Mechanisms of innate immunity
Innate immunity of the host performs two most impor-tant functions: it kills invading microbes and it activates acquired (adaptive) immune processes. Innate immunity unlike adaptive immunity, however, does not have any memory and does not improve after re-exposure to the same microorganism. The innate immunity is primarily dependent on four types of defensive barriers: (a) anatomic barriers, (b) physiologic barriers, (c) phagocytosis, and (d) inflammatory responses.
Mucous membranes form a large part of outer cover-ing of gastrointestinal, respiratory, genitourinary, and many other tracts of human host. A number of nonspecific defense mechanisms act to prevent entry of microorganisms through mucous membrane.
· Saliva, tears, and mucous secretions tend to wash away potential invading microorganisms, thereby preventing their attachment to the initial site of infections. These secre-tions also contain antibacterial or antiviral substances that kill these pathogens.
· Mucus is a viscous fluid secreted by the epithelial cells of mucous membranes that entraps invading microorganisms.
· In lower respiratory tract, mucous membrane is covered by cilia, the hair-like protrusions of the epithelial cell mem-branes. The synchronous movement of cilia propels mucus-entrapped microorganisms from these tracts.
· In addition, nonpathogenic organisms tend to colonize the epithelial cells of mucosal surfaces. These normal flora generally compete with pathogens for attachment sites on the epithelial cell surface and for necessary nutrients.
· Gastric acidity is an innate physiologic barrier to infection because very few ingested microorganisms can survive the low pH of stomach contents.
· Lysozyme, interferon, and complement are some of the soluble mediators of innate immunity. Lysozyme has anti-bacterial effect due to its action on the bacterial cell wall. Interferons are secreted by cells in response to products of viral infected cells. These substances have a general antiviral effect by preventing the synthesis of viral structural proteins. Complement is a group of serum-soluble substances that when activated damage the cell membrane.
· There are certain types of molecules that are unique to microbes and are never found in multicellular organisms. The ability of the host to immediately recognize and com-bat invaders displaying such molecules is a strong feature of innate immunity.