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.