(d) Antigen-presenting cells present the hapten-peptide complex to sensitized Th1 cells, which secrete cytokines and attract macrophages; the macrophages are activated and secrete mediators of inflammation that cause skin lesions.
Figure 18.7 Poison Oak Dermatitis Is an Example of Type IV Hypersensitivity: Delayed Cell-Mediated
(e) Characteristic skin lesions appear after 24 hours, reaching their peak at 48-72 hours after exposure to the plant.
cytokines when they come into contact again with the same antigen. These cytokines cause inflammatory reactions that attract macrophages to the site. The macrophages then release mediators that add to the inflammatory response, resulting in allergic dermatitis.
Familiar examples of contact hypersensitivity (contact allergy or contact dermatitis) are poison ivy and poison oak (figure 18.7) and allergic reactions to the nickel of metal jewelry, the chromium salts in certain leather products, or components of some cosmetics. In the case of poison oak or ivy, a hapten from an oily product of the plant is responsible. With a metal, a soluble salt of the metal acts as a hapten.
Latex products are a frequent cause of contact hypersensitivity reactions, and they also can cause IgE-mediated reactions. Latex, a product of the rubber tree, contains a plant protein that readily induces sensitization. Many products contain latex, such as fabrics, elastics, toys, and contraceptive condoms, but latex gloves probably account for most latex sensitization. Gloves are used extensively by health care and laboratory workers, food preparers, and many others. Typically, a person will notice redness, itching, and a rash on the hands after wearing gloves. To prevent the reaction, latex gloves should be replaced by vinyl or other synthetic gloves. Topical cortisone-like medications are effective treatment.
The causative substance in contact hypersensitivity is commonly detected by patch tests, in which suspect substances are applied to the skin under an adhesive bandage. Positive reactions reach their maximum in about 3 days and consist of redness, itching, and blisters of the skin. Figure 18.8 shows a severe contact hypersensitivity skin rash.
Delayed Hypersensitivity in Infectious Diseases
The role of cell-mediated immunity in combating intracellular infections through the cell-destroying activity of activated
macrophages and T lymphocytes was discussed previously. Although these functions are protective, tissue damage or hypersensitivity also results. These infections may be caused by viruses, mycobacteria and certain other bacteria, protozoa, and fungi. They include leprosy, tuberculosis, leishmaniasis, and herpes simplex, among many others. In particularly slowly progressing infections, delayed hypersensitivity causes extensive cell destruction and progressive impairment of tissue function, such as the damaged sensory nerves in leprosy. The immune response is a two-edged sword, protecting on the one side, but causing damage on the other. ■ immunity to intracellular infections, p. 409 ■ leprosy, p. 670
Was this article helpful?
Do you hate the spring? Do you run at the site of a dog or cat? Do you carry around tissues wherever you go? Youre not alone. 51 Ways to Reduce Allergies can help. Find all these tips and more Start putting those tissues away. Get Your Copy Of 51 Ways to Reduce Allergies Today.