Mechanism
A patch test relies on the principle of a type IV hypersensitivity reaction.
The first step in becoming allergic is sensitization. When the skin is exposed to an allergen, the antigen presenting cells (APCs) - also known as Langerhans cell or Dermal Dendritic Cell - eat up substance (phagocytoze) and break it into smaller pieces. This is where a substance is recognized by immune cells in the skin. They then put parts of the substance onto their surface (technically holds the part of the molecule on the surface in the major histocompatibility complex type two (MHC-II). Once this is done the APC moves down the lymphatic system to a lymph node where it presents this part of the substance (what we now call an antigen) to a particular immune cell called a CD4+ T-cell or T-helper cell. The T-cell, if it recognizes the substance as dangerous, expands in number and sends out more of itself to the skin, at the site of antigen exposure. When the skin is again exposed to the antigen, the memory t-cells in the skin recognize the antigen and produce cytokines (chemical signals) which cause more T-cells to migrate from blood vessels. This starts a complex immune cascade leading to skin inflammation, itching and the typical rash of contact dermatitis. In general, it takes 2 to 4 days for a response in patch testing to develop. The patch test is really just induction of a contact dermatitis in a small area.
Interestingly, the size of the molecule necessary to be picked up and recognized is ten times the size of the largest molecule that can pass through the skin. Therefore, it is likely that an antigen (like nickel) when it has passed through the skin, combines with something else before it is recognized.
Read more about this topic: Patch Test (medicine)
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