectly influence the immune response through the release of certain hormones and neurotransmitters, it is not directly involved in the process.

Option 3: Spleen. The spleen is an organ located in the abdominal cavity and serves as a storage site for red and white blood cells. It also filters the blood, removing old or damaged cells, and plays a role in immune responses by producing antibodies. Therefore, the spleen is involved in the immune response.

Option 4: Lymph nodes. Lymph nodes are small, bean-shaped structures located throughout the body and are an essential part of the immune system. They filter lymph fluid, containing immune cells that help