Correct Response: D. White blood cells are the cells of the immune system whose primary responsibility is to protect the body from disease. They move through the body in the vessels of the circulatory and lymphatic systems. Neurotransmitters (A) are chemicals released by nerve cells that propagate signals between nerve cells or from a nerve cell to a muscle or organ. Although hormones of the reproductive and endocrine systems (B) and nutrients released by the digestive system (C) also travel throughout the body by means of the circulatory system, these molecules are not part of the immune system.
