7. Defence

Abstract

This chapter describes how the human body protects its internal conditions against micro-organisms and the environment. The body’s first line of defence is the mechanical barrier provided by the skin, a part of the integumentary system. When the integument is breached, the body’s immediate priority is to seal the hole by coagulation of the leaking blood. The next line of defence is chemical: the secretions that cover the surfaces of eyes and the inside of the nose contain a variety of proteins that attack bacteria. Within the blood and fluids that bathe internal tissues are proteins of the complement system. The chapter then considers the innate and the adaptive immune systems.