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

stimulates effects of histamine by occupying all H receptor sites.

blocks effects of histamine by competing and occupying H1 receptor sites.

blocks effects of histamine by competing and occupying H2 receptor sites.

stimulates effects of histamine by increasing amount of H2 receptor sites.