The biological significance of immunity

The classical definition of immunity as the resistance of the body to disease, views the immune system in simple mechanistic terms. In this brief overview, possible reasons and consequences of the presence of an effective immune system across a range of invertebrate and vertebrate animals are considered. Topics discussed include the forces favouring the development of immunity, such as the acquisition of the colonial habit and terrestrial mode of life by many primitive animals, the constant threats of microbial and macrobial invasion, and the need to eliminate somatic mutations. The consequences of immunity in terms of the development of autoimmunity and hypersensitivity reactions, as well as the interaction of the immune system with the brain, the neuroendocrine organs and environmental factors, are also examined. Finally, the possibilities that the delicate balance maintained between many parasites and the immune systems of their hosts results in the spread of disease, and may also determine the maintenance of sexual reproduction and the choice of mate, are discussed.