ABSTRACT
The immune system is an integrated host defence system, involving numerous cells and molecules, that defends the body against invading organisms, foreign antigens, and abnormal host cells (e.g. tumour cells). It is also actively involved in the rejection of transplanted organs and tissues and in a variety of autoimmune diseases. Immunity towards pathogens is crucial for the survival of the species. Over the million of years of coexistence with external threats from pathogens, animals have developed sophisticated defence systems (immunity) against these pathogens. In this chapter, a short description of the action of the immune system in humans is presented.
