An allergy is an abnormal response by the immune system to a foreign substance (called an allergy) that is normally harmless. Often, allergies begin in childhood but can develop at anytime. An estimated 50 million people have an allergy in the United States.
Symptoms vary and can range from mild to severe. The most common symptoms consist of sneezing, watery eyes, and nasal congestion, breathing difficulties (as with asthma), and skin rashes.
Another symptom includes itchy and swollen skin (as with hives or urticaria). Urticaria is a common provocative agent or agents in susceptible subjects,ingested foods, such as shellfish, injected sera or antibiotics such as penicillin. Urticaria allergic reactions develop very suddenly, usually last a few days, and leaves no visible trace.

Conjunctivitis is another allergy typically known as "pink eye", which is inflammation of the conjunctiva and normally occurs in countries with low standards of hygiene.
Anaphylaxis is a hypersensitive state of the body to a foreign protein. It can cause swelling of the body tissue, breathing difficulties, cramps, vomiting and a rapid drop in blood-pressure. If not treated immediately, anaphylaxis can be fatal.
The breathing difficulties in asthma are characterized by wheezing and difficulty in expiration because of muscular spasm in the bronchi. Resent advances in immunology reveal that mast cells in bronchial walls produce immunoglobulin when encountering pollen grains; when more are inhaled, alveolar mast cells burst, causing an asthmatic attack. However, new anti-asthmatic drugs can prevent this, such ans an inhaler, which is breathe directly into the lungs. These types of anti-asthmatic drugs relax the muscles around the airways to relieve coughing, wheezing, and shortness of breath.
All allergies develop in the same way. When the immune system - the body's defense mechanism against toxins, bacteria, and viruses - first mistakes a substance that is harmless for one that is harmful. This response stimulates the production of a special type of antibody, or protein, called immunoglobulin E (IgE). IgE antibodies then attach themselves to mass cells or white blood cell (found mainly in the respiratory and digestive systems). When the person is exposed to the same substance again, the antibodies signal the mast cells or white blood cells to release chemical that cause allergic reactions or symptoms.
Though people can be allergic to many things, more allergic to inhaled substances

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