"Allergies are the immune system's specific reaction to a normally harmless substance such as pollens, certain foods, or medicines." "There are two main types, seasonal and perennial. The seasonal variety occurs at specific times of the year, while the perennial variety can happen year-round. In many cases, successful treatment involves medication, reducing contact with specific allergens, or shots."
It is a very important fact which should be in everyone’s knowledge, that those things which are harmless to us might turn dangerous if our immune system shows hypersensitivity to them i.e. if we become allergic. And it takes almost 10 days after first contact when we become sensitive to something. But in case of allergy the time span is relatively short, our body shows reaction within few minutes or it might take a day or two, after coming in contact with that particular thing.