Physical barriers are the first line of defense against infection. The skin and mucous membranes provide a continous surface which must be breached and back this up with mechanical protection through cilia and mucous. Physiological factors such as pH, temperature and oxygen tension limit microbial growth. The acid environment of the stomach combined with microbial competion from the commensal flora inhibits gut infection. Protein secretions into external body fluids such as lysozyme also help resist invasion.
Phagocytic cells are critical in the defense against bacterial and simple eukaryotic pathogens.
The acute inflammatory response which has been described in previous lectures is a key part of the innate immune system. Many infections, especially where small wounds are the route of entry, are eliminated by the combination of complement and recruitment of phagocytes, which flow from the acute inflammatory response. A defining aspect of the innate immune system is that it carries no memory of an encounter with a foreign organism. This can be an enormous range of substances from simple chemicals, sugars, small peptides to complex protein complexes such as viruses. The small antigens are not, however immunogenic in themselves and need to be coupled to a carrier to elicit an immune response.