The immune system comprises you body's defense against invasion. There are three lines of defense, each line of defense becoming more specific for a particular invader. Secretions from exocrine glands on epithelial tissue. They prevent the overgrowth of bacteria. These cells originate in bone marrow from stem cells.
Phagocytes engulf foreign cells and other substances by endocytosis.
Some circulate in the blood and can squeeze through the walls of capillaries to get to the invaders. Some live in lymph nodes and spleen. 1 in 100 cells in your body is a white blood cell. The complement system has a set of plasma proteins that enhance nonspecific and specific immune responses. They are secreted in an inactive form and activated where they are needed.