A client frequently takes diphenhydramine (Benadryl) to alleviate his upper respiratory symptoms associated with allergies. Which of the following mechanisms of action applies to diphenhydramine (Benadryl)?

Stimulates salivation by increasing its receptor sites.

Blocks the effects of histamine by competing and occupying H4 receptor sites.

Stimulates the effects of histamine by occupying all H receptor sites.

Blocks histamine receptors and inhibits acetylcholine by binding to H1 receptors.