An allergy is a strong reaction to a substance. Many things can cause allergies. The most common cause is pollen  , which is usually produced by trees in the spring, grasses in the summer and weeds in the fall, as part of their reproductive process. Other causes include organisms, chemicals, plants and dead skin particles from dogs and cats can also cause allergic reactions. So can insect stings and some food. The most common kind of allergic reactions is itchy   , watery eyes and a blocked or watery nose. Allergies can also cause red, itchy skin. Some reactions can be life-threatening, for example, when breathing passages become blocked. It is not always easy to avoid allergies. Drugs may offer an effective treatment. Another treatment used in some cases is called immunotherapy. A patient is injected with small amounts of the allergy-causing substance. The idea is that larger and larger amounts are given over time until the patient develops resistance in his body. In the United States, experts estimate that up to 8% of young children have food allergies. Every year these allergies cause about 30,000 cases with severe reactions that require immediate treatment. It can result in breathing trouble and in some cases death. It is said that about 100 to 200 people will die. It is said that most of the reactions are caused by peanuts and tree nuts like walnuts. People can also be allergic to medicines. The American Academy of Allergy, Asthma and Immunology says about 10% of bad reactions to commonly used medicines are allergic. In other words, a person's immune system overreacts and produces an allergic reaction. The most common reactions include skin rashes, itching, breathing problems and swelling in areas such as the face. Allergies seldom occur in winter because   _  .
plants don't usually flower in cold weather people usually wear thick clothes pollen is active below freezing point the weather is too cold for viruses