Lesson 3.4: Hypersensitivity, Immunodeficiency, and Autoimmunity

Introduction

In this lesson, students will explore the complex interplay between hypersensitivity reactions, immunodeficiencies, and autoimmune diseases. Understanding these concepts is crucial for grasping the underlying principles of immunology as they pertain to various clinical conditions. By the end of this lesson, students should be able to classify hypersensitivity reactions, describe the characteristics of primary and acquired immunodeficiencies, and explain the mechanisms leading to autoimmunity.

Learning Objectives

• Understand the four types of hypersensitivity reactions and the corresponding disorders and mechanisms.
• Distinguish between primary and acquired immunodeficiencies, including their characteristic infections.
• Explain the mechanisms of autoimmunity and the failure of self-tolerance.
• Classify hypersensitivity reactions and provide clinical examples.
• Connect different immunodeficiency patterns to the infections they predispose to.

Section 1: Hypersensitivity Reactions

Hypersensitivity reactions are exaggerated immune responses that can cause tissue damage and dysfunction. They are classified into four types based on the underlying mechanisms and the type of immune response involved. Understanding each type and its prototypical disorders is essential for recognizing and managing allergies and immune-mediated diseases.

1.1 Type I Hypersensitivity

Type I hypersensitivity, also known as immediate hypersensitivity, is mediated by IgE antibodies. When a susceptible individual encounters an allergen, the immune system mounts a response involving mast cells and basophils, which release histamines and other mediators, leading to symptoms such as itching, swelling, and bronchoconstriction.

Prototypical Disorders

• Allergic rhinitis (hay fever)
• Asthma
• Anaphylaxis

Mechanism

1. Sensitization: Initial exposure to an allergen results in the production of IgE antibodies.
2. Re-exposure: On subsequent exposure, the allergen binds to IgE on mast cells, triggering degranulation and the release of inflammatory mediators.

Worked Example

Consider a patient who experiences severe allergic reactions to pollen each spring. Upon first exposure, the patient’s immune system produces IgE against pollen. On re-exposure, a substantial amount of histamine is released, causing nasal congestion and wheezing. This illustrates the mechanism of type I hypersensitivity well.

1.2 Type II Hypersensitivity

Type II hypersensitivity involves IgG or IgM antibodies directed against antigens on cell surfaces or extracellular matrix. This leads to cellular damage via mechanisms such as complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC).

Prototypical Disorders

• Hemolytic anemia
• Goodpasture syndrome
• Rheumatic fever

Mechanism

1. Antibodies bind to specific antigens on target cells.
2. Complement activation or recruitment of immune cells results in cell damage.

Worked Example

In hemolytic anemia, antibodies directed against red blood cell antigens lead to their destruction. The binding of IgG to red blood cells activates complement, resulting in lysis, which can cause symptoms of anemia.

1.3 Type III Hypersensitivity

Type III hypersensitivity is characterized by the formation of immune complexes that precipitate in tissues, activating complement and leading to inflammation and tissue damage. It generally involves IgG antibodies.

Prototypical Disorders

• Systemic lupus erythematosus (SLE)
• Serum sickness
• Post-streptococcal glomerulonephritis

Mechanism

1. The formation of immune complexes between antigens and antibodies.
2. Deposition of these complexes in tissues leads to activation of complement and recruitment of leukocytes.

Worked Example

In post-streptococcal glomerulonephritis, immune complexes deposited in the kidneys lead to inflammation, resulting in symptoms such as hematuria and proteinuria.

1.4 Type IV Hypersensitivity

Type IV hypersensitivity is a delayed-type hypersensitivity response mediated by T cells rather than antibodies. This type of reaction often takes hours to days to develop after exposure to an antigen.

Prototypical Disorders

• Contact dermatitis
• Tuberculosis skin test reaction
• Type 1 diabetes mellitus

Mechanism

1. T cell activation upon exposure to an antigen.
2. Release of cytokines that recruit macrophages and cytotoxic T cells, leading to tissue damage.

Worked Example

In contact dermatitis caused by poison ivy, T cells sensitized to the urushiol oil in the plant mount a response upon re-exposure, resulting in an itchy rash that develops days after contact.

Section 2: Immunodeficiencies

Immunodeficiencies are conditions where the immune system's ability to fight infectious diseases is compromised. They can be classified into primary (congenital) and acquired immunodeficiencies.

2.1 Primary Immunodeficiencies

These are often genetic and can involve defects in B cells, T cells, or phagocytic cells. They can present early in life and often lead to recurrent infections.

Examples of Conditions

• Severe combined immunodeficiency (SCID): characterized by both B and T cell dysfunction, leading to severe susceptibility to infections.
• X-linked agammaglobulinemia: results from a mutation affecting B cell development, causing recurrent bacterial infections.

2.2 Acquired Immunodeficiencies

Acquired immunodeficiencies develop due to external factors such as infections (e.g., HIV), malnutrition, or the effects of medications (e.g., chemotherapy).

Characteristic Infections

• HIV infection leads to progressive loss of CD4+ T cells, predisposing individuals to opportunistic infections such as Pneumocystis jirovecii pneumonia and certain cancers.

Connection of Patterns to Infections

Different immunodeficiencies predispose individuals to specific types of infections:

• B cell deficiencies typically result in increased susceptibility to encapsulated bacteria.
• T cell deficiencies lead to increased risk of viral infections and intracellular pathogens.

Section 3: Autoimmunity

Autoimmunity occurs when the immune system mistakenly attacks the body’s own tissues, which can arise due to failures in self-tolerance.

3.1 Mechanisms of Autoimmunity

The mechanisms of autoimmunity can involve genetic predisposition, environmental factors, and breakdowns in normal immune regulation. Factors contributing to autoimmunity include:

• Genetic susceptibility to autoimmune diseases (e.g., HLA allele associations)
• Molecular mimicry, where foreign antigens resemble self-antigens
• Dysregulation of immune tolerance mechanisms

3.2 Examples of Autoimmune Diseases

• Rheumatoid arthritis: characterized by chronic inflammation of joints.
• Type 1 diabetes mellitus: involves the immune destruction of insulin-producing beta cells in the pancreas.
• Multiple sclerosis: involves damage to myelin sheaths in the central nervous system.

Mechanism of Autoimmune Disease Development

1. Loss of self-tolerance: Typically, the immune system distinguishes between self and non-self antigens. Failures in this process can lead to the initiation of an autoimmune response.
2. Clinical Manifestations: Depending on the target of the autoimmune response, symptoms can vary widely from joint pain in rheumatoid arthritis to hyperglycemia in type 1 diabetes.

Conclusion

In this lesson, students has learned about hypersensitivity reactions, the different types of immunodeficiencies, and the mechanisms behind autoimmunity. These concepts are not only foundational to understanding immunology but are also critical in clinical applications, especially in the diagnosis and management of various diseases. Recognizing the interactions between these components of the immune system will aid in the thorough understanding of disease pathology and treatment.

Study Notes

• Type I Hypersensitivity: IgE-mediated; examples include asthma and anaphylaxis.
• Type II Hypersensitivity: IgG or IgM-mediated; examples include hemolytic anemia and Goodpasture syndrome.
• Type III Hypersensitivity: immune complex-mediated; conditions include SLE and post-streptococcal glomerulonephritis.
• Type IV Hypersensitivity: T cell-mediated; examples are contact dermatitis and type 1 diabetes.
• Primary Immunodeficiencies: Genetic disorders leading to increased infection susceptibility.
• Acquired Immunodeficiencies: Develop due to external factors such as HIV or medications.
• Autoimmunity: Immune response against self; caused by genetic and environmental factors.