Table of Contents
Gell and Coombs' classification
Gell and Coombs' classification is a widely used system to categorize hypersensitivity (allergic) reactions into four types based on their underlying mechanisms and timing of the response. This classification helps in understanding and managing various allergic diseases.
Type I hypersensitivity (or immediate hypersensitivity). It is also known as an immediate reaction and involves immunoglobulin E (IgE) mediated release of antibodies against the soluble antigen. This result in mast cell degranulation and release of histamine and other inflamatory mediators. Some examples; allergic asthma, Food allergy and Penicillin allergy etc.
Type II hypersensitivity (or Antibody - Mediated Cytotoxic Hypersensitivity. It is an antibody mediated process in which IgG and IgM antibodies are directed against antigens on cells (such as circulating red blood cell) or extracellular material (such as basement membrane). This subsequently leads to cell lysis, tissue damage or loss of function through mechanisms such as (i) Complement activation via the classical complement pathway, (ii) Antibody dependent cellular cytotoxicity, (iii) Anti - receptor activity. Some examples are Autoimmune hemolytic anemia, Goodpasture syndrome and Graves disease etc.
Type III hypersensitivity (or immune complex - mediated hypersensitivity). It occurs when there is accumulation of immune complexs (antigen - antibody complexes) that have not been adequately cleared by innate immune cells, giving rise to an inflammatory response and attraction of leukocytes. There are three steps that lead to this response. The first step is immune complex formation, which involves the binding of antigens to antibodies to form mobile immune complexes. The second step is immune complex deposition, during which the complexes leave the plasma and are deposited into tissues, and the third step is the inflammatory reaction, during which the classical pathway is activated and macrophages and neutrophils are recruited to the affected tissues.
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Type IV hypersensitivity, often called delayed type hypersensitivity is a type of hypersensitivity reaction that can take a day or more to develop. Unlike the other types, it is not humoral (not antibody mediated) but rather is a type of cell - mediated response. This response involves the interaction of T cells, monocytes and macrophages. Example; Allergic contact dermatitis and Tuberculin reaction.
Brief description of various types of hypersensitivities
Hypersensitivity is an exaggerated or inappropriate immune system response to a substance or antigen that usually wouldn't cause an immune response. It can lead to allergy symptoms that range from mild to life - threatening.
There are four traditional classification for hypersensitivity reactions, and these include Type I, Type II, Type III and Type IV reaction:
Type I hypersensitivities, is also known as an immediate reaction and involves immunoglobulin E (IgE) mediated release of antibodies against the histamine and other inflamatory mediators.
Type II hypersensitivity is also known as cytotoxic reactions and engages IgE and IgM antibodies, leading to the complement system activation and cell damage or lysis.
Type III hypersensitivity is also known as immune complex reaction and involves IgE, IgM, and sometimes IgA antibodies. The build up of these immune complexes results in complement system activation, which leads to polymorphonuclear leukocytes (PMNs) chemotaxis and eventually causing tissue damage.
Type IV hypersensitivity is also known as delayed - type and involves of T- cell - mediated reactions. T- cells or macrophages are activated as a result of cytokine release, leading to tissue damage.
Autoimmunity and Immunodeficiency
Autoimmunity
Autoimmunity is the system of immune responses of an organism against its healthy cells and tissues. A disease which results due to this kind of response is called an autoimmune disease.
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The higher vertebraes are capable of recognising foreign antigens. Due to the memory - based acquired immunity, their immune system could very well distinguish between its cells and the foreign organisms. However, due to a genetic condition or some other unknown reason, the body attacks its own cells. The resulting diseases are called autoimmune diseases. The most common example of an autoimmune disease is rheumatoid arthritis (RA).
Immunodeficiency
Immunodeficiency refers to a state where the immune system's ability to fight infections diseases and cancer is compromised or entirely absent. It can be broadly classified into two main types:
1. Primary (Congenital) Immunodeficiency:
These are genetic disorders present from birth. Examples include sever combined immunodeficiency (SCID) , common variables immunodeficiency (CVID).
2. Secondary (Acquired) immunodeficiency:
These occur due to external factors affecting the immune system. Causes include infections like HIV/AlDS , and chronic disease like diabetes and cancer.
Symptoms of immunodeficiency can vary widely but often include frequent infections, chronic infections, and infections that are unusually severe or difficult to treat.
FAQs
What is the immune system?
The immune system is the body's defense mechanism against infections, diseases, and foreign substances. It comprises various cells, tissues, and organs that work together to identify and eliminate harmful invaders.
What is hypersensitivity?
Hypersensitivity refers to an exaggerated or inappropriate immune response to an antigen, which can cause tissue damage and disease.
What is autoimmunity?
Autoimmunity occurs when the immune system mistakenly attacks the body's own cells and tissues, leading to autoimmune diseases such as type 1 diabetes, multiple sclerosis, and rheumatoid arthritis.
What causes autoimmune diseases?
The exact cause is unknown, but a combination of genetic, environmental, and hormonal factors is believed to contribute. Infections and certain medications can also trigger autoimmunity.
