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Antihistamines

Allergies are one of the most common types of acquired health conditions as the causes include various factors. Most of these factors are related to the environment such as dust, pollen, virus and chemicals. Allergic reaction signifies the defense mechanism of the immune system to a foreign particle. Allergic reactions if untreated can lead to bigger complications such as bronchitis and sinusitis. Antihistamines were first synthesized by Daniel Bovet. They play a major role in controlling the release of histamines in the body which are produced as response to an allergen. Histamines are chemicals which are released from the mast cell damage. These chemicals initiate the process of allergic reaction in the blood and tissues which causes discomfort. Histamines have three types of receptors H1, H2 and H3 which are specific for different organ systems. The H1 receptor is associated with vasodilation and smooth muscle stimulation, H2 receptor is associated with the cardiac stimulation and H3 is associated with the feedback inhibition process regulated by the central nervous system.


Types of antihistamines

Antihistamines mainly act upon the receptors of the type of histamine released. Each antihistamine has a tendency to compete in binding to the receptors to inhibit the process of allergic stimulation caused in the organ system. There are two major classes of antihistamines. The first generation and the second generation are classified based on their sedative effect on the patient during the course of their action. The first generation of antihistamines are sedatives as they act upon the parasympathetic nervous system in preventing the release of acetylcholine. Their anticholinergic properties enable them to act fast in providing relief within a few minutes. People who experience motion sickness are often suggested to take antihistamines because it suppresses the ear balance discomfort.

The most common ingredients present in these medications are chlorpheneramine, doxylamine, brompheneramine and dimenhydrinate. Most of these first generation antihistamines are recommended in allergic reactions such as hay fever, viral infection, pollen and dander as they suppress the central nervous system. Patients experience drowsiness for a period of time during the first generation antihistamine treatment.

Second generation antihistamines are not sedative in nature and they have large molecular size and electrostatic nature which makes them noncompeting to cross the blood and brain barrier. The response pertaining to their action is comparatively slow as they do not act upon the central nervous system. Studies have shown the increased administration of non-sedative antihistamines in the recent times. Some of these second generation antihistamines have been banned from the market due to their adverse side effects such as arrhythmia. Most of these antihistamines are prescribed during seasonal allergies.

Side effects of antihistamines

The common side effect of most first generation antihistamines is drowsiness or dizziness. The other side effects include nausea, feeling of dryness in mouth and nostrils and sometimes blurred vision. These happen because of the inhibition of secretions from salivary, nasal and lachrymal glands during the onset of an allergic response. The suppression of central nervous system activity may initiate confusion, disorientation and sometimes hallucination in patients.

Rashes, chest congestion and difficulty in breathing have also been reported as adverse effects of antihistamine administration. In case of H2 receptor antihistamines, diarrhea is significant side effect along with headache. Most of the side effects caused by antihistamine intake subside within a few hours. However increased sensitivity to a certain type of antihistamine has to be immediately reported to the physician.

Anaphylaxis

Anaphylaxis is a hypersensitive reaction due to contact through allergens. These allergens induce an immediate immune response that is life threatening in most occasions. Anaphylactic reactions produce various clinical symptoms such as urticaria, respiratory congestion and gastrointestinal disturbances. Anaphylactic response is caused by certain type of compounds which may include proteins, pollen, venom, hormones and also some food extracts. The immunoglobulin E or IgE is responsible for the onset of anaphylactic reaction when exposed to an allergen.

Mechanism of anaphylactic reaction

A series of reactions take place in the body when anaphylaxis is triggered. The IgE which has a half-life of two days, binds to the fragment crystallizable or FC receptor part of the basophils and mast cells. This process is activated when the person comes in contact with an allergen which in turn mediates cellular de granulation and their release of biogenic amines such as histamine and serotonin. The effects of these biogenic amines may include the smooth muscles which generally constrict at the bronchiolar region and broncho capillary venules. It also causes the arteriole dilation.

Anaphylactic responses are of different types and are associated with allergen sources of different kinds such as pollen, food, insects and also some drugs. It happens because of the antigen contact to which the patient had been previously exposed. This type of antigen is also called an allergen. Systemic anaphylaxis is a serious condition in which the mast cells of the connective tissue become activated resulting in dangerous effects such as airway obstruction, swelling of the epiglottis and ultimately suffocation. Systemic anaphylaxis proceeds into another condition called anaphylactic shock which happens because of suffocation induced by the respective allergen. In addition to these, anaphylaxis may also result because of extreme physical exercise. This condition is also called exercise induced urticaria. Allergies to food like eggs and milk are common among infants. Other than these, underlying conditions such as hay fever, asthma and eczema may also induce anaphylaxis if untreated.


Anaphylactic shock is widely reported in people after consuming foods such as peanuts and Brazil nuts. Such food products induce the anaphylactic reaction in an instant causing severe discomfort and sometimes death. Many people also experience anaphylactic reactions because of penicillin administration. Some reactions of anaphylaxis also include excess production of mucous in the mouth, nose and throat. Atopic patients are highly susceptible to anaphylaxis. Patients having an underlying atopic condition produce high amount of IgE antibodies and hence they must be monitored carefully.

Diagnosis of Anaphylaxis

Anaphylactic reaction is diagnosed based on clinical symptoms such as skin rash (insect bites), hoarse voice, chest congestion, breathing difficulty, diarrhea, vomiting and abdominal cramps. Identification of the source is the predominant factor in the diagnosis of anaphylactic reactions. Thorough examination of the cause with immediate life saving measures must be done. Anaphylaxis can be fatal if not managed properly.

Anaphylaxis Treatment

Avoid foods that induce the condition. In the case of anaphylactic reaction, medical attention must be sought immediately. Oral medications such as Benadryl or diphenhydramine are given. EpiPen or injectable epinephrine is the drug of choice to control the anaphylactic reaction. Intravenous antihistamine drugs and oral steroids are given for a period of time to prevent the reoccurrence of the anaphylactic reactions.


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Bibliography / Reference

Collection of Pages - Last revised Date: August 20, 2019