Allergic reactions

An allergic reaction occurs when the immune system responds in an abnormal fashion to a substance that is foreign to the body. The immune system normally protects the body from harmful agents like bacteria and toxins. The body's abnormal response to a substance that is usually harmless (called an allergen) is called a hypersensitivity reaction, or an allergic reaction. Generally, these reactions are due to interactions between the allergen and the family of proteins, IgE (immunoglobulin E), resulting in activation of cells in the body called mast cells and basophils. These cells release chemical messengers in the body that cause the symptoms of an allergic reaction.
Many substances in the environment can become allergens but only in genetically susceptible people. Dust mites, molds, animal hair or danders, pollens, medications, foods, and insect venoms are examples of common allergens. Reactions may be in the nose (hay fever), eyes (conjunctivitis), chest (asthma), or it can be systemic (anaphylaxis), meaning it can involve the entire body.
Allergic rhinitis is an inflammation of the nasal membranes (along with membranes of the eyes, eustachian tubes, middle ear, sinuses, and throat) due to an allergic reaction. It is the most common cause of inflammation in the nose (rhinitis). Approximately 20% of the U.S. population is believed to suffer from allergic rhinitis
 
 What Are the Risks of Allergies?In rare cases, an allergic reaction can be life-threatening, such as with asthma or anaphylaxis . However, most allergic reactions  are less serious.
Allergic rhinitis can be associated with other conditions, including asthma, atopic dermatitis, and nasal polyps. It can also have complications such as otitis media, eustachian tube dysfunction, sinusitis, and conjunctivitis. Symptoms of allergic rhinitis can in some cases contribute to or worsen learning difficulties, sleep disorders, and fatigue.

 Treatments  Used for Allergies
Although it is not always possible, one should attempt to avoid contact with known or suspected allergens. (If you have cats and are allergic to cat dander, it is best to reconsider having cats.) At first, some nonprescription antihistamines or decongestants may be tried to decrease nasal congestion (that "stuffed up" feeling), a runny nose, and itching or watery eyes. If symptoms persist, a doctor or health-care professional may prescribe a prescription-strength medication. The following sections discuss some common medications for allergies and hay fever.