What is a hapten? what does it do in the immune system?

Makes it go nuts: A hapten is molecule that is too small on its own to be recognized by the immune system, but attaches to larger molecules, such as proteins, making them trigger substantial immune reactions. Common with allergic contact dermatitis seen with plants (poison ivy: urushiol), metals (nickel, chromium), and other chemicals. Also occurs with drug allergy (penicillin: the major determinant).