Question 8 of 14

(demo test)

Type I hypersensitivity is caused by IgE bound to mast cells and basophils (not eosinophils), such as in immediate allergic reaction and atopy, not with contact dermatitis, which is type IV delayed hypersensitivity.

According to the Gell and Coombs classification of immunologic reactions:

Type I hypersensitivity (immediate reaction- within 1 hour):  Antigen exposure causes IgE-mediated activation of mast cells and basophils (not eosinophils), with release of vasoactive substances such as histamine, prostaglandins and leukotrienes.

Examples:  anaphylaxis, angioedema, bronchospasm, urticaria (hives), and allergic conjunctivitis.  Answer "A" is not correct because contact dermatitis is not mediated by histamine.

Type II hypersensitivity:  (antibody dependent cytotoxicity)  An antigen or hapten closely associated with a cell binds to antibody, leading to cell or tissue injury. 

Examples:  Destruction of RBCs in hemolytic anemia, thrombocytopenia, neutropenia, ocular cicatrial pemphagoid, Wegener’s granulomatosis  (c-ANCA)

Type III hypersensitivity (immune complex deposition)   Damage is caused by deposition of antigen-antibody complexes in vessels or tissue.  This deposition also causes complement activation and/or recruitment of neutrophils. 

Examples:  serum sickness, systemic lupus erythematosis, Stevens-Johnson Syndrome

Type IV hypersensitivity  (delayed hypersensitivity):   Antigen exposure activates T cells, which then mediate tissue injury.  

Examples:   Contact dermatitis (due to poison ivy, metals in jewelry, etc), TB skin test, transplant rejection 

To remember the 4 categories, trying using the mnemonic “ACID” (Consider that your skin would be fairly hypersensitive to someone pouring ACID on it).

The “A” stands for Allergic/Anaphylactic (Type I)

The “C” stands for Cell-mediated (Type II)

The “I” is for Immune Complex Deposition (Type III)

The “D” stands for ‘Delayed’ (Type IV)