Allergic Reactions – Four Types

The immune system is an integral part of human protection against disease but the normally protective immune mechanisms can sometimes cause detrimental reactions in the host. All types of allergic reactions are caused by the hypersensitivity of the immune system to an allergen. Any item, chemical or substance that causes an allergic reaction is an allergen.

Different immunoglobulins (Igs) are involved in different types of allergic reactions. The main groups of Igs are IgE, IgG, IgA, IgM and IgD.


The four main types of allergic reactions are listed below along with brief explanation:


1. Type I (IgE-mediated and anaphylactic)

Type I is most commonly associated with allergic reactions to drugs such as chemotherapy medicine.  These reactions are immediate and may occur within seconds or few minutes, especially if the body has been exposed to the foreign substance before and has been ‘sensitized’.

Examples of this type of reaction are hay fever, allergic asthma, hives (urticaria), food allergies etc.


2. Type II (Cytotoxic, cell reactions)

With Type II reactions, the antibodies produced during an immune response recognise and bind to antigens (structural components of cell surfaces).  This antibody/antigen complex then activates ‘classical’ pathways in the immune system to cause inflammation at the site. This creates a defect on the cell’s surface leading to breaking open of the cell and eventually killing it.

Examples of this type of allergic reaction are transfusion reactions, autoimmune hemolytic anemia.


3. Type III (Immune-complex)

In Type III reactions, immune complexes are formed in the circulation and deposit in various tissues where they may trigger the classical pathways in the immune system.  This process may occur in hours to days from the triggering substance.

Examples of this type of allergic reaction are serum sickness, systemic lupus erythematosus immune-complex glomerulonephritis (a disorder of the kidney).


4. Type IV (cell-mediated)

This type of reaction is a delayed reaction (2-3 days) and involves activation of the T-cells of the immune system. The foreign substance is presented to the T-cells of the immune system, which recognises them and sets off a series of reactions that eventually work to destroy the targeted cells.

Examples of this type of reaction are contact dermatitis (poison ivy), rejection of a transplanted organ etc.