A patient is given antihistamines due to an allergic reaction where vascular permeability is increased, causing watery eyes. Which of the following describes how antihistamines affect a signal transduction pathway? Antihistamines block histamine from binding to the appropriate receptor.
Antihistamines reduce histamine in the blood by binding to and destroying it.
Antihistamines reduce the amount of histamine in the bloodstream by binding to and inactivating histamine.
Antihistamines produce a second messenger that interrupts and reverses the direction of the signaling pathway.

Antihistamines affect a signal transduction pathway by blocking histamine from binding to the appropriate receptor (First option).

Explanation:

Antihistamines are a group of drugs used in allergic reactions mediated by the immune system. Specifically, antihistamines are competitive inhibitors of histamine binding to its specific receptor, the histamine H1 receptor.

Histamine is an inflammatory mediator, being one of the substances of the humoral immune response. It is secreted by mast cells and is responsible for the production of allergic effects, such as nasal congestion, sneezing, skin rash, and red, watery eyes.  Antihistamines block histamine from binding to the appropriate receptor, preventing these effects.

    The other options are not correct, because they do not correspond to the effects of antihistamines, that is, it does not bind to histamine, destroy it or render it inactive..