Hypersensitivity reactions: T-cell-mediated, type IV --- delayed-type hypersensitivity.

Hypersensitivity reactions: T-cell-mediated, Hypersensitivity reactions: T-cell-mediated, type IV --- delayed-type hypersensitivitytype IV --- delayed-type hypersensitivity

General characteristics

• No apparent response until 24 - 72 hours after challenge

• Erythema (redness) & induration (raised thickening)• Mononuclear (monocytes/macrophages) infiltration, b

ut no neutrophils• Tuberculin reaction• Two stages

– Sensitization– Elicitation : Activation of TH1 cells

Figure 16.1(A) Type IV delayed-type hypersensitivity reaction (tuberculin reaction)- gross appearance showing induration and erythema 48 hours after tuberculin test. [Courtesy of Dr. A. Gottlieb, Tulane University Medical School.] (B) Type IV delayed-type hypersensitivity reaction-histologic picture showing dermal mononuclear cell infiltrate and perivascular cuffing. [Courtesy of Dr. M. Stadecker, Tufts University Medical School.]

Figure 16.2The DTH reaction. Stage of sensitization by antigen involves presentation of antigen to T cells by antigen-presenting cells, leading to the release of cytokines and differentiation of T cells to TH1 cells. Challenge with antigen involves antigen presentation to TH1 cells by antigen-presenting cells, leading to TH1 activation, release of cytokines, and recruitment and activation of macrophages.

Overview of the DTH caused by intracellular bacteria

Figure 16.3The effect of IFN- on peritoneal macrophages. (A) Normal macrophages in culture as they are just beginning to adhere. (B) Macrophages that after activation with IFN- have adhered, spread out with development of numerous pseudopodia, and grown larger. More lysosomal granules are also visible. [Courtesy of Dr. M. Stadecker, Tufts University Medical School.]

Survival of the intracellular pathogen Listeria monocytogenes

Figure 16.4(A) Type IV contact sensitivity reaction-gross appearance of reaction to poison ivy. [Courtesy of Dr. M. Stadecker, Tufts University Medical School.] (B) Type IV contact hypersensitivity reaction-histologic appearance showing intraepithelial blister formation and mononuclear infiltrate in the dermis. [Courtesy of Dr. M. Stadecker, Tufts University Medical School.] (C) Cutaneous basophil reaction showing basophils and some mononuclear cells 24 hours after skin test. [Courtesy of Dr. M. Stadecker, Tufts University Medical School.]