The innate immunity provides the initial stage in the defense against infection in the body. The physiologic barriers that contribute to innate immunity include temperature, pH, and various soluble and cell associated molecules that mediate against infection in the body.
Let us consider a simple fact that it has been observed that many species are not vulnerable to certain diseases simply because their normal body temperature restrains the growth of pathogens. Chickens, for example, have innate immunity to anthrax because their high body temperature inhibits the growth of the bacteria. Similarly gastric acidity is an innate physiologic obstacle to infection because very few ingested micro-organisms can stay alive in the low pH of the stomach contents. One reason newborns are susceptible to some diseases that do not bother adults is that their stomach contents are less acidic than those of adults.
A variety of soluble factors add to innate immunity; amongst them are the soluble proteins lysozyme, interferon, and their complements.
• Lysozyme: It is a hydrolytic enzyme that is found in mucous secretions and in tears. It is able to disintegrate the peptidoglycan layer of the bacterial cell wall.
• Interferons: These comprise a group of proteins that are produced by virus-infected cells. Among the many functions of the interferons is the ability to attach to nearby cells and encourage a generalized antiviral state.
• Complements: These are a group of serum proteins that circulate in an inactive state. A variety of precise and imprecise immunologic mechanisms can alter the inactive forms of complement proteins into an active state with the ability to damage the membranes of pathogenic organisms, either annihilating the pathogens or facilitating their clearance. Complements may also function as an effector system that is triggered by binding of antibodies to certain cell surfaces, or it may be activated by reactions between complement molecules and certain components of microbial cell walls.
• Collectins: Recent studies on collectins show that these surfactant proteins may kill certain bacteria directly by disrupting their lipid membranes or, alternatively, by stimulating the bacteria to enhance their susceptibility to phagocytes.
Many of the molecules concerned in innate immunity have the abilities of pattern recognition—the aptitude to distinguish a given class of molecules. Because there are certain types of molecules that are unique to microbes and never found in multicellular organisms, the ability to immediately distinguish and battle invaders displaying such molecules is the feature of innate immunity. Molecules with pattern recognition aptitudes may be soluble, like lysozyme and the complement components, or they may be cell-associated receptors.
Among the class of receptors designated the toll-like receptors (TLRs), TLR2 distinguishes the lipopolysaccharide (LPS) found on Gram-negative bacteria. It has long been documented that systemic exposure of mammals to relatively small quantities of purified LPS leads to an acute inflammatory response. The system for this reaction is via a TLR on macrophages that identifies the LPS and draws a variety of molecules in an inflammatory response upon exposure. When a TLR is exposed to the LPS upon a local invasion by a Gram-negative bacterium, the contained response by the TLR results in the elimination of the bacterial challenge.