Disorders in Immunity: Hypersensitivity, Autoimmunity, and Immunodeficiency

Chapter 17: Disorders in Immunity

Overview

This chapter explores the various disorders of the immune system, focusing on hypersensitivity reactions, autoimmune diseases, and immunodeficiency states. Understanding these conditions is essential for recognizing how the immune system can malfunction, leading to disease.

The Immune Response: A Two-Sided Coin

Immunopathology

• Immunopathology is the study of disease states associated with overreactivity or underreactivity of the immune response.

• Overreactivity includes allergy, hypersensitivity, and autoimmunity.

• Underreactivity (hyposensitivity) includes immunodeficiency diseases, where immune function is incomplete, suppressed, or destroyed.

Overview of Disorders of the Immune System

• Primary Immunodeficiency: Congenital, often genetic, such as Severe Combined Immunodeficiency (SCID).

• Secondary Immunodeficiency: Acquired after birth, e.g., AIDS.

• Hypersensitivities: Overreactions of the immune system, classified into four types.

Hypersensitivity States

Hypersensitivity reactions are classified based on the immune mechanisms involved and the clinical manifestations.

Type	Systems and Mechanisms Involved	Examples
Type I: Immediate hypersensitivity	IgE mediated; involves mast cells, basophils, and allergic mediators	Anaphylaxis, allergies such as hay fever, asthma
Type II: Antibody-mediated	IgG, IgM antibodies plus complement act upon cells and cause cell lysis; includes some autoimmune diseases	Blood group incompatibility, pernicious anemia, myasthenia gravis
Type III: Immune complex-mediated	Antibody-mediated inflammation, circulating IgG complexes deposited in basement membranes of target organs; includes some autoimmune diseases	Systemic lupus erythematosus, rheumatoid arthritis, serum sickness, rheumatic fever
Type IV: T-cell-mediated	Delayed hypersensitivity and cytotoxic reactions in tissues; includes some autoimmune diseases	Infection reactions, contact dermatitis, graft rejection

Key Definitions and Concepts

• Allergen: An antigen that triggers a hypersensitivity reaction.

• Autoimmunity: An inappropriate immune response against self-antigens.

• Immunodeficiency: A state where immune function is incomplete, suppressed, or destroyed.

Examples and Applications

• Type I Example: Hay fever and anaphylaxis are classic IgE-mediated allergic responses.

• Type II Example: Blood transfusion reactions due to ABO incompatibility.

• Type III Example: Post-streptococcal glomerulonephritis, where immune complexes deposit in the kidneys.

• Type IV Example: Contact dermatitis from poison ivy exposure.

Additional info:

• Type I hypersensitivity is rapid and can be life-threatening (e.g., anaphylaxis).

• Type II and III involve antibody-mediated mechanisms, but differ in whether the antibodies target cells directly (Type II) or form complexes that deposit in tissues (Type III).

• Type IV is delayed and mediated by T cells rather than antibodies.