In 1963, two British immunologists, Robert Coombs and Philip Gel, classified hypersensitivity reactions into four different types.
- Type I – Immediate
- Type II – Cytotoxic
- Type III – Immune complex-mediated
- Type IV – Cell-mediated reactions or delayed hypersensitivity
Type I - Immediate
Type I reactions are mediated by proteins called IgE antibodies, which are produced by the immune system. These are produced in response to the allergens such as pollen, animal dander, dust mites, or even certain foods. This causes the release of histamine and other chemicals causing inflammation and swelling.
Type I Immediate allergies include:
- Bronchial asthma
- Allergic rhinitis
- Allergic dermatitis
- Atopic eczema
- Food allergies
- Allergic conjunctivitis (eye inflammation)
- Angioedema (a swelling that is similar to hives, but under the skin instead of on the surface)
- Anaphylaxis (a life-threatening reaction that requires immediate treatment. Common with bee stings and severe nut allergies)
Type II - Cytotoxic
This type of allergic reaction is mediated by proteins called IgG and IgM antibodies. The antibodies involved in Type II reactions damage cells by activating a component of immunity called the complement system.
Type II allergic reactions can be seen in certain conditions like:
- Autoimmune hemolytic anemia
- Immune thrombocytopenia
- Autoimmune neutropenia
Type III - Immune complex-mediated
These reactions are also mediated by IgM and IgG antibodies, and sometimes IgA. These antibodies react with the allergen to form immunocomplexes. The build-up of these immunocomplexes result in complement system activation, which leads to polymorphonuclear leukocytes (PMNs) chemotaxis, and eventually to tissue damage.
Type III allergic reactions can be seen in:
- Serum sickness
- Arthus reaction - usually occurs after vaccination
Type IV - Cell-mediated reactions
Cell-mediated reactions do not involve antibodies. Instead, eosinophils, monocytes, or lymphocytes called T-cells are activated by the antigen. These reactions typically take 48-72 hours or longer to appear after contact with the allergen. T-cells are activated as a result of cytokine release, leading to tissue damage.
Many long-term infectious diseases, such as tuberculosis and fungal infections show cell-mediated reactions. Certain skin sensitivity reactions, such as contact dermatitis, belong to this type. Examples include: