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Chapter: Microbiology and Immunology: Immunity

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.

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.

Anatomic barriersAnatomic barriers include skin andmucous membrane. They are the most important components of innate immunity. They act as mechanical barriers and prevent entry of microorganisms into the body. The intact skin prevents entry of microorganisms. For example, breaks in the skin due to scratches, wounds, or abrasion cause infection. Bites of insects harboring pathogenic organisms (e.g., mosquitoes, mites, ticks, fleas, and sandflies), introduce the pathogens into the body and transmit the infection. Skin secretes sebum, which prevents growth of many microorganisms. The sebum consists of lactic acid and fatty acids that maintain the pH of skin between 3 and 5, and this pH inhibits the growth of most microorganisms.

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.

Physiologic barriersThe physiologic barriers that contributeto innate immunity include the following:

·           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.

PhagocytosisPhagocytosis is another important defensemechanism of the innate immunity. Phagocytosis is a process of ingestion of extracellular particulate material by certain specialized cells, such as blood monocytes, neutrophils, and tis-sue macrophages. It is a type of endocytosis in which invading microorganisms present in the environment are ingested by the phagocytic cells. In this process, plasma membrane of the cell expands around the particulate material, which may include whole pathogenic microorganisms to form large vesicles called phagosomes.

Inflammatory responsesTissue damage caused by a woundor by an invading pathogenic microorganism induces a com-plex sequence of events, collectively known as the inflamma-tory responses. The end result of inflammation may be the activation of a specific immune response to the invasion or clearance of the invader by components of the innate immune system. The four cardinal features of inflammatory responses are rubor (redness), calor (rise in temperature), dolor (pain), and tumor (swelling).

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