- Describe the three subtypes of type IV hypersensitivity.
- Explain how T cells contribute to tissue damage in type IV hypersensitivity.
Using delayed hypersensitivity to test for tb
Austrian pediatrician Clemans von Pirquet (1874–1929) first described allergy mechanisms, including type III serum sickness. B. Huber “100 Jahre Allergie: Clemens von Pirquet–sein Allergiebegriff und das ihm zugrunde liegende Krankheitsverständnis.” Wiener Klinische Wochenschrift 118 no. 19–20 (2006):573–579. His interest led to the development of a test for tuberculosis (TB), using the tuberculin antigen, based on earlier work identifying the TB pathogen performed by Robert Koch. Pirquet’s method involved scarification, which results in simultaneous multiple punctures, using a device with an array of needles to break the skin numerous times in a small area. The device Pirquet used was similar to the tine test device with four needles seen in [link] .
The tips of all the needles in the array are coated with tuberculin, a protein extract of TB bacteria, effectively introducing the tuberculin into the skin. One to 3 days later, the area can be examined for a delayed hypersensitivity reaction, signs of which include swelling and redness.
As you can imagine, scarification was not a pleasant experience, C.A. Stewart. “The Pirquet Test: Comparison of the Scarification and the Puncture Methods of Application.” Archives of Pediatrics&Adolescent Medicine 35 no. 3 (1928):388–391. and the numerous skin punctures put the patient at risk of developing bacterial infection of the skin. Mantoux modified Pirquet’s test to use a single subcutaneous injection of purified tuberculin material. A positive test, which is indicated by a delayed localized swelling at the injection site, does not necessarily mean that the patient is currently infected with active TB. Because type IV (delayed-type) hypersensitivity is mediated by reactivation of memory T cells, such cells may have been created recently (due to an active current infection) or years prior (if a patient had TB and had spontaneously cleared it, or if it had gone into latency). However, the test can be used to confirm infection in cases in which symptoms in the patient or findings on a radiograph suggest its presence.
Hypersensitivity pneumonitis
Some disease caused by hypersensitivities are not caused exclusively by one type. For example, hypersensitivity pneumonitis (HP) , which is often an occupational or environmental disease, occurs when the lungs become inflamed due to an allergic reaction to inhaled dust, endospores, bird feathers, bird droppings, molds, or chemicals. HP goes by many different names associated with various forms of exposure ( [link] ). HP associated with bird droppings is sometimes called pigeon fancier’s lung or poultry worker’s lung —both common in bird breeders and handlers. Cheese handler’s disease , farmer’s lung , sauna takers' disease , and hot-tub lung are other names for HP associated with exposure to molds in various environments.