What is the mechanism of action of the antihistamine loratadine?

Loratadine primarily acts as a selective peripheral H1 receptor antagonist. This means that it specifically blocks the H1 receptors for histamine, which are involved in allergic reactions and symptoms such as itching, sneezing, and nasal congestion. By preventing histamine from binding to these receptors, loratadine effectively reduces the physiological effects of histamine release during an allergic response without penetrating the blood-brain barrier significantly, which helps avoid central nervous system effects commonly associated with first-generation antihistamines, such as sedation.

The other options provided involve mechanisms not applicable to loratadine. For instance, H2 receptor agonists would relate to gastrointestinal acid secretion, while serotonin reuptake inhibitors pertain to antidepressant effects and are unrelated to the action of antihistamines. Lastly, inhibiting histamine precursor synthesis does not reflect the action of antihistamines like loratadine, which directly blocks the receptor's action rather than affecting histamine synthesis.