Atopy Patch Test – Mechanism and Procedure
Atopy patch test is a diagnostic test for atopic dermatitis, also known as allergic contact dermatitis. This test confirms the suspected allergen. It is based on the principle of delayed hypersensitivity reaction of skin cells towards the allergens. First we will give information about skin patch before proceeding to mechanism and procedure.
A skin patch is a sticky patch that is put on the surface of the skin to release drugs (here it is allergens) into the bloodstream.
The mechanism is somewhat difficult to understand. But it is based on the immune system of the body. An allergic substance is recognized by skin immune cells as antigen presenting cells. When they are placed on the skin, these cells eat up substances which pass through the skin. They chop the substances into smaller pieces. Then parts of the substances are out onto the surface. Then antigen presenting cells move down the lymphatic system and then to a lymph node.
Here it presents the substance to an immune cell, the T-cell. If the T-cell recognizes the substance as hazardous, it sends out many more immune cells to the place where the antigen is coming from. This is what is exactly meant by the immune response of the skin which the person gets in contact dermatitis.
In this process, a little quantity of each of 25 to 150 allergic substances are placed in individual square plastic chambers and are applied to the upper back. Skin patch is significant here. These patches are stuck in place using special hypoallergenic adhesive tape. These patches should not be disturbed for at least 48 hours.
The process takes about an hour but one needs to go to the clinic for 2 or 3 times. Dermatologists interpret the result in 6 grades: negative, irritant reaction, equivocal/uncertain, weak positive, strong positive and extreme reaction. Nickle is the most frequent allergen that is studied using this procedure.