HistamineWhen you have an allergy to something, you exhibit certain common symptoms. Sneezing, itchy and watery eyes, an itchy throat, a runny nose, and itchy rashes are all possible symptoms of an allergy. Essentially this is an overreaction by the body's immune system to substance, or substances, that would not normally provoke such a response in other people. Common substances such as pollen, dust particles, animal dander (pieces of dry skin, hair and fur), and mould spores can prompt a severe reaction in people who are allergic to them. These substances are collectively referred to as "allergens". Allergens usually contain protein, an organic compound that consists of hydrogen, oxygen, and nitrogen. Penicillin is an example of a non-protein allergen that needs to be bound to a protein once it enter the body in order to cause an allergic reaction. When the body encounters allergens, it releases a natural chemical called histamine, among others, that is produced by 'mast cells' in the body. These special 'mast cells', or tissue cells, are part of the immune system and are found in the deep vascular inner layer of the skin, outside of the capillaries that circulate red blood cells throughout the body. Mast cells do not usually circulate in the blood stream but settle in the connective tissue in the dermis. Antigens are what normally trigger an immune response in the body. Allergens are a specific type of antigen. The difference between the two being that, while allergens are harmless to those without an allergy, antigens are universally harmful and include types bacteria and viruses. If you are allergic to something then you have antibodies present in your system that will target that specific antigen. The mast cell reacts to the presence of antigens that attach to the mast cell's Immunoglobulin E (IgE) antibodies and cause granules which contain histamine, as well as other vasoactive chemicals, within the mast cell to be released into the surrounding tissue. IgE antibodies can also be found in circulation, bound to basophils (blood cells). These potent inflammatory mediators trigger inflammatory materials to be released which in turn cause fluids to leak from the capillaries and the white blood cells creating, amongst other things, a red, itchy welt on the surface of the skin. Histamine is an agonist that is present in all animal tissues. It is a neutral substance that is usually found inside cells. Once it is released from the cells, however, it serves as a vasoactive chemical. Because of these chemical properties, histamine is responsible for most of the symptoms commonly associated with allergies. Histamine is produced by the immune system in response to tissue damage, causing the blood vessels to dilate, allowing other mediators of inflammation to enter the area. The red colouration on the surface of the skin is caused by the dilation of the capillaries. The itchiness is the result of histamine's effect on nerves in close proximity, functioning as a neurotransmitter. The swelling, or edema, is a consequence of protein-rich fluids leaking into nearby skin after the blood vessels relax and dilate. Histamine also increases bodily secretions, including mucus. Most allergies are not a danger to life. Anaphylactic shock, however, is a serious, potentially life-threatening condition that can occur if a person is exposed to an antigen that they are especially sensitised to, like bee or wasp venom, causing a rapid allergic reaction involving multiple parts of the body. In these cases, the body's immune system mistakenly overreacts to something that is not life-threatening, and, thinking that it poses great danger, releases excessive amounts of histamine. In these cases histamine can cause severe systemic changes. The smooth muscle tissue, also known as "involuntary muscles", in the lungs and the gastrointestinal system contract, resulting in hypotension (low blood pressure) and possible stomach pains, vasodilation (dilation of blood vessel walls), and tachycardia (increased heart rate). This results in an inadequate amount of blood being pumped to the body's cells because of the drop in blood pressure. Other symptoms of anaphylactic shock can include respiratory problems, itching, urticaria or "hives", fainting, and swelling (angioedema) of the mucous membranes or throat.AntihistaminesThe best way to combat these symptoms is to use an antihistamine that, as the name suggests, blocks the amount of histamine that is produced by the body. Histamine is the primary effector substance released by the mast cell in response to an allergen. Antihistamines block H1 (histamine, type 1) receptors found on blood vessel endothelial cells and smooth muscle cells. The smooth muscles are located in the breathing passages, internal organs, intestines, and blood vessels of the body. Antihistamines are antagonist in that they prevent histamines from binding to the receptors in cells by essentially beating them to it. Antihistamines are competitive inhibitors that bind to the receptors first because they are more numerous. The IgE antibodies will still be developed and the mast cells will still be activated due to the allergy. Because the mast cells release other chemicals apart from histamine, an antihistamine can only diminish an allergic reaction and cannot block it completely. Antihistamines can temporarily relieve the symptoms of allergies, hayfever, upper respiratory complaints and those symptoms usually associated with the common cold, especially when combined with a decongestant. A combined nasal spray is also useful in treating vasomotor rhinitis, which has symptoms that also include a chronic stuffy nose but is triggered by factors like emotional stress, pregnancy and thyroid disease. Antihistamines and decongestant combinations do not treat the cause of the allergy or the cold but they do alleviate the symptoms. The use of antihistamines to treat hayfever will also help to relieve asthma and reduce the severity of its symptoms. Antihistamines are available in tablet, nasal spray, cream, and eye drop form. The eye drops are useful for the red and itchy eyes often caused by airborne allergens like pollen. In these cases, application of antihistamine directly to the eye will provide more effectively relief than oral antihistamines. Because histamine is a major component of many stings and venom, antihistamines are commonly used to treat stings and bites. Antihistamines, H1 and H2 varieties, have also been used to reduce stiffness, control gastric secretion, motion sickness and vomiting, treat duodenal ulcers, and even as a treatment for Parkinson's disease, Crohn's disease, and acute multiple sclerosis.