T cells possess specialized chemical “receptors” on their outer surfaces, giving them the ability to recognize foreign substances. For a drug to react with a receptor, it must first be presented to the surface of the T cell by another cell in a way that holds the reactive portion of its structure in the correct position for interaction to take place. This often requires that the drug “hook a ride” on a larger chemical that clamps it into the correct position by forming a chemical bond with the drug. If the drug is not chemically reactive enough to produce a chemical bond, it can sometimes be modified by the body's metabolism into a form that can then effectively bond with the larger chemical. Recently, researchers have recognized a third way that a drug can react with T cells, even if it cannot form a chemical bond with the “carrier” chemical. This involves a much looser connection (known as a labile connection) with the T cell or the carrier. A labile connection is based on the structure of the drug rather than its chemical reactivity. In this method of reaction, the drug tucks into the receptor structure without forming a chemical bond but is still able to stimulate the immune receptor. The manner of drug presentation (chemical bond vs. labile connection) may determine what kinds of hypersensitivity reaction can occur in the skin. Drug hypersensitivity reactions may kill cells in various layers of the skin, forming blisters or severe destruction; alternatively, they may be characterized by inflammation. The actual form of the skin reaction depends on the specific chemicals released by the T cell in response to the offending drug.