An antihistamine is a type of pharmaceutical drug that opposes the activity of histamine receptors in the body.Antihistamines are subclassified according to the histamine receptor that they act upon: the two largest classes of antihistamines are H1-antihistamines and H2-antihistamines.n H3-antihistamine is a classification of drugs used to inhibit the action of histamine at the H3 receptor. H 3 receptors are primarily found in the brain and are inhibitory autoreceptors located on histaminergic nerve terminals, which modulate the release of histamine.
U.S. Secret Service. U.S. Secret Service, federal law-enforcement agency within the United States Department of Homeland Security tasked with the criminal investigation of counterfeiting and other financial crimes. After the assassination of Pres.
Authority/ Statutory Mandate. By law, the Secret Service is authorized to protect: 1  The president, the vice president, (or other individuals next in order of succession to the Office of the President), the president-elect and vice president-elect.  The immediate families of the above individuals.