Which group of major parts and organs make up the immune system?

The immune system is primarily composed of the lymphatic system and several key organs that play vital roles in the body's defense against pathogens. The lymphatic system includes lymph nodes and vessels that transport lymph, a fluid containing infection-fighting white blood cells. The spleen filters blood and helps to remove old or damaged blood cells while being a site for immune cell activation. Tonsils and the thymus are also essential for producing and maturing immune cells, particularly T cells. Bone marrow is crucial because it is the primary site of new blood cell production, including red blood cells, white blood cells, and platelets, which are vital for immune responses.

The other groups mentioned do not encompass the major components specifically associated with the immune system. For instance, the respiratory system is focused on gas exchange, the nervous system is involved in transmitting signals throughout the body, and the digestive system is responsible for breaking down food and nutrient absorption. These systems have functions entirely separate from the immune responses essential for defending against infections and diseases. This distinction confirms why the group that includes the lymphatic system, spleen, tonsils, thymus, and bone marrow is the correct answer in relation to the immune system.