What Is Patch Test
A patch test is a method used to determine whether a specific substance causes allergic inflammation of a patient’s skin. Any individual suspected of having allergic contact dermatitis or atopic dermatitis needs patch testing.
Patch testing helps identify which substances may be causing a delayed-type allergic reaction in a patient, and may identify allergens not identified by blood testing or skin prick testing. It is intended to produce a local allergic reaction on a small area of the patient’s back, where the diluted chemicals were planted. The chemicals included in the patch test kit are the offenders in approximately 85–90 percent of contact allergic eczema, and include chemicals present in metals (e.g., nickel), rubber, leather, formaldehyde, lanolin, fragrance, toiletries, hair dyes, medicine, pharmaceutical items, food, drink, preservative, and other additives.
How Does Patch Test Works?
A patch test relies on the principle of a type IV hypersensitivity reaction.
The first step in becoming allergic is sensitization. When skin is exposed to an allergen, the antigen presenting cells (APCs) – also known as Langerhans cell or Dermal Dendritic Cell – phagocytize the substance, break it down to smaller components and present them on their surface bound to major histocompatibility complex type two (MHC-II)) molecules. The APC then travels to a lymph node, where it presents the displayed allergen to CD4+ T-cell, or T-helper cell. The T-cell undergoes clonal expansion and some clones of the newly formed antigen specific sensitized T-cells travel back to 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.
How Much Time Patch Test Takes?
In general, it takes 2–4 days for a response in patch testing to develop. The patch test is just induction of a contact dermatitis in a small area.