Given below are four sentences in jumbled order. Pick the option that gives their correct order.
A. An allergy is a reaction of the immune system to a substance which is normally harmless.
B. These antibodies then attach themselves to cells, which contain histamine, which inflames the tissues.
C. One-third of us are affected by an allergy at some point in our lives.
D. The immune system in allergy sufferers makes antibodies against harmless substances because it believes them to be dangerous.