11. Innate host defences to infectious disease

Questions

What are the innate (non-specific) defences?

What is the role of phagocytes in infection?

Describe the sources and roles of cytokines.

The skin and mucous membranes are physical barriers that form the first defence against infection (Fig. 3.11.1). They have high cell turnover, with superficial cells, which may have become colonized by pathogens, constantly being shed. Most sites are further protected by secretions of mucus, which trap microorganisms and prevent them sticking directly to epithelial cells, and substances such as lysozyme (a powerful degradative enzyme) and lactoferrin (an iron-binding protein that makes the essential acquisition of iron by bacteria difficult). Environments may be made more hostile still by an acid pH, such as in the stomach, vagina or urine, or by alkaline pH, such as in the duodenum. It is also difficult for microorganisms to rest on epithelial surfaces because of the peristalsis of gut contents, periodic flushing of the urethra with urine and the muco-ciliary escalator of the respiratory tract. The normal bacterial flora at many of these sites can also be protective against incoming pathogens because of several factors:

scavenging of all available nutrients

maintenance of hostile conditions, e.g. Lactobacillus spp. metabolism contributes to the acid pH of the vagina

natural production of ‘antibiotics’ (bacteriocins)

direct cell-to-cell contact inhibition

ensuring ‘fitness’ of specific immunity by subclinical stimulation at mucosal sites.