Antihistamines compete with histamine at which type of receptor sites?

Antihistamines are specifically designed to block the action of histamine, a compound involved in various physiological responses, particularly in allergic reactions. They achieve this by competing with histamine at H1 receptors and, in some cases, H2 receptors. H1 receptors are primarily involved in mediating allergic symptoms such as itching, sneezing, and inflammation, while H2 receptors are related to gastric acid secretion in the stomach.

By binding to these receptors, antihistamines can effectively mitigate the effects of histamine, providing relief from allergy symptoms and other histamine-mediated conditions. This mechanism of action highlights why the correct answer is associated with H1 and H2 receptor sites, as these are the specific receptors that antihistamines target to exert their therapeutic effects.