The human immune system is responsible for protecting the body from bacteria and viruses. In some cases, the immune system will defend against substances that do not normally pose a threat to the body. These substances are called allergens, and when the body reacts to them it is called an allergic reaction. Some allergic substances can cause allergies, such as: pet dander; insect venom; certain foods, including nuts or shellfish; certain medications, such as penicillin or aspirin; certain plants; pollen or molds. These species can easily cause the body to have an allergic reaction. Symptoms of a mild allergic reaction include: hives (red spots on the skin); itching; nasal congestion (called rhinitis); rash; raspy throat; watery or itchy eyes Severe allergic reactions may cause the following symptoms: abdominal cramps or pain; chest pain or tightness; diarrhea; difficulty swallowing; dizziness (vertigo); fear or anxiety; flushed face; nausea or vomiting; heart palpitations; face, eyes or tongue swelling; weakness; wheezing; difficulty breathing; confusion. Doctors often apply a small amount of a suspected allergen to the skin and observe the reaction. The substance may be applied to the skin (patch test), by small punctures through the skin (skin prick test), or by injection under the skin.