Type 4 delayed hypersensitivity seen to a range of things including:
- Nickel (for example in jewellery, belt buckles, fastenings)
- Limonene, found in many cleaning products and cosmetics
- Sodium lauryl sulfate, found in cleaning products and cosmetics
- Lanolin and other wool products
Mechanism is complicated as metals are clearly not proteins so not identified by HLA class 2 as happens in type 1 allergy. Presumably happens through toll like receptors.
Testing is by patch testing, done by dermatology.