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Acquired immunity is the other major arm of the hosts

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Acquired immunityis the other major arm of the host’sIS and is the most highly evolved. It is also the most specificand allows the IS to have memory of pathogens it hasencountered previously. The acquired system is present onlyin vertebrates. The termacquiredrefers to the fact that theimmunity is acquired via specific contact with a pathogen oraberrant cell. The termadaptiverefers to the ability to adaptto and destroy new complex pathogens, although it mustfirst react to them through complex recognition processes.Acquired immunity is specific in recognition of newpathogens and has specific responses, depending on the typeof pathogen it encounters.The acquired IS uses antibodies as specific immuneeffectors. Antigen specificity and uniqueness determinethe particular antibody that will bind to it. The antigen-antibody complex is a three-dimensional interaction thatdoes not allow recognition of near misses. For example,antibodies against one blood group antigen do not reactagainst another blood group antigen. An antigen that anantibody is made against is sometimes referred to as itsantitheticalantigen. The fact that acquired immunity hasmemory means medical histories of patients that requiretransfusions are absolutely critical. Antibodies do notalways remain in plasma at levels observable with serologictesting, and if antigen-positive RBC units are transfused ina sensitized patient, the second antibody response againstthe transfused cell antigens can be more vigorous, result-ing in intravascular RBC hemolysis.Cells and Organs of the ImmuneSystemThe different types of immune system cells can be distin-guished by the membrane markers they possess. These arereferred to asclusters of differentiation (CD)markers andare detected by immunotyping methods. The immunizationprocess requires many different types of cells and tissues,including phagocytic granulocytes of the innate system andmonocytes and macrophages.Lymphocytes are also important in acquired immunity.They are divided into two major types, theT lymphocyte(T cell) and theB lymphocyte(B cell). The T lymphocytematures in the thymus gland and is responsible for makingcytokines and destroying virally infected host cells. B lym-phocytes mature in the bone marrow, and when stimulatedby an antigen, evolve intoplasma cellsthat secrete antibody.Natural killer cells are a type of lymphocyte that plays a role50PART IFundamental ConceptsTable 3–3Cytokines and Their Functions—cont’dCYTOKINESOURCESTIMULATORY FUNCTIONInterferonsIFN-aLeukocytesAntiviral; expression MHC IIFN-bFibroblastsIFN-gTOtherTGF-bT, BLIFTAdapted from Delves, PJ, Martin, SJ, Burton, DR, Roitt, IM: Roitt’s Essential Immunology, 8th ed. Wiley-Blackwell, Malden, 2011.APC = antigen-presenting cells; BM = bone marrow; CSIF = cytokine synthesis inhibitory factor, Fc = immunoglobulin Fc receptor for IgE; G-CSF = granulocyte colony-stimulating factor;GM-CSF = granulocyte monocyte/macrophage colony-stimulating factor; IFN = interferon; IL = interleukin; LIF = leukocyte inhibitory factor; MC = mast cell; M-CSF = monocyte colony-stimulating factor; MHC = major histocompatibility complex; Mf = macrophage; NK = natural killer cells; PGE2= prostaglandin E2; T = T lymphocyte; TGF = transforming growth factor;TNF = tumor necrosis factor.

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