itching, a stimulation of free nerve endings, usually at the junction of the dermis and epidermis of the skin, that evokes a desire to scratch. It has been suggested that an itch is a subthreshold sensation of pain; however, although both itch and pain sensations share common nerve pathways, they are generally considered distinct sensory types. Itching evokes a range of sensations, from a tickling that is easily relieved to pathologic itching, which generally indicates a dermatologic or systemic disease.
The release of histamine from cells in the epidermis is usually considered to incite most sensations of itching. Scratching may temporarily relieve itching by interrupting the rhythm of nerve impulses or by inflicting transitory damage to the nerves. Persistent scratching produces redness, papules, and crusting of the skin.
Why do scabs itch?Injuries that break skin can be painfully memorable.
Itching associated with skin lesions (such as vesicles and papules) generally indicates a dermatologic disease. Generalized itching without skin lesions can occur in systemic diseases such as those that occur in metabolic and endocrine conditions, malignant cancers, drug reactions, and renal, blood, and liver diseases. Excessive drying of the skin caused by pregnancy, old age, or weather often produces itching. A psychological cause for itching is considered only after all other possibilities have been sufficiently excluded.
No generally accepted therapy exists for the relief of itching, although many have been suggested. Topical anesthetics and other drugs usually have temporary or minor effects. Emollients, applied and blotted dry, may be helpful when applied to wet skin. Antihistamines may also be helpful. At present the choice of therapy usually depends on the underlying cause of the itching, and the focus is primarily on curing the underlying disease rather than the symptom.
Pathways of complement activationThe main function of complement proteins is to aid in the destruction of pathogens by piercing their outer membranes (cell lysis) or by making them more attractive to phagocytic cells such as macrophages (a process known as opsonization). Some complement components also promote inflammation by stimulating cells to release histamine and by attracting phagocytic cells to the site of infection.
histamine, biologically active substance found in a great variety of living organisms. It is distributed widely, albeit unevenly, throughout the animal kingdom and is present in many plants and bacteria and in insect venom. Histamine is chemically classified as an amine, an organic molecule based on the structure of ammonia (NH3). It is formed by the decarboxylation (the removal of a carboxyl group) of the amino acidhistidine.
English scientists George Barger and Henry H. Dale first isolated histamine from the plant fungus ergot in 1910, and in 1911 they isolated the substance from animal tissues. Plants that produce histamine include stinging nettles; the histamine in the hairlike structures on nettle leaves is partly responsible for the swelling and itching produced by contact with them. Histamine is also the irritating ingredient present in the venom of many species of wasps and bees.
In humans histamine is found in nearly all tissues of the body, where it is stored primarily in the granules of tissue mast cells. The blood cells called basophils also harbour histamine-containing granules. Once released from its granules, histamine produces many varied effects within the body, including the contraction of smooth muscle tissues of the lungs, uterus, and stomach; the dilation of blood vessels, which increases permeability and lowers blood pressure; the stimulation of gastric acid secretion in the stomach; and the acceleration of heart rate. Histamine also serves as a neurotransmitter, carrying chemical messages between nerve cells.
Why scratching an itch actually makes it worseUnderstanding the chemical process of itching and why scratching is futile.
The effect histamine has on blood vessels is crucial to its role in the immune response, which is most clearly observed in inflammation—i.e., the local reaction of bodily tissues to injury caused by physical damage, infection, or allergic reaction. Injured tissue mast cells release histamine, causing the surrounding blood vessels to dilate and increase in permeability. This allows fluid and cells of the immune system, such as leukocytes (white blood cells) and blood plasma proteins, to leak from the bloodstream through the vessel walls and migrate to the site of tissue injury or infection, where they begin to fight the infection and nourish and heal the injured tissues.
In an allergic reaction—the immune system’s hypersensitivity reaction to usually harmless foreign substances (called antigens in this context) that enter the body—mast cells release histamine in inordinate amounts. Immune system proteins called antibodies, which are bound to mast cells, bind to the antigens to remove them, but in the process the mast cells are stimulated to release their histamines. This causes the visible symptoms of a localized allergic reaction, including runny nose, watery eyes, constriction of bronchi, and tissue swelling. Histamine also contributes to generalized allergic conditions such as anaphylaxis, a severe, immediate, and often fatal response to exposure to a previously encountered antigen. Anaphylaxis is especially common in susceptible individuals following insect stings.
Scombroid fish poisoning, or histamine fish poisoning, is a type of allergic reaction that occurs following the consumption of fish contaminated with high amounts of histamine. This happens when fish are not refrigerated properly, resulting in the breakdown of fish flesh by bacteria, which produce histamine in the process. Types of fish typically affected include bluefish, mahi-mahi, tuna, skipjack, and mackerel. Symptoms of histamine poisoning include facial flushing, headache, nausea, and vomiting.
antihistamines, including BenadrylAntihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H1 receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.
Histamine works by binding to histamine receptors on the surface of cells. There are four kinds of receptors, called H1, H2, H3, and H4. The activity of histamine can be blocked by various chemical drugs called antihistamines, which prevent the binding of histamine to these receptors. Conventional antihistamines used to treat allergies block H1 receptors and thus are called H1antagonists. H2antagonists are those drugs, such as cimetidine (Tagamet), that inhibit gastric acid secretion and are used to help heal peptic ulcers.
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