Allergies occur when the body has a hypersensitive reaction to a substance that is actually harmless. The reason for this is a malfunction of the body's own defense system against disease - the immune system. It can cause you to sneeze or have a runny or stuffy nose. Allergies also often cause skin rashes and itching, red or watery eyes. However, even more severe reactions can occur, such as allergic shock or allergic asthma. There are many different types of allergies. Common triggers include pet dander, dust, mold, preservatives, medications, cosmetics, metals, or food. Hay fever is an allergy to pollen from plants. Allergic reactions can vary in severity. Some allergies exist from birth, others develop later in life. Infants who are breastfed for a longer time seem to develop allergies less often than babies who are weaned early.