Innate Immunity

An inflammatory response is the body's reaction to tissue damage or infection by foreign substances. It is a complex phenomenon composed of two very distinct types of immunity: innate and acquired. In innate immunity, the body utilizes natural and physiologic barriers to initiate cellular repair, to protect itself from invasion, and to attempt to limit the spread of pathogen through activation of a complex inflammatory cascade.

The natural barriers of innate immunity include the skin, mucous membranes, hair, and the acidic environments of the stomach and sweat. Examples of the physiologic responses include the coughing reflex, the release of antifungal and antibacterial secretions from skin glands, the beating of respiratory cilia, the production of hydrolytic enzymes in tears and saliva, the production of chemical signals known as cytokines, and the stimulation of specific innate cellular components which aid in the fight against infection. Effective innate immunity does not rely on previous interaction with the pathogen and is therefore a nonspecific cellular immunity.

Phagocytosis, characterized by host cell migration, recognition, attachment, engulfment, and processing of pathogens, is the main mechanism of innate immunity. In this process, host cells are protected from invasion and attempt to control the spread of a pathogen. The primary cells involved are neutrophils and macrophages. Neutrophils are considered the first responders during inflammation and are often replaced by macrophages arriving at the exposure site within 48 hours. Activation of this process often follows exposure of the body to specific bacterial components, like lipopolysaccharide (LPS), a polysaccharide on certain bacterial cell membranes.

The body also responds to bacterial peptides, membrane components like peptidoglycan and techoic acid, and foreign nucleic acids. Each of these substances contains unique sequences known as pathogen-asso-ciated molecular patterns, or PAMPs, that the body recognizes as foreign. The body possesses its own recognition machinery in the form of toll-like receptors, or TLRs, found on host innate immune cells, and these are used to recognize and bind these PAMPS. Initiation of downstream cellular responses then occurs and leads to the production of exudate and specific chemical messages such as interferon, interleukins 1 and 6, and tumor necrosis factor.

Another process by which host cells are protected from invasion and attempt to control the spread of a pathogen is via complement activation. A complement is an immune complex comprised of three activating pathways: the leptin pathway, the alternative pathway, and the classical pathway, all of which allow for the destruction of the invading pathogen.

The typical signs and characteristics of the innate immune response include heat, redness, swelling, pain, and loss of function. The heat and redness accompany vascular dilatation while swelling is the result of the escape of fluid, plasma protein, and cellular accumulation within the area of exposure. Pain results from released chemical mediators and nerve compression. The severity of the pathogen, the immune status of the host, and the site of exposure determine the intensity of each symptom. Without such a complex response, the body would be unable to effectively fight off infection and disease.