Summary of Hypersensitivity Reactions

Summary of Hypersensitivity Reactions

Introduction

Hypersensitivity reactions are exaggerated or inappropriate immune responses to antigens, resulting in tissue damage or disease. These reactions are classified into four main types based on their immunological mechanisms and clinical manifestations.

Type I: Immediate Hypersensitivity

• Definition: Also known as anaphylactic or allergic reactions, Type I hypersensitivity is mediated by IgE antibodies and mast cells.

• Cause: Occurs when IgE antibodies bind to mast cells and basophils, leading to degranulation and release of histamine and other mediators.

• Onset: Immediate; seconds to minutes after exposure to the allergen.

• Cells Involved: Mast cells, basophils.

• Examples: Hay fever, asthma, anaphylaxis.

• Treatment: Localized reactions treated with antihistamines; systemic reactions (anaphylaxis) treated with epinephrine.

Type II: Cytotoxic Hypersensitivity

• Definition: Antibody-mediated cytotoxic reactions involving IgG or IgM antibodies directed against cell surface antigens.

• Cause: Antibodies bind to antigens on cell surfaces, activating complement and leading to cell lysis or phagocytosis.

• Onset: Minutes to hours following exposure.

• Cells Involved: Phagocytes, complement system.

• Examples: Transfusion reactions, hemolytic disease of the newborn (Rh incompatibility).

• Treatment: Cross-matching of blood types, administration of Rh immunoglobulin (RhoGAM).

Type III: Immune Complex-Mediated Hypersensitivity

• Definition: Immune complex-mediated reactions occur when antigen-antibody complexes deposit in tissues, triggering inflammation.

• Cause: Formation of soluble antigen-antibody complexes that activate complement and attract neutrophils.

• Onset: Several hours after exposure.

• Cells Involved: Neutrophils.

• Examples: Serum sickness, Arthus reaction.

• Treatment: Anti-inflammatory and immunosuppressive drugs.

Type IV: Delayed Hypersensitivity

• Definition: Cell-mediated (T-cell) hypersensitivity reactions that do not involve antibodies.

• Cause: Sensitized T cells recognize antigen and release cytokines, recruiting macrophages and causing tissue damage.

• Onset: Hours to days after exposure.

• Cells Involved: Cytotoxic T lymphocytes (CTLs).

• Examples: Contact dermatitis (e.g., poison ivy), tuberculin skin test.

• Treatment: Corticosteroids and immunosuppressive drugs.

Comparison Table: Types of Hypersensitivity Reactions

Type	Name	Mechanism	Onset	Cells Involved	Examples	Treatment
I	Immediate (Anaphylactic)	IgE-mediated mast cell degranulation	Seconds to minutes	Mast cells, basophils	Hay fever, asthma, anaphylaxis	Antihistamines, epinephrine
II	Cytotoxic	Antibody (IgG/IgM) and complement-mediated cell lysis	Minutes to hours	Phagocytes, complement	Transfusion reactions, Rh incompatibility	RhoGAM, cross-matching
III	Immune Complex	Antigen-antibody complex deposition	Several hours	Neutrophils	Serum sickness, Arthus reaction	Anti-inflammatory drugs
IV	Delayed (Cell-mediated)	T cell-mediated tissue damage	Hours to days	CTLs	Contact dermatitis, tuberculin test	Corticosteroids

Additional info: Hypersensitivity reactions are important in clinical immunology and are relevant to the study of infectious diseases, autoimmunity, and allergy management.