The immune system

This chapter focuses on the immune system of animals. This immune system protects against pathogenic invaders, wherein there is an innate immune response in which phagocytes engulf invading pathogens. The chapter explores the adaptive immune response. This responds to macromolecules, mainly proteins and complex polysaccharides, but not to small foreign molecules unless attached to large ones. Foreign macromolecules are indicative of an invader and are called antigens because antibodies are generated in response to them. The chapter includes two arms of the adaptive immune response: the humoral in which antibodies prevent pathogens in the blood and tissue fluids from entering cells and aid their removal from the body, and cell-mediated immunity in which cytotoxic T cells destroy infected cells. Cells of the immune system, known as lymphocytes, recirculate through the blood and lymphoid tissues to the sites of potential infection.