Type IV Hypersensitivity: Pathogenesis and clinical findings Definition: Unique because it is entirely T-Cell mediated; exposure to allergen causes development of allergen-specific T-Cell response. Allergic reaction develops several days after exposure, because T-Cell activation takes time. Initial exposure to allergen (typically a drug) -> Macrophage engulfs antigen -> Antigen presentation on MHC II molecule -> Particular CD4+ Th cells recognize antigen on allergen's surface as potentially harmful -> Th Cells activate macrophages, mast cells or CTLs -> Inflammation and destruction of involved issue => • Stevens-Johnson Syndrome • Contact Dermatitis (Ex. Poison ivy) #TypeIV #Type4 #HypersensitivityReaction #Allergy #Immunology #pathophysiology
