As previously stated, autoimmune diseases are the results of the immune system attacking "self antigens." Several different immunological mechanisms can be responsible. The immunological mechanisms are often described as "hypersensitivity reactions." There are four types of hypersensitivity reactions, three of which are found in autoimmune diseases:
- Type I Hypersensitivity: Type I reactions are primarily due to the class IgE antibodies. This group is not responsible for autoimmunity; it involves mainly reactions to environmental antigens, such as occurs in allergies.
- Type II Hypersensitivity: The mechanism of Type II autoimmune hypersensitivities is a cytotoxic one in which class IgM or IgG antibodies attach directly to self tissues and can destroy those tissues by either triggering the complement cascade or by triggering antibody-dependent cell-mediated cytotoxicity (ADCC) which leads to activation of natural killer cells (NK cells) or CD8 T cells. These antibodies can also sometimes directly interfere with cell function. Examples of autoimmune diseases caused by this mechanism are Goodpasture's syndrome, Autoimmune Hemolytic Anemia, and Grave's Disease, among others.
- Type III Hypersensitivity: These autoimmune reactions are mediated by class IgG antibodies forming antigen/antibody complexes that deposit in various tissues in the body. They can then go on to trigger the complement system or attract neutrophils, leading to inflammation. An example of this mechanism is Systemic Lupus Erythematosus (SLE).
- Type IV Hypersensitivity: The mechanism of this type of autoimmunity is cell-mediated by pre-sensitized CD4 and CD8 T cells. Some autoimmune diseases have a type II or III autoantibody component and a type IV cell-mediated component, such as Type I Diabetes, Rheumatoid Arthritis, and others.
Autoimmune diseases can also be grouped by location:
- Organ-specific autoimmune disease: The autoantibodies attack a specific organ.
- Example: Hashimoto thyroiditis, where autoantibodies damage the thyroid gland.
- Non-organ specific/systemic autoimmune disease: The autoantibodies attack multiple organ systems.
- Example: Systemic lupus erythematosus (SLE), where the autoantibodies may attack any organ in the body.
