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involved in local immune response as well as regulating physiological function in the gut and acting as a neurotransmitter
mediates inflammation, anaphylaxis, gastric acid secretion, and neurotransmission
increases permeability of capillaries to white blood cells and other proteins to allow them to engage foreign invaders


low concentration in the plasma and body fluids, but high concentration in the CSF
predominant storage in mast cells so high concentration in the skin, bronchial mucosa, and intestinal mucosa
stored in basophils in the blood

histamine release

immunologic mechanism
mast sells are sensitized by IgE antibodies attached to their membranes and degranulate when exposed to the appropriate antigen

histamine and the cardiovascular system

causes vasodilation, increased capillary permeability, lower systemic blood pressure, increases Ca influx and heart rate and triple response of Lewis
triple response of Lewis; localized red spot at the site of injection, brighter red flush or "flare", and wheal formation

histamine and endocrine glands

physiological regulator of gastric acid secretion (H2 receptor action)
increases salivary, lacrimal, and other exocrine secretions (H1 receptor action)

histamine and the CNS

causes pain, itching, and indirect effects to the peripheral nerve endings

H1 receptor antagonists

inhibit effects of histamine on smooth muscles, especially the constriction of respiratory smooth muscle
block increased capillary permeability and block formation of edema and wheal
suppress both itch and flare
do not suppress gastric secretion or other histamine-evoked salivary, lacrimal, and other exocrine secretions


sudden, severe, potentially fatal, systemic allergic reaction that can involve various areas of the body such as skin, respiratory tract, GI tract, and cardiovascular system

H1 receptor antagonists

first generation i.e. diphenhydramine causes central nervous depression; diminished alertness, slowed reaction times, somnolence
first generation have anticholinergic side effects i.e. dry mouth, constipation, blurred vision (double vision), urinary retention (difficult or painful urination), increased intraocular pressure; second generation do not have these side effects

first generation adverse effects

sedation, impaired coordination, dizziness, sleepiness, fatigue, blurred vision, increased intraocular pressure, loss of appetite, nausea, vomiting, epigastric distress, constipation, diarrhea, dry mouth, induced cough, urinary retention, frequency, and dysuria
take with food to avoid GI effects

first generation warnings

caution when performing tasks requiring mental alertness
caution in patients with asthma, CV disease, glaucoma, urinary obstruction, stenotic peptic ulcer, and thyroid dysfunction
do not use in children under four
do not use to sedate children
may cause excitation in young children


first generation
used for chronic uticaria (hives), angioedema, and nocturnal pruritis


first generation
available OTC
used for rhinitis or allergic symptoms including hives


first generation
available OTC
used for allergic symptoms caused by histamine including nasal allergies and allergic dermatosis, adjunct to epinephrine in anaphylaxis, night time sleep aid, motion sickness, antitussive, parkinsonian syndrome, pain and itching associated with insect bites, minor cuts, and burns, and rashes due to poison ivy, poison oak, and poison sumac


first generation
available OTC
used for nausea, vertigo, and vomiting associated with motion sickness


first generation
most potent H1 antagonist
causes relatively less drowsiness, but still causes some sedation; more suitable agent for daytime use


first generation
used for skin allergies (antipruritic action) and as a mild anxyolytic
has a tranquilizer and antihistamine effect
CNS-depressant activity may contribute to its prominent antipruretic activity


first generation
used for motion sickness although promethazine, diphenhydramine, and scopolmine are more effective


first generation
H1 blocking
anticholinergic, sedative, antimotion sickness, antiemetic
adverse effect; extravasation after IV injection causes gangrene


second generation
causes minimal anticholinergic effects, negligible penetration into the brain, and higher drowsiness than others


second generation
intranasal spray used for seasonal allergic rhinitis and vasomotor rhinitis; 1 or 2 sprays per nostril twice daily
ophthalmic used for seasonal allergic conjunctivitis; 1 drop into affected eye twice daily
adverse effects; bitter taste, epistaxis, headache, fatigue, somnolence

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