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Delayed (Type IV) Hypersensitivity

Why does repeated exposure to poison ivy tend to produce more severe symptoms? Poison ivy (Rhus radicans) is a common North American plant that produces an oil that cause skin rashes on many people following contact. Sensitivity to the oil varies widely from person to person. There is a fairly consistent pattern that sensitivity increases with repeated exposure. Each exposure causes the immune system to produce more T cells specific to the molecule in the oil. The T cells provoke an immune response causing the reaction on the skin. Therefore repeated exposures over a relatively short time period tend to cause increasingly severe reactions as the number of cells increases. Because the reaction depends on the production of more T cells the rash occurs a day or so after contact.

View the animation below, then complete the quiz to test your knowledge of the concept.

1On first exposure to antigen, T helper cells
A)become activated and increase in number
B)cause inflammation
C)cause skin lesions
D)attract more macrophages

2When T helper cells are exposed for the second time to hapten-peptide on antigen presenting cells, they
A)lyse cells using perforin
B)release cytokines which attract more macrophages
C)make antibodies
D)kill macrophages

3Skin lesions due to hypersensitivity appear
A)24 hours after first exposure to antigen
B)48–72 hours after first exposure to antigen
C)24 hours after second exposure to antigen
D)3 weeks after second exposure to antigen

4A hapten is an antigen that is capable of causing antibody production on its own.

5Delayed hypersensitivity is due to the effects of the humoral part of the immune system.

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