Vaccines and Hypersensitivities

Overview

This study guide summarizes key concepts from Chapters 18 and 19 of a college-level Microbiology course, focusing on the principles of vaccines, types of immunity, and immunological hypersensitivity reactions. These topics are essential for understanding how the immune system protects against pathogens and how it can sometimes cause harm through inappropriate responses.

Vaccines: Principles and Types

Vaccine Materials

Vaccines contain antigens that mimic pathogens to stimulate protective immunity without causing disease.

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

• Attenuated (live, weakened) vaccines: Use live pathogens that have been weakened so they do not cause disease in healthy individuals; often produce stronger, longer-lasting immunity.

• Toxoid vaccines: Contain inactivated toxins produced by bacteria (e.g., diphtheria, tetanus); stimulate immunity to the toxin rather than the pathogen itself.

• Subunit/recombinant vaccines: Include only specific antigens (proteins) from the pathogen, often produced using recombinant DNA technology.

• Polysaccharide conjugate vaccines: Combine polysaccharide antigens with proteins to enhance immune response, especially in young children.

Example: The influenza vaccine is available in both inactivated and live attenuated forms.

Principal Vaccines Used in the United States

Bacterial Diseases

• DTP/DTaP: Diphtheria, Tetanus, Pertussis (toxoid and subunit components)

• Haemophilus influenzae type b (Hib): Conjugate vaccine

• Meningococcal: Conjugate vaccine

• Pneumococcal: Conjugate and polysaccharide vaccines

Viral Diseases

• Measles, Mumps, Rubella (MMR): Attenuated virus

• Poliomyelitis: Inactivated virus (Salk) and attenuated virus (Sabin)

• Rabies: Inactivated virus

• Hepatitis B: Antigenic fragments (recombinant vaccine)

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

• SARS-CoV-2: RNA vaccine (mRNA encoding viral spike protein)

Example: The SARS-CoV-2 mRNA vaccine delivers genetic instructions for the spike protein, prompting the immune system to produce antibodies.

Types of Immunity

Immunity can be acquired naturally or artificially, and can be active or passive.

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

• 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 preformed antibodies (e.g., gamma globulin injections).

Example: Gamma globulin injections provide immediate, short-term protection against certain diseases.

Immunological Disorders: Hypersensitivity Reactions

Overview

Hypersensitivity reactions are inappropriate or exaggerated 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 the release of histamine and other mediators upon exposure to an allergen.

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

• Degranulation: Release of chemical mediators (histamine, leukotrienes) upon subsequent exposure.

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

Example: Peanut allergy can cause rapid, life-threatening anaphylaxis.

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 phagocytes.

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

• Mechanism: Antibody binds to target cell, complement activation leads to cell destruction.

Example: Hemolytic disease of the newborn occurs when maternal anti-Rh antibodies attack fetal red blood cells.

Type III: Immune Complex-Mediated Hypersensitivity

Type III reactions result from the formation of antigen-antibody complexes that deposit in tissues, triggering inflammation and tissue damage.

• Examples: Systemic lupus erythematosus (SLE), rheumatoid arthritis.

• Mechanism: Immune complexes activate complement, attract neutrophils, and cause tissue injury.

Example: SLE involves antibodies against self-antigens, leading to widespread immune complex deposition.

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

Type IV hypersensitivity is mediated by T cells rather than antibodies. The response is delayed (12–48 hours) after exposure to antigen.

• First exposure: T cells become sensitized and proliferate.

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

• Examples: Allergic contact dermatitis (e.g., poison ivy), tuberculin skin test, organ transplant rejection.

Example: Tuberculin skin test for Mycobacterium tuberculosis infection involves intradermal injection of purified protein derivative (PPD) and measurement of induration after 48–72 hours.

Procedure for Tuberculin Skin Test

• Inject intradermally 0.1 ml of 5 TU PPD tuberculin

• Produce wheal 6–10 mm in diameter

• Read reaction 48–72 hours after injection

• Measure only induration (not redness)

• Record reaction in millimeters

Organ Transplant Rejection

Type IV hypersensitivity is a major mechanism of organ transplant rejection, primarily mediated by T cells recognizing foreign antigens on transplanted tissue.

Summary Table: Types of Hypersensitivity Reactions

Key Terms and Definitions

• Antigen: Any substance that can induce an immune response.

• Antibody: Protein produced by B cells that binds specifically to antigens.

• Immunization: The process of inducing immunity, either by natural exposure or vaccination.

• Hypersensitivity: Excessive or inappropriate immune response to an antigen.

• Passive immunity: Immunity acquired by receiving preformed antibodies.

• Active immunity: Immunity developed by exposure to antigen and subsequent immune response.

Equations and Immunological Principles

• Antigen-Antibody Complex Formation:

• Complement Activation (Classical Pathway):

Applications and Clinical Relevance

• Vaccines: Prevent infectious diseases by inducing protective immunity.

• Hypersensitivity testing: Used to diagnose allergies and immune disorders (e.g., skin prick test, tuberculin test).

• Transplant medicine: Understanding hypersensitivity is crucial for managing organ rejection.

Additional info: Some details, such as the mechanisms of vaccine action and the clinical procedures for hypersensitivity testing, have been expanded for clarity and completeness.