Important initial barriers to infection are physical (e.g. the skin), enhanced by substances secreted by the body, such as saliva and tears, that contain molecules that can neutralise bacteria. The internal mucosal tissues (e.g. lungs & airways, and the gut) are coated with mucus that is able to trap potential infectants. In the airways, mobile ciliate hairs work together to transport contaminants away from vulnerable areas. Tissues such as the skin, mucosal surfaces and airways also contain populations of immune cells that can respond to infectants that breach these physical defences.

In its most complex forms, the immune system consists of two branches: the innate immune system that utilises certain ‘hard-wired’ strategies to provide a rapid, general, response when alerted by certain typical signals of infection (essentially forming a first-line of defence); and the adaptive immune system that is able to develop highly specific responses (and a persistent ‘immune memory’) to target infection with extraordinary accuracy. Both systems work in close cooperation and, to an important extent, the adaptive immune system relies upon the innate immune system to alert it to potential targets, and shape its response to them.