The correct option is B Brain
Lymph nodes are oval-shaped lymphoid organ and stores B, T and other immune cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells.
The spleen is a lymphoid organ acts as a blood filter. The spleen serves a valuable role in immune function because it purifies the blood and helps the immune system with recognizing and attack foreign bodies.
The thymus is a specialized organ of the immune system, Within the thymus, T-cells mature and these T-cells are critical to the adaptive immune system.
The brain is not involved in the elicitation of the immune response.
Thus, the correct answer is option A.