Ans- As given in NCERT notes for Class 12 chemistry chapter 16, the characteristics of histamines are:

• potent vasodilator

• contracts the smooth muscles in the bronchi & gut

• relaxes other muscles

• responsible for the nasal congestion

• responsible for an allergic response to pollen.

Antihistamines interfere with the natural action of histamine. They do so by competing with histamine for binding sites of the receptor protein.