CBSE Class 12-science Answered

Receptors are proteins that bind to ligands like hormones and are responsible for mediating the effect of the ligand. For a ligand the different cells have receptors with different characteristics. 

Histamine receptors are of 4 different types.  H₁ receptors are present on air passages, blood vessels while stomach cells express H₂ receptors.

The H₂ receptor antagonists (H₂RA) are a class of drugs used to block the action of histamine on parietal cells (specifically the histamine H₂ receptors) in the stomach, decreasing the production of acid by these cells.

The common antihistamines block the binding of histamine to H₁ receptors but not H₂ receptors and therefore do not affect the secretion of gastric juice.