Description
Hypersensitivity reactions are overactive responses of the immune system to certain antigens. These reactions can result in a variety of symptoms and conditions, many of which can be severe or life-threatening. Hypersensitivity reactions are classified into four types (Type I, II, III, IV) based on the mechanism of the immune response.
Type I – Immediate
This type of reaction is also known as an immediate or anaphylactic reaction occurring within minutes of exposure to the antigen. It involves immunoglobulin E (IgE) antibodies, which bind to the allergen and then to an Fc receptor on mast cells and basophils, leading to degranulation and release of inflammatory mediators such as histamine.
- Allergic Rhinitis – Inflammatory response of nasal mucosa to allergens like pollen, dust mites.
- Asthma – Chronic inflammation and constriction of bronchial tubes due to allergens.
- Anaphylaxis – Severe, systemic reaction with multi-organ symptoms.
- Atopic Dermatitis (Eczema) – Chronic skin condition causing itchy,
- Food allergies – Allergic reactions to specific food proteins, including those found in peanuts, tree nuts, milk, and shellfish.
Type II – Cytotoxic
These reactions, also known as cytotoxic hypersensitivity reactions, occur when antibodies (IgG or IgM) bind to antigens on the cell surface, leading to cell damage or death, often through the activation of the complement system or through Fc receptor-mediated phagocytosis.
- Hemolytic transfusion reactions – Occur when transfused blood cells are attacked by recipient’s immune system.
- Goodpasture’s syndrome – Autoimmune disorder where antibodies attack the lungs and kidneys.
- Rheumatic fever – A delayed reaction to Streptococcus bacteria causing inflammation in heart, joints, skin, and brain.
- Graves’ disease – Autoimmune disorder where antibodies stimulate thyroid, causing hyperthyroidism.
- Myasthenia gravis – Disorder in which antibodies block or destroy muscles’ receptor sites, causing muscle weakness.
Type III – Immune Complex-Mediated
Type III reactions, also known as immune complex-mediated reactions, occur when antigens bind to antibodies (IgG or IgM), forming immune complexes that deposit in tissues and blood vessels, activating complement and causing inflammation.
- Systemic lupus erythematosus (SLE) – Chronic autoimmune disease where immune complexes deposit in various organs.
- Post-streptococcal glomerulonephritis – Kidney inflammation following a streptococcal infection.
- Serum sickness – Reaction to antiserum derived from a non-human animal source, causing fever, joint pain, rash, and swollen lymph nodes.
- Arthus reaction – Local vasculitis due to deposition of immune complexes following an injection of antigen.
- Rheumatoid arthritis – Chronic inflammatory disorder affecting the small joints in hands and feet due to immune complex deposition.
Type IV – Delayed
Also known as delayed-type hypersensitivity reactions, these occur when antigen-presenting cells activate T-cells, leading to cell-mediated immune responses that result in tissue damage. Symptoms usually take 48-72 hours to develop after exposure to the antigen
- Tuberculin skin test (PPD) – Local skin reaction to the injection of tuberculin, indicating exposure to tuberculosis.
- Contact dermatitis – Skin reaction resulting from exposure to allergens or irritants.
- Chronic transplant rejection – Late stage of organ transplant rejection involving T-cell mediated damage.
- Sarcoidosis – Formation of granulomas in various organs due to T-cell response.
- Coeliac disease – Gluten triggers an immune response that damages the small bowel lining.