Types of Hypersensitivity

Hypersensitivity refers to an exaggerated or abnormal response of the immune system to a substance that is normally harmless. This can lead to various types of immune reactions and associated symptoms. There are four main types of hypersensitivity reactions, commonly referred to as Types I, II, III, and IV, each characterized by distinct mechanisms and immune responses.

1. Type I Hypersensitivity (Immediate Hypersensitivity):
Type I hypersensitivity, also known as immediate hypersensitivity or allergic reactions, involves the rapid release of mediators such as histamine in response to exposure to an allergen. This type of reaction is associated with conditions like allergic rhinitis (hay fever), asthma, and anaphylaxis. Examples of allergens include pollen, animal dander, insect venom, and certain foods. Symptoms usually occur within minutes of exposure and can range from mild itching and sneezing to severe respiratory distress and anaphylactic shock.

2. Type II Hypersensitivity (Cytotoxic Hypersensitivity):
In Type II hypersensitivity reactions, antibodies (usually IgG or IgM) bind to antigens on the surface of target cells, leading to their destruction by immune cells or complement activation. This can result in various autoimmune diseases, blood transfusion reactions, and hemolytic diseases of the newborn. Examples include autoimmune hemolytic anemia and Rh incompatibility in newborns.

3. Type III Hypersensitivity (Immune Complex-Mediated Hypersensitivity):
Type III hypersensitivity involves the formation of immune complexes—antigen-antibody complexes—that can deposit in various tissues, triggering inflammation and tissue damage. This type of reaction is associated with conditions like systemic lupus erythematosus (SLE), rheumatoid arthritis, and certain forms of vasculitis. Immune complexes can accumulate in the kidneys, joints, and blood vessels, leading to tissue injury and chronic inflammation.

4. Type IV Hypersensitivity (Delayed-Type Hypersensitivity):
Type IV hypersensitivity, also known as delayed-type hypersensitivity, is characterized by a delayed immune response that typically occurs 24 to 72 hours after exposure to an antigen. This type of reaction involves the activation of T cells, particularly CD4+ T helper cells and CD8+ cytotoxic T cells. Type IV hypersensitivity is responsible for contact dermatitis (e.g., poison ivy reaction), tuberculin skin tests, and certain autoimmune diseases like multiple sclerosis. It plays a role in graft rejection as well.

It's important to note that while these classifications provide a framework for understanding hypersensitivity reactions, real-life scenarios can be more complex, often involving a combination of immune mechanisms. Furthermore, each type of hypersensitivity reaction can vary in its clinical presentation and severity.

If you suspect you have hypersensitivity-related symptoms or concerns, it's advisable to consult a medical professional for proper diagnosis and management.