Histamine, Hypersensitivity, and Antihistamines

Vocabulary flashcards covering key terms related to histamine biology, hypersensitivity reactions, and antihistamines as discussed in the lecture.

Histamine

A biogenic amine released from mast cells during allergic reactions; binds to H1 and H2 receptors to mediate effects such as vasodilation, increased capillary permeability, bronchoconstriction, gastric acid secretion, wakefulness, appetite, and itch.

Mast cell

Immune cells that store histamine and other mediators and release them upon exposure to allergens, triggering hypersensitivity reactions.

H1 receptor

Histamine receptor primarily involved in allergic symptoms; activation causes vasodilation, edema, itching, and bronchoconstriction and is the main target of antihistamines.

H2 receptor

Histamine receptor in gastric mucosa that stimulates gastric acid secretion.

Vasodilation

Widening of blood vessels; a primary histamine effect on the endothelium leading to redness and warmth.

Increased capillary permeability

Histamine-induced leakage of fluid from capillaries, contributing to swelling (edema) during allergic reactions.

Wheal

Raised, itchy swollen skin area produced by histamine during the wheal-and-flare reaction.

Flare

Redness that radiates outward from the initial wheal in a histamine-mediated skin reaction.

Triple response of Lewis

Skin reaction pattern: red spot, red flare, and wheal that occurs after histamine exposure or skin testing.

Allergic rhinitis

Inflammation of the nasal mucosa due to allergen-triggered histamine release; symptoms include runny nose, sneezing, and itchy, watery eyes.

Urticaria

Localized itching and wheals (hives) caused by histamine release in the skin.

Angioedema

Deep tissue swelling (often lips, tongue, or throat) that can be life-threatening; may be triggered by drugs or ACE inhibitors.

Anaphylaxis

Systemic, life-threatening hypersensitivity reaction requiring urgent treatment.

First generation antihistamines

H1 antagonists that readily cross the blood–brain barrier; sedating with prominent anticholinergic effects; examples include diphenhydramine, chlorpheniramine, brompheniramine, clemastine, hydroxyzine, dimenhydrinate, promethazine, and cyproheptadine.

Second generation antihistamines

H1 antagonists with limited central nervous system penetration; non-sedating or minimally sedating with longer half-lives; examples include cetirizine, loratadine, desloratadine, and fexofenadine; some ophthalmic options include olopatadine.

Diphenhydramine

A classic first-generation antihistamine; sedating; used for allergic symptoms, motion sickness, and insomnia.

Cetirizine

A second-generation antihistamine; commonly used for allergic rhinitis with minimal sedation.

Loratadine

A second-generation antihistamine; non-sedating and usually dosed once daily.

Fexofenadine

A second-generation antihistamine; non-sedating and long-acting, commonly dosed once daily.

Olopatadine

Ophthalmic antihistamine used to treat allergic conjunctivitis and available as an eye drop.

Ketotifen

Antihistamine with mast cell–stabilizing properties; used for ocular itching and allergic conjunctivitis; can cause eye irritation in some patients.

Azelastine

Nasal and ophthalmic antihistamine; can have mast cell stabilizing properties and is used for rhinitis and conjunctivitis.

Scopolamine

Antimuscarinic agent (often in patch form) used to prevent motion sickness; crosses into the CNS and can contribute anticholinergic effects.