type 4 hypersensitivity reaction and TB testing

Delayed hypersensitivity is a cell-mediated reaction initiated by T-lymphocytes and mediated by effector T-cells and macrophages (choice C is correct). The interferon-gamma-release assays (IGRAs) are blood tests that measure ex vivo T-lymphocyte release of interferon-gamma after stimulation by antigens specific for Mycobacterium tuberculosis. This response involves the interaction of antigens with the surface of lymphocytes. Sensitized lymphocytes can produce cytokines, such as interferon-gamma, that may affect the function of other cells. This type of reaction takes 48 to 72 hours or longer after contact with the antigen to fully develop. Because these antigens are not found on M. bovis-BCG or most nontuberculous mycobacteria, IGRAs are more specific than the TST, yielding fewer false-positive results. In addition, most antigens are not identified in most nontuberculous mycobacteria, except for M. kansasii, M. szulgai, and M. marinum. IGRAs have little advantage over the TST in sensitivity, and both methods have reduced sensitivity in immunocompromised hosts, including those with severe TB disease.

In a cytotoxic reaction (antibody-dependent), the antibody reacts directly with the antigen that is bound to the cell membrane, inducing cell lysis through complement activation (choice A is incorrect). Cytotoxic reactions are mediated by IgG and IgM. Immune complex reaction measures the forming of antigen-antibody (IgG and IgM) that activates complement, which results in polymorphonuclear leucocytes (PMN) chemotaxis and activation that then releases tissue damaging enzymes (choice B is incorrect). The immediate hypersensitivity or anaphylactic reaction is systemic or localized, as in allergic dermatitis (eg, hives, wheal, and erythema reactions) (choice D is incorrect). The reaction is the result of an antigen cross-linking with membrane-bound IgE antibody of a mast cell or a basophil. Histamine, serotonin, bradykinin, and lipid mediators (eg, platelet activating factor, prostaglandins, and leukotrienes) are released during the anaphylactic reaction. These released substances have the potential to cause tissue damage.