What is the primary effect of antihistamines on the body?

The primary effect of antihistamines on the body is that they block the effects of histamine. Histamine is a chemical released by the body during allergic reactions and plays a significant role in various physiological processes such as inflammation, gastric acid secretion, and neurotransmission. When histamine binds to its receptors, it can cause symptoms such as itching, sneezing, runny nose, and other allergic responses. Antihistamines work by antagonizing these receptors, preventing histamine from exerting its effects, and thereby alleviating allergic symptoms.

While other options may reference various bodily functions, they do not correctly describe the action of antihistamines. For example, antihistamines do not intrinsically increase blood pressure, stimulate the immune system, or enhance the action of leukotrienes; their main function is specifically to block histamine's actions, particularly in the context of allergy and inflammation.