The immune system, designed to protect the body, can sometimes overreact, leading to hypersensitivity reactions that cause tissue damage or disease. This diagram outlines the four types of hypersensitivity—Type I, II, III, and IV—each involving distinct mechanisms and immune components, primarily mediated by B cells or T cells. Exploring these reactions provides a deeper understanding of how the immune system can both defend and, in some cases, harm the body under specific conditions.
Key Labels in the Hypersensitivity Diagram
This section breaks down each labeled component, offering insight into the mechanisms of hypersensitivity reactions.
Allergen: This environmental trigger, such as pollen or food proteins, binds to IgE on mast cells, initiating Type I hypersensitivity. Its presence leads to allergic responses ranging from localized to systemic effects.
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Allergen-specific IgE: This antibody, produced by B cells, attaches to Fc receptors on mast cells and basophils, triggering degranulation upon allergen binding. It is central to the immediate allergic response in Type I hypersensitivity.
Fc receptor for IgE: Located on mast cells, this receptor binds allergen-specific IgE, facilitating the release of histamine and other mediators. Its activation is a key step in the allergic cascade of Type I reactions.
Fc receptor for IgG: Found on cytotoxic T cells or phagocytes, this receptor binds IgG-coated target cells, aiding in Type II hypersensitivity. It enhances the destruction of cells marked by antibodies.
Surface antigen: Present on target cells, this molecule is recognized by antibodies or immune complexes, triggering hypersensitivity reactions. It serves as a marker for immune attack in Types II and III.
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Target cell: This cell, coated with antibodies or immune complexes, is destroyed in Type II hypersensitivity via complement or cytotoxic T cells. Its destruction can lead to conditions like transfusion reactions.
Complement activation: This process amplifies Type II and III hypersensitivity by recruiting immune components like neutrophils. It enhances tissue damage through the formation of the membrane attack complex.
T cell: In Type II, this cytotoxic cell destroys antibody-coated target cells, while in Type IV, Th1 cells secrete cytokines. Its role varies depending on the hypersensitivity type.
Immune complex: Formed by antigen-antibody binding in Type III, these complexes deposit in tissues, triggering inflammation. They attract neutrophils, leading to tissue damage.
Neutrophil: This phagocytic cell is recruited by complement activation in Types II and III, releasing enzymes that damage tissues. Its involvement exacerbates hypersensitivity reactions.
Free-floating immune complex: Circulating antigen-antibody complexes in Type III hypersensitivity deposit in blood vessels or tissues. They initiate inflammation and complement activation.
Sensitized Th1 cell: In Type IV, this T helper cell recognizes antigens and releases cytokines to activate macrophages. Its action drives delayed-type hypersensitivity.
Activated macrophage: Stimulated by Th1 cytokines in Type IV, this cell accumulates at the site, causing tissue damage. It plays a key role in chronic inflammatory responses.
Cytotoxic T cell: In Type IV, this cell directly kills antigen-presenting cells, contributing to hypersensitivity. It is involved in conditions like contact dermatitis.
Cytokines: Secreted by Th1 cells in Type IV, these signaling molecules activate macrophages and cytotoxic T cells. They mediate the inflammatory response.
Degranulation: This process in Type I involves mast cells releasing histamine and other mediators upon IgE cross-linking. It leads to symptoms like anaphylaxis or allergies.
Antigen: In Types III and IV, this molecule binds antibodies or is presented to T cells, initiating hypersensitivity. Its role varies by reaction type.
Mechanisms of Type I Hypersensitivity
Type I hypersensitivity involves immediate allergic reactions. It is driven by IgE-mediated responses.
- Allergen binding to allergen-specific IgE on mast cells triggers degranulation.
- Released mediators cause localized symptoms like rhinitis or systemic anaphylaxis.
- Common triggers include pollen, food allergens like peanuts, and shellfish.
- The response occurs within minutes, reflecting its rapid onset.
- Histamine release leads to vasodilation and increased vascular permeability.
Exploring Type II Hypersensitivity
Type II hypersensitivity targets specific cells for destruction. It involves antibody-mediated cytotoxicity.
- Fc receptor for IgG on T cells binds antibody-coated target cells.
- Complement activation enhances cell lysis, as seen in transfusion reactions.
- Red blood cell destruction occurs in mismatched blood transfusions.
- Erythroblastosis fetalis results from maternal-fetal blood group incompatibility.
- This type requires antibody specificity for surface antigens.
Understanding Type III Hypersensitivity
Type III hypersensitivity results from immune complex deposition. It leads to tissue inflammation.
- Free-floating immune complexes deposit in tissues, activating complement.
- Neutrophil recruitment causes damage in conditions like glomerulonephritis.
- Rheumatoid arthritis and systemic lupus erythematosus are common examples.
- Inflammation arises from enzyme release by neutrophils.
- Chronic exposure to antigens exacerbates this reaction.
Insights into Type IV Hypersensitivity
Type IV hypersensitivity is a delayed T cell-mediated response. It involves cellular immunity.
- Sensitized Th1 cells secrete cytokines to activate activated macrophages.
- Cytotoxic T cells contribute to tissue damage in contact dermatitis.
- Common conditions include tuberculin reactions and type 1 diabetes.
- The response develops over 24-48 hours, differing from immediate types.
- Macrophage accumulation drives chronic inflammation.
Clinical Relevance and Management
Hypersensitivity reactions have significant clinical implications. Understanding them aids in treatment.
- Allergen avoidance is key for managing Type I reactions.
- Immunosuppressants reduce complement activation in Types II and III.
- Cytokines modulation helps control Type IV inflammation.
- Desensitization therapy can mitigate IgE-mediated allergies.
- Monitoring neutrophil activity guides therapy in immune complex diseases.
In conclusion, the immune hypersensitivity diagram illustrates the diverse ways the immune system can overreact, from allergen-driven Type I to T cell-mediated Type IV. Each type highlights unique mechanisms involving B cells, T cells, and complement, offering insights into associated diseases. Recognizing these patterns is essential for developing targeted treatments to mitigate their impact on health.
