25.34: Hypersensitivities

Hypersensitivity, also known as a hypersensitivity reaction or allergic reaction, is a condition where the body's immune system reacts abnormally to a foreign substance. Such substances, that cause hypersensitivity are referred to as an allergen, could be something typically harmless to most people, like pollen or certain foods.

Types of Hypersensitivities

Hypersensitivity reactions are categorized into four types: Type 1, Type 2, Type 3, and Type 4. Each type has a distinct mechanism associated with specific allergens and reactions.

Type 1 Hypersensitivity

• In Type 1, or immediate hypersensitivity, such as asthma or food allergies, IgE antibodies are expressed on the surface of mast cells following initial exposure. During reexposure, the antigen binds to the IgE antibody on the mast cell surface, resulting in degranulation of histamine and inflammatory mediators.
• Common examples include hay fever, asthma, hives, and food allergies. These reactions can range from mild, such as a runny nose or itchiness, to severe, potentially life-threatening reactions known as anaphylaxis.
• Treatment often involves avoiding the allergen, using medications to control symptoms, and in severe cases, carrying an epinephrine autoinjector for emergency use.

Type 2 Hypersensitivity

• Type 2 hypersensitivity, also known as cytotoxic hypersensitivity, may involve the destruction of host cells by the immune system. The mechanism involves IgG or IgM antibodies binding to antigens on the surface of cells, leading to dysfunction or their destruction through complement activation or antibody-dependent cell-mediated cytotoxicity.
• Examples of diseases caused by Type 2 hypersensitivity include autoimmune hemolytic anemia, Goodpasture syndrome, and transfusion reactions. Treatment typically involves managing symptoms and suppressing the immune response with medications like corticosteroids.

Type 3 Hypersensitivity

• Type 3 hypersensitivity, also known as immune complex hypersensitivity, involves the formation of immune complexes (antigen-antibody complexes) that deposit in tissues and cause inflammation and tissue damage.
• Examples of Type 3 hypersensitivity reactions include systemic lupus erythematosus, post-streptococcal glomerulonephritis, and serum sickness. These conditions are often managed with anti-inflammatory medications and immune suppressants.

Type 4 Hypersensitivity

• Type 4 hypersensitivity, also known as delayed hypersensitivity, is the only type of hypersensitivity that is not antibody-mediated but is instead a cell-mediated response. It involves T cells reacting to antigens, leading to inflammation and cell damage.
• Examples of Type 4 hypersensitivity reactions include contact dermatitis (such as poison ivy rash), tuberculosis skin tests, and transplant rejection. Treatment often involves avoiding the allergen and using topical corticosteroids to reduce inflammation.

Hypersensitivity reactions represent an overactive immune response to substances that are typically harmless. They can lead to various conditions, ranging from mild allergies to severe autoimmune diseases. Understanding the types of hypersensitivities and their mechanisms is crucial for effective treatment and management of these conditions. While avoidance of allergens is a common strategy, medical interventions such as anti-inflammatory medications, immunosuppressants, and emergency treatments like epinephrine are often necessary.