The organ in the human body primarily associated with the immune system is the spleen. The spleen plays a crucial role in filtering blood and helping to produce immune cells such as lymphocytes. It is involved in the body's defenses by capturing and destroying pathogens and old or damaged blood cells. The spleen also helps regulate the number of red blood cells in the bloodstream. When the body is under threat from infections or diseases, the spleen becomes active in producing immune responses and mobilizing immune cells to combat these threats effectively.