M4 - Vaccines and Hypersensitivities: Immunological Principles and Disorders

Vaccines and Hypersensitivities

Introduction

This study guide covers the principles of vaccination and hypersensitivity reactions, focusing on immunological mechanisms, types of vaccines, immunity, and selected immunological disorders. These topics are central to Chapters 18 and 19 in a college-level Microbiology course.

Vaccines: Principles and Types

Vaccine Materials

Vaccines contain antigens that mimic pathogens to stimulate an immune response without causing disease.

• Killed (inactivated) vaccines: Contain pathogens that have been killed; cannot cause infection but still elicit immunity.

• Attenuated vaccines: Use live pathogens that have been weakened; provide strong, long-lasting immunity but may not be suitable for immunocompromised individuals.

• Toxoid vaccines: Contain inactivated toxins (e.g., tetanus toxoid) to induce immunity against toxin-mediated diseases.

• Subunit vaccines: Include only specific parts of the pathogen, such as proteins or polysaccharides, reducing risk of adverse reactions.

• Recombinant/component vaccines: Use genetically engineered antigens to stimulate immune responses.

Example: The hepatitis B vaccine is a recombinant subunit vaccine containing hepatitis B surface antigen.

Principal Vaccines Used in the United States

Bacterial Diseases

• DTP/DTaP: Diphtheria, Tetanus, and Pertussis (acellular components for pertussis).

• Haemophilus influenzae type b (Hib): Conjugated polysaccharide vaccine.

• Meningococcal and Pneumococcal vaccines: Conjugated polysaccharide antigens.

Viral Diseases

• Measles, Mumps, Rubella (MMR): Attenuated viruses.

• Poliovirus: Inactivated (Salk) and attenuated (Sabin) vaccines.

• Rabies: Inactivated virus.

• Human papillomavirus (HPV): Antigenic fragments (recombinant vaccine).

• Hepatitis B: Recombinant subunit vaccine.

• SARS-CoV-2: mRNA vaccine encoding spike protein antigen.

Example: The MMR vaccine protects against measles, mumps, and rubella using live attenuated viruses.

Immunization Trends

Vaccination programs have dramatically reduced the incidence of diseases such as measles in the United States, as shown by epidemiological data.

Passive Immunization

Passive immunization involves administering preformed antibodies from another source.

• Sources: Human or animal gamma globulin, pooled serum, or special preparations.

• Advantages: Immediate protection, useful for immunocompromised individuals or post-exposure prophylaxis.

• Disadvantages: Short-lived immunity, possible allergic reactions, and serum sickness.

Example: Rabies post-exposure prophylaxis includes passive immunization with rabies immune globulin.

Types of Immunity

Summary of Immunity Types

• Naturally acquired active immunity: Results from infection or normal exposure to antigens.

• Naturally acquired passive immunity: Transfer of antibodies from mother to child via placenta or breast milk.

• Artificially acquired active immunity: Induced by vaccination.

• Artificially acquired passive immunity: Administration of antibodies (e.g., gamma globulin injections).

Additional info: Active immunity is generally longer-lasting than passive immunity because it involves memory cell formation.

Hypersensitivity Reactions (Immunological Disorders)

Overview

Hypersensitivity reactions are immune responses to antigens (allergens) that result in tissue damage. They are classified into four types:

1. Type I: Anaphylactic

2. Type II: Cytotoxic

3. Type III: Immune complex-mediated

4. Type IV: Cell-mediated (delayed type)

Type I: Anaphylactic Reactions

Type I hypersensitivity involves IgE antibodies binding to mast cells and basophils, leading to degranulation and release of histamine and other mediators.

• Allergy development: Sensitization to allergen, production of IgE antibodies, binding of antigen to IgE on mast cells.

• Degranulation: Release of chemical mediators (histamine, leukotrienes, prostaglandins).

• Symptoms: Local (hay fever, asthma) or systemic (anaphylactic shock).

Example: Peanut allergy can cause systemic anaphylaxis, requiring immediate medical intervention.

Type II: Cytotoxic Reactions

Type II hypersensitivity involves IgG or IgM antibodies binding to cell surface antigens, activating complement and leading to cell lysis or damage by macrophages.

Reaction	Antibody	Complement Activation	Cell Damage
Transfusion reaction	IgG/IgM	Yes	Hemolysis
Hemolytic disease of the newborn	IgG	Yes	Destruction of fetal RBCs

Example: Rh incompatibility between mother and fetus can cause hemolytic disease of the newborn.

Type III: Immune Complex-Mediated Hypersensitivity

Type III reactions occur when immune complexes (antigen-antibody) deposit in tissues, triggering inflammation and tissue damage.

• Examples:

  • Systemic lupus erythematosus (SLE): Antibodies to cell nucleus components deposit in organs.

  • Rheumatoid arthritis: Antibodies to rheumatoid factor cause chronic joint inflammation.

Additional info: Immune complex deposition activates complement and attracts neutrophils, leading to tissue injury.

Type IV: Cell-Mediated (Delayed-Type) Hypersensitivity

Type IV reactions are mediated by T cells, not antibodies, and typically occur 12–48 hours after exposure to antigen.

• First exposure: T cells become sensitized and proliferate.

• Second exposure: Sensitized T cells release lymphokines, activate macrophages, and cause inflammation.

• Examples:

  • Allergic contact dermatitis (e.g., poison ivy, nickel allergy).

  • Tuberculin skin test for Mycobacterium tuberculosis exposure.

  • Organ transplant rejection (primarily T cell-mediated).

Example: The Mantoux tuberculin skin test is a diagnostic tool for tuberculosis exposure, based on a Type IV hypersensitivity reaction.

Clinical Applications: Tuberculin Skin Test

• Inject 0.1 ml of 5 TU PPD tuberculin intradermally.

• Read reaction 48–72 hours after injection; measure induration in millimeters.

• Positive reaction indicates prior exposure or infection.

Additional info: The test is used to screen for latent tuberculosis infection and guide further diagnostic evaluation.

Summary Table: Hypersensitivity Types

Type	Mechanism	Antibody/T Cell	Examples
I (Anaphylactic)	IgE-mediated mast cell degranulation	IgE	Allergic rhinitis, asthma, anaphylaxis
II (Cytotoxic)	Antibody-mediated cell lysis	IgG/IgM	Transfusion reaction, hemolytic disease of newborn
III (Immune Complex)	Immune complex deposition	IgG/IgM	SLE, rheumatoid arthritis
IV (Cell-Mediated)	T cell-mediated delayed response	T cells	Contact dermatitis, TB skin test, transplant rejection

Key Equations and Concepts

• Antibody-Antigen Binding: $Ag + Ab \rightarrow AgAb$ (Antigen plus antibody forms immune complex)

• Complement Activation (Type II): $AgAb + C' \rightarrow Cell~Lysis$ (Immune complex plus complement leads to cell lysis)

Conclusion

Understanding vaccines and hypersensitivity reactions is essential for preventing infectious diseases and managing immunological disorders. Mastery of these concepts is critical for success in microbiology and related health sciences.