Anti-allergic agent
Anti-allergic agents are medications used to treat allergic reactions. Anti-allergic agents have existed since 3000 B.C in countries such as China and Egypt. It was not until 1933 when antihistamines, the first type of anti-allergic agents, were developed. Common allergic diseases include allergic rhinitis, allergic asthma and atopic dermatitis with varying symptoms, including runny nose, watery eyes, itchiness, coughing, and shortness of breath. More than one-third of the world's population is currently being affected by one or more allergic conditions.
Commonly used anti-allergic agents include antihistamines, decongestants, corticosteroids, leukotriene pathway modifiers and mast cell stabilizers. Antihistamines and decongestants are generally the first-line treatment in mild to moderate allergic reactions. Corticosteroids are generally indicated for moderate cases. In severe cases, adrenaline is used to reduce swelling and aid breathing. Allergen immunotherapy is an alternative treatment considered in some patients, with a success rate of 80%-90% in reducing symptoms, but requiring a much longer duration of therapy. The choice of medications depends on the disease to be treated, its severity and patient factors.
Allergy
, also known as type 1 hypersensitivity, are a type of immune response which takes place when our body's protective mechanism defends us against triggers called allergens.Common causes
An allergic reaction can be caused by direct contact with an allergen. For example, through consuming a certain food, inhalation of pollens or dust mites, or direct contact with a certain material. A family history of allergies also leads to a higher risk of developing allergic diseases.| Allergen | Examples |
| Environmental | Airborne: pollen, pet dander, mould, dust mites, smoke Skin: plants, cosmetics, cleaning detergents, insect stings, metals such as nickel and chromium |
| Food | Milk, eggs, nuts, shellfish, soy and more |
| Medications | Antibiotics, sulfonamides, NSAIDs, chemotherapy drugs and more. |
Mechanism
When a person comes in contact with a specific trigger, their immune system produces antibodies called immunoglobulin E molecules that bind to mast cells and triggers the release of histamine and other chemicals that guards our immune system. Histamine increases the level of vascular permeability and increases the swelling and dilation of vessels. This induces allergic symptoms, such as runny nose, sneezing, watery eyes and itching.Types
Allergic reactions can range from mild to severe. Common allergic diseases include allergic rhinitis, allergic asthma, allergic conjunctivitis and atopic dermatitis. Each condition presents its own set of symptoms. In severe cases, certain allergies can lead to a life-threatening reaction called anaphylaxis. This is a medical emergency that requires immediate treatment, as it can be fatal if not treated properly.| Type of allergic disease | Common signs and symptoms |
| Allergic rhinitis | Sneezing, runny or stuffy nose, itching, watery eyes |
| Allergic asthma | Coughing, wheezing, chest tightness, shortness of breath |
| Allergic conjunctivitis | Redness, itching, and swelling of the eyes |
| Atopic dermatitis | Itching, redness, and flaking of the skin |
| Anaphylaxis | Flushing, shortness of breath, dizziness, loss of consciousness |
Symptoms can vary in severity from person to person. Proper diagnosis and management are crucial as allergies can affect daily activities, sleep quality, work or school performance, and mood, directly impacting an individual's quality of life.
Management
The most effective management of allergies is avoiding allergens to reduce the risk of an allergic reaction. If the allergic reaction continues, medications may be needed to help reduce symptoms.Common anti-allergic agents
Antihistamines
s, specifically H1-antihistamines, are medicines which provide relief for allergic symptoms such as runny nose, sneezing, itching, and watery eyes from seasonal allergies. They are usually the first line of medications prescribed by a general practitioner, or a pharmacist for allergies in a community pharmacy. H1-antihistamines are further split into three groups known as the first-generation, second-generation and third-generation antihistamines. Another type of antihistamines known as H2-antihistamines are used to treat gastrointestinal conditions caused by excessive stomach acid.Mechanism
H-1 antihistamines work by inhibiting histamine from binding to H-1 receptors. Histamine receptors expressed in smooth muscles, vascular endothelial cells, the heart, and the central nervous system. This prevents a range of cellular signaling cascades that lead to vasodilation and increased membrane permeability from happening, thus preventing allergic symptoms.
First-generation antihistamines can easily cross the blood-brain barrier into the central nervous system to reach the H-1 receptors within, often causing drowsiness. Second-generation antihistamines selectively bind to the peripheral H-1 receptors outside the blood-brain barrier, therefore they are less likely to cause sedation. First-generation antihistamines usually last around 4–6 hours whilst second-generation antihistamines work for 12–24 hours. Third generation antihistamines are metabolites of the second-generation antihistamines but without cardiac toxicity.
Indications and route of administration
Antihistamines are generally indicated for the alleviation of conditions such as allergic rhinitis, allergic conjunctivitis, or atopic dermatitis. Other indications may include nausea and vomiting. Antihistamines are usually for short-term treatment. Chronic allergies, such as allergic asthma may include health problems which antihistamines alone cannot treat. The routes of administration of antihistamines are usually oral, intranasal, intraocular and topical.
| Class of antihistamines | Examples | Uses | Route of administration |
| First generation | Diphenhydramine, Chlorpheniramine, Brompheniramine, Promethazine | Allergic rhinitis, allergic conjunctivitis, atopic dermatitis, urticaria, itching | Oral, topical, intraocular |
| Second generation | Cetirizine, Loratadine, Acrivastine, Azelastine | Allergic rhinitis, allergic conjunctivitis, atopic dermatitis, urticaria, itching | Oral, intranasal |
| Third generation | Levocetirizine, Desloratadine, Fexofenadine | Allergic rhinitis, allergic conjunctivitis, atopic dermatitis, urticaria, itching | Oral |
Side effects
Side effects generally exist more in first-generation antihistamines, for example drowsiness, blurred vision and dry mouth. Other common side effects for all generations include gastrointestinal side effects, headaches and tiredness.
Antihistamines are generally safe to use during pregnancy with second-generation antihistamines being more preferred due to their lack of sedation.
Decongestants
are medicines which relieve nasal obstruction due to inflammation in the upper respiratory tract. They are also often used as the first-line therapy for nasal congestion from common cold or allergic rhinitis.Mechanism
Decongestants work by binding to the alpha-adrenergic receptors in the sympathetic nervous system on the smooth muscle cells situated on the blood vessels in nasal passages. This causes vasoconstriction, helps reduce blood flow to the nasal mucosa and decreases swelling. Decongestants help open up nasal passages thus allowing for easier breathing.
Indications and route of administration
Decongestants are mainly used in conditions like common cold, allergic rhinitis, and sinusitis. They provide temporary relief from symptoms of nasal congestion. Decongestants are available in oral and intranasal forms. Naphazoline and oxymetazoline are common topical decongestants, whilst pseudoephedrine is the most common example of oral decongestant used to reduce nasal congestion. Topical decongestants have a faster onset of action compared with oral ones.
Side effects
Topical decongestants should not be used for longer than a week as prolonged use may result in rhinitis medicamentosa.
Common side effects for oral decongestants include insomnia, hypertension, and difficulty in urination.
Avoid use during pregnancy as it may cause vasoconstriction of uterine arteries thus reducing fetal blood supply.
Combination therapy with antihistamines
Antihistamines and decongestants can be used as a combination to treat nasal congestion, runny nose, and sneezing symptoms caused by common cold and hay fever.
Some examples include:
- Pseudoephedrine + Loratadine
- Pseudoephedrine + Cetirizine
- Pseudoephedrine + Fexofenadine
Corticosteroids