Dendritic cells also phagocytose and function as anti-gen-presenting cells (APCs) and act as important mes-sengers between innate and adaptive immunity. Mastcells and basophils share many salient features witheach other and both are instrumental in the initiation ofacute inflammatory responses, such as those seen inallergy and asthma. Unlike mast cells, which generallyreside in the connective tissue surrounding blood ves-sels, basophils reside in the circulation. Eosinophils aregranulocytes that possess phagocytic properties and playan important role in the destruction of parasites that aretoo large to be phagocytosed. Along with mast cells andbasophils, they also control mechanisms associated withallergy and asthma. NK cells (also known as large granu-lar lymphocytes [LGLs]) play a major role in the rejec-tion of tumours and the destruction of cells infected byviruses. Destruction of infected cells is achieved throughthe release of perforins and granzymes from NK-cellgranules which induce apoptosis (programmed celldeath) . The main characteristics and functions of thecells involved in the innate immune response are sum-marized in Figure 1.Innate immunity can be viewed as comprising fourtypes of defensive barriers: anatomic (skin and mucousCellImage% in adultsNucleusFunctionsLifetimeMain targetsMacrophage*VariesVariesxPhagocytosisxAntigen presentation to T cellsMonths – years xVariousNeutrophil40-75%Multi-lobedxPhagocytosisxDegranulation (discharge of contents of a cell)6 hours – few daysxBacteriaxFungiEosinophil1-6%Bi-lobedxDegranulationxRelease of enzymes, growth factors, cytokines8-12 days (circulate for 4-5hours)xParasitesxVarious allergic tissuesBasophil< 1%Bi- or tri-lobedxDegranulationxRelease of histamine, enzymes, cytokinesLifetime uncertain; likely a few hours –few daysxVarious allergic tissuesLymphocytes (T cells)20-40%Deeply staining, eccentricT helper (Th) cells (CD4+): immune response mediatorsCytotoxic T cells (CD8+): cell destructionWeeks to yearsxTh cells: intracellular bacteriaxCytotoxic T cells: virus infected and tumour cellsxNatural killer cells: virus-infected and tumour cellsMonocyte2-6%Kidney shapedDifferentiate into macrophages and dendritic cells to elicit an immune responseHours – days xVariousFigure 1Characteristics and function of cells involved in innate immunity[1,3,4]. *Dust cells (within pulmonary alveolus), histiocytes(connective tissue), Kupffer cells (liver), microglial cells (neural tissue), epithelioid cells (granulomas), osteoclasts (bone), mesangial cells (kidney)Warringtonet al.Allergy, Asthma & Clinical Immunology2011,7(Suppl 1):S1Page 2 of 8
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membrane), physiologic (temperature, low pH and che-mical mediators), endocytic and phagocytic, and inflam-matory. Table 1 summarizes the non-specific host-defense mechanisms for each of these barriers.
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