The most basic allergy symptoms are similar to those of a common cold - snuffling, itchy eyes, and sneezing. An allergy can also cause skin rashes, hives or weals such as contact dermatitis or eczema; these are often caused in reaction to medications.
Hay fever is one example of a very common minor allergy - large percentages of the population suffer from hayfever symptoms in response to airborne pollen. Asthmatics are often allergic to dust mites.
Most allergies are minor annoyances, but they are not something to take lightly. An allergy can also be extremely life-threatening if it is severe, causing anaphylactic shock and a total shutdown of the airways, circulation and every function of the body.