The cellular response to infection – production of new factors.

When cells of the innate immune system encounter pathogenic products they can respond in ways other than phagocytosis. Cells can be stimulated to synthesise and/or secrete an enormous variety of new products. Some of these products may be directly involved in killing pathogens. Other products are involved indirectly in recruiting other cell types to try and eliminate the pathogen. An important group of proteins that can be secreted in response to pathogenic stimuli are known as cytokines.

Recognition by phagocytes. Phagocytes must distinguish microbes and dead host cells from healthy host cells so that healthy host cells are not phagocytosed. Phagocytes have receptors on their surface that recognise sugars present on microbes or sugars that are newly expressed on dead or damaged host cells. These sugars are not present on healthy host cells and therefore the host cells are not phagocytosed.