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Hypersensitivity Reactions: Immune-Complex Reactions

Hypersensitivity Reactions: Immune-Complex Reactions

Type III hypersensitivity reactions occur when antigen–antibody complexes form and activate the complement system. Normally, these complexes help the clearance of antigens by phagocytes and red blood cells. However, when large numbers of immune complexes are present, they can deposit in tissues—particularly in the walls of blood vessels—leading to inflammation and tissue injury. These deposits trigger complement activation and neutrophil recruitment, resulting in serum...

Drug Toxicity: Allergic Reactions

Drug Toxicity: Allergic Reactions

Drug-related allergies are immune-mediated responses triggered by the administration of pharmacological agents. These hypersensitivity reactions are classified based on the immune mechanisms involved. The four primary types—Type I, II, III, and IV—are mediated by different immunological pathways and exhibit distinct clinical manifestations.Type I Hypersensitivity/ IgE-Mediated Reactions: Immunoglobulin E (IgE) immediately mediates Type I hypersensitivity reactions. Upon initial...

Allergic Drug Reactions

Allergic Drug Reactions

Allergic reactions related to drugs are hypersensitivity responses driven by the immune system and bear no connection to the drug's therapeutic action. While drugs in isolation do not trigger an immune response, they can interact with endogenous proteins to form antigens. These antigens stimulate lymphocytes to produce antibodies. IgE-type antibodies attach themselves to mast cells. Upon subsequent exposure to the same stimulus, the antigen-antibody interaction is initiated, unleashing...

Hypersensitivity Reactions: Delayed Hypersensitivity Reactions

Hypersensitivity Reactions: Delayed Hypersensitivity Reactions

Delayed-Type Hypersensitivity (DTH), or Type IV hypersensitivity, is a cell-mediated immune response. It occurs when T cells, rather than antibodies, mediate a reaction to specific antigens. It is characterized by a delayed onset (1-2 days) and involves the recruitment of macrophages to the inflammation site.The initiation of a DTH response begins with the sensitization of T cells. During this phase, which lasts at least 1-2 weeks, antigen-specific T cells are activated, clonally expanded, and...

Hypersensitivity Reactions: Cytolytic Reactions

Hypersensitivity Reactions: Cytolytic Reactions

Type II hypersensitivity involves IgG and IgM antibodies targeting cell surface antigens, leading to cell destruction. This can occur through complement activation, antibody-dependent cell-mediated cytotoxicity (ADCC), or acting as opsonins for phagocytosis. When excessive, these reactions cause significant tissue damage.Drug-induced hemolytic anemia is a common example, where drugs like penicillin or cephalosporins bind to red blood cells, forming drug-protein complexes. These complexes...

Hypersensitivities

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...

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Updated: Mar 31, 2026

Humanized Mediator Release Assay as a Read-Out for Allergen Potency

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Type 1 reaction masquerading clinically as ENL: A Case Report.

Ashish Khodke, Vanaja P Shetty

|October 28, 2015

Summary

This case study highlights a Type 1 leprosy reaction mimicking Erythema Nodosum Leprosum (ENL). Histopathology revealed heightened T-cell activity, characteristic of Type 1 reactions, not ENL.

Area of Science:

Dermatology
Immunology
Infectious Diseases

Background:

Leprosy presents complex immunological reactions.
Type 1 and Type 2 reactions require accurate differentiation for appropriate management.
Erythema Nodosum Leprosum (ENL) is a common Type 2 reaction, but other conditions can mimic its clinical presentation.

Observation:

A 29-year-old man with lepromatous leprosy presented with recurrent episodes clinically resembling Erythema Nodosum Leprosum (ENL).
The patient had a history of recurrent Type 2 reactions and was treated with thalidomide for two years.
Skin biopsies were taken during two separate episodes, two months apart.

Findings:

Histopathological examination revealed heightened cell-mediated immunity (CMI), indicative of a Type 1 reaction.

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Basophil Activation Test for Investigation of IgE-Mediated Mechanisms in Drug Hypersensitivity

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Crucially, the histology showed no evidence of ENL, despite the clinical presentation.

This suggests a Type 1 reaction can clinically mimic ENL.

Implications:

Accurate differentiation between leprosy reaction types is crucial for appropriate treatment.
This case underscores the importance of histological correlation in diagnosing leprosy reactions.
Recognizing atypical presentations can prevent misdiagnosis and guide effective therapeutic strategies.