The definition of the term allergy is broad and includes several different immunological diseases. Most allergy diseases can be assigned to 4 basic types of allergy. The classification is based on 2 criteria: on the one hand, the reaction mechanism and on the other hand, the reaction time, i.e. the time that elapses after contact with the allergy until the first symptoms occur. 

Type I: immediate type, anaphylactic reaction

This allergy type is characterized by symptoms occurring immediately or within a few minutes. Usually, the skin or mucous membranes are affected. The allergens of type I allergies are almost always proteins. In terms of numbers, type I allergies are the most prevalent.
Examples: hay fever, animal dander, insect sting allergies, latex allergies, dust mite allergies, food allergy, allergies to drugs (local anesthetics, antibiotics), allergic asthma, hives, angioedema, anaphylactic shock

Type II: allergies of the cytotoxic type

With type II allergies, the body's own cells become so damaged that they are classified by the body as allergens. Specific antibodies are bound directly to the surface of the cells thereby making the cell itself an antigen that must be fought. Type II allergies are very rare. Examples: hemolytic transfusion incidents, rhesus incompatibility in newborns, autoimmune hemolytic anemias, myasthenia gravis, agranulocytosis, blood group incompatibility

Type III: immune complex type allergies

Medication allergies are often manifested by type III reactions. In this case, foreign substances dissolved in the blood, for ex. medications, bond with antibodies and then with the blood vessel walls in the smaller blood vessels, for instance in the skin or kidneys. This leads to local inflammatory reactions, which can usually be seen on the skin in the form of a rash (exanthema), possibly in conjunction with bleeding, damage to the skin, and itching. Typical type III allergic diseases are: vasculitis (vascular inflammation), serum sickness, alveolitis (inflammation of the alveoli), farmer's lung, nephritis (inflammation of the kidneys), arthritis (inflammation of the joints)

Type IV: cellular immune response, delayed type allergy

Characteristic of type IV allergies is the delayed appearance of the allergic reaction. Like with type II, allergens or foreign substances attach to body cells and activate specific T lymphocytes, which recognize and fight these cells. This leads to damage of the surrounding tissue. With this "first contact", the T cells memorize the defense response that is taking place and as soon as they come into contact with the same allergen, they trigger acute allergic reactions. However, since the (re)activation of the T lymphocytes and cell migration take some time, the allergic reaction doesn't occur immediately but only 12 to 72 hours after contact with the allergen.  

Examples of type IV reactions: transplant rejection, contact allergies, for ex. nickel allergy, tuberculin reaction (TBC test), drug-related exanthema

Source: Prof. Dr. med. Dr. h. c. Torsten Zuberbier, Dezember 2013