ANTIHISTAMINES

Histamine To understand antihistamines you have to slightly understand what it is blocking Histamine – There are 4 different histamine receptors. H 1 – H 4 – We will be focusing on H1 H1 receptors are located in the lower respiratory tract and skin

Histamine Background – Most tissue histamine is sequestered and bound in granules in mast cells or basophils; the histamine content of many tissues is directly related to their mast cell content. The bound form of histamine is biologically inactive, but many stimuli can trigger the release of mast cell histamine, allowing the free amine to exert its actions on surrounding tissues.

Mast Cell Mast cell Noun: a cell filled with basophil granules, found in numbers in connective tissue and releasing histamine and other substances during inflammatory and allergic reactions.

– Nervous system Powerful stimulant of sensory nerve endings, especially those mediating pain and itching The H1 mediated effect is an important component of the urticarial response and reactions to insect and nettle stings – Urticaria: Another name for hives. Raised, itchy areas of skin that are usually a sign of an allergic reaction.

The first time the body is exposed to an antigen it produces immunoglobin E antibodies that attach to mast cells Antigen: a substance that prompts the generation of antibodies and that can produce an immune response – These sensitized mast cells are found in tissues of the GI tract, skin and respiratory tract Where did we say H1 receptors were located?

With the second exposure to the antigen, the antigen binds to the immunoglobulin E antibodies on the mast cells. This binding causes a release of receptor sites in tissues, causing an allergic response that commonly includes coughing, sneezing, wheezing and urticaria – Urticaria: also known as hives; red welts that arise on the surface of the skin; often attributable to an allergic reaction but may have nonallergic causes

Histamine can produce a drop in blood pressure because it can dilate small blood vessels and capillaries Histamine also causes bronchoconstriction, making it difficult to breathe. In extremely severe reactions, the airways can become swollen and close; this is called anaphylactic shock, which can result in death if not treated immediately

When an allergic reaction takes place, antihistamines decrease the release of histamines by binding the antigen to the mast cells – Allergic reactions can be blocked in two ways Mast cells are prevented from releasing histamine Histamine 1 receptors are blocked form interacting with histamine

Antihistamine drugs specifically block histamine 1 receptors. The agents are commonly found in cold and cough medications because of their ability to dry secretions

Two generations of antihistamines – 1 st generations are considered nonspecific antihistamines. They bind to histamine 1 and histamine 2 receptors. Many can cause sedation because of their effect on the central nervous system; others decrease N/V and motion sickness. – 2 nd generation affect histamine 1 receptors specifically thus they don’t cause the same amount of sedation

Antihistamines are used to decrease inflammation and irritation from allergens. Allergens are also know as antigens. Medications that can be taken to prevent an allergic response by stabilization of the mast cell membranes include> cetirizine(Zyrtec) and fexofenadine (Allegra).

1 st generation meds examples 1 st generation were selective or nonselective? – Bromphenirmaine maleate : Lohist, Lodrane Rx – Chlorphenirmine: Chlor-Trimeton allergy relief OTC – Cyproheptadine: Pericatin Rx – Diphenhydramine: Benadryl OTC

2 nd generation med examples Selective or nonselective? – Cetirizine : Zyrtec OTC – Desloratidine: clarinex Rx – Fexofenadine: Allergra OTC – Loratidine : Claritin OTC