Allergy An allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person’s immune system reacts to normally harmless substances in the environment. A substance that cause a reaction is called an allergen .
allergy Depending on the individual, allergen and mode of introduction the symptoms can be system wide, localized to particular body systems. Mild allergies like hay fever are very common & causes such as red eye , itchiness , runny nose , eczema & hives . In some people severe allergies to environmental or dietary allergens or medications may result in life threatening reactions called anaphylaxis .
allergen An allergen is an antigen capable of stimulating a type- I hypersensitivity reaction in atopic individuals through Immunoglobulin E (Ig E) responses. Sensitivities vary widely from one person to another.
Common allergens
Mechanism of allergic reaction The mechanism of allergic reactions is broken down into two successive phases: Phase I : It is a sensitization phase during which the immune system identifies the substance as an allergen. Phase II : When the organism comes into contact with the allergen again, the allergic phase is triggered.
Mechanism of allergic reaction
histamine Histamines are the ‘ tissue amine ’ present mostly within storage granules of mast cells. As part of an immune response to foreign pathogens, histamine is produced. Histamine increases the permeability of the capillaries to WBC & some proteins, to allow them to engage pathogens in the infected tissues.
histamine Mast cells are especially numerous at sites of potential injury – nose, mouth, feet, internal body surfaces & blood vessels. Non mast cell histamine is found in several tissues, including the brain, epidermis & growing regions. Histamine combines with the specific cellular Histamine receptors present on the cell membrane & exerts its biological effect.
Histamine receptors
Histamine antagonist A histamine antagonist is a drug that inhibits the action of histamine by blocking its attachment to histamine receptors.
H1 antagonist An H1 antagonist is a histamine antagonist of the H1 receptor that serves to reduce or eliminate effects mediated by histamine during allergic reactions. Clinically, H1 antagonists are used to treat allergic reaction.
aladin Aladin consists fexofenadine hydrochloride as an active ingredient. Fexofenadine is a new non-sedating antihistamine. It works by being an antagonist to the H1 receptor. Fexofenadine, like other 2 nd & 3 rd – generation antihistamines, does not readily cross the blood-brain barrier, & so causes less drowsiness then 1 st generation antihistamine.
pharmacodynamics Fexofenadine works by blocking H1 receptors. It does not prevent the actual release of histamine from mast cells, but prevents it binding to its receptors. Blockage prevents the activation of the H1 receptors by histamine preventing the symptoms associated with allergies from occurring.
pharmacokinetics Absorption Absorbed rapidly from the GI tract(oral). Peak plasma concentration after 2-3 hrs. Distribution Enters breast milk. 60-70 % bound to plasma proteins. Metabolism Intestinal mucosa (5% of total dose) Hepatic (0.5-1.5%) Excretion mainly via faces, via urine(10% of dose) Elimination half-life: 14 hrs.
Indication & dosage Oral Seasonal allergic rhinitis Adult : 120mg once daily Child : 6-11 yrs.: 30mg BID Chronic idiopathic urticarial Adult : 180mg once daily Child : 6 month -<2 yrs.: 15mg BID; ≥ 2yrs.: 30mg BID Take the table on an empty stomach, at least 1 hour before or 2 hours after a meal.
Drug interaction Co-admin with ketoconazole or erythromycin may increase plasma levels of fexofenadine. Bioavailability may be increased by verapamil . Efficacy may be reduced by rifampin .
Precaution Pregnancy Lactation Renal impairment Children < 6 years