Vaccines and Hypersensitivities: Immunological Principles and Disorders

Vaccines and Hypersensitivities

Overview

This study guide covers the principles of vaccination, types of immunity, and immunological hypersensitivity reactions. It is based on Chapters 18 and 19 of a standard microbiology curriculum, focusing on the mechanisms, examples, and clinical relevance of vaccines and hypersensitivity disorders.

Vaccines: Principles and Types

Vaccine Materials

• Inactivated (killed) vaccines: Contain pathogens that have been killed so they cannot cause disease 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 but stimulate strong immunity.

• Toxoid vaccines: Contain inactivated toxins (toxoids) produced by bacteria, used to protect against diseases caused by bacterial toxins (e.g., tetanus, diphtheria).

• Subunit vaccines: Include only parts of the pathogen, such as proteins or polysaccharides, to stimulate immunity (e.g., acellular pertussis, HPV).

• Conjugate vaccines: Combine a weak antigen with a strong antigen to enhance the immune response (e.g., Haemophilus influenzae type b).

• Recombinant/component vaccines: Use genetically engineered antigens (e.g., hepatitis B surface antigen).

Example: The Hepatitis B vaccine is a recombinant vaccine containing the surface antigen produced in yeast cells.

Principal Vaccines Used in the United States

Bacterial Diseases

• DTP/DTaP: Diphtheria, Tetanus, and Pertussis (acellular or whole-cell components)

• Haemophilus influenzae type b (Hib): Conjugate vaccine

• Meningococcal: Conjugate vaccine for Neisseria meningitidis

• Pneumococcal: Conjugate and polysaccharide vaccines for Streptococcus pneumoniae

Viral Diseases

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

• Varicella (chickenpox): Attenuated virus

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

• Rabies: Inactivated virus

• Hepatitis A: Inactivated virus

• Hepatitis B: Recombinant antigen

• Human papillomavirus (HPV): Recombinant antigen

• SARS-CoV-2: mRNA and viral vector vaccines

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

SARS-CoV-2 RNA Vaccine

• Uses messenger RNA encoding the spike protein of the virus.

• Host cells produce the spike protein, which is recognized as an antigen by the immune system, stimulating both antibody and T cell responses.

Example: Pfizer-BioNTech and Moderna COVID-19 vaccines are mRNA vaccines.

Passive Immunization

• Involves administration of preformed antibodies from another individual or animal.

• Provides immediate, short-term protection.

• Sources: gamma globulin, antitoxins, antivenoms.

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

• Disadvantages: no long-term immunity, possible serum sickness.

Example: Rabies immune globulin is given after exposure to rabies virus.

Types of Immunity

Summary Table

• Naturally acquired active immunity: Exposure to pathogen through infection.

• Naturally acquired passive immunity: Transfer of antibodies from mother to child (placenta, breast milk).

• Artificially acquired active immunity: Vaccination with antigens.

• Artificially acquired passive immunity: Injection of antibodies (gamma globulin).

Key Point: Active immunity involves the host's own immune response and provides long-term protection; passive immunity is immediate but short-lived.

Hypersensitivity Reactions (Immunological Disorders)

Overview

Hypersensitivity reactions are immune responses to antigens (allergens) that cause tissue damage. They are classified into four types based on the mechanism and timing of the response.

Type I: Anaphylactic (Immediate) Reactions

• Involves IgE antibodies bound to mast cells and basophils.

• Upon exposure to allergen, cross-linking of IgE triggers degranulation and release of histamine and other mediators.

• Symptoms: vasodilation, increased vascular permeability, smooth muscle contraction (bronchospasm), hives, anaphylaxis.

• Examples: allergic rhinitis, asthma, food allergies, systemic anaphylaxis.

Example: Peanut allergy causing rapid onset of hives and difficulty breathing.

Type II: Cytotoxic Reactions

• Involves IgG or IgM antibodies directed against cell surface antigens.

• Activates complement, leading to cell lysis or phagocytosis.

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

Reaction	Antibody	Complement	Cells Affected	Example
Transfusion reaction	IgM	Yes	RBCs	ABO incompatibility
Hemolytic disease of the newborn	IgG	Yes	Fetal RBCs	Rh incompatibility

Example: A Rh-negative mother develops antibodies against Rh-positive fetal red blood cells, leading to hemolysis in the fetus.

Type III: Immune Complex-Mediated Hypersensitivity

• Involves formation of antigen-antibody complexes that deposit in tissues.

• Triggers complement activation and inflammation.

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

Example: In SLE, immune complexes deposit in the kidneys, joints, and skin, causing inflammation and tissue damage.

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

• Mediated by T cells, not antibodies.

• Response is delayed (12–48 hours after exposure).

• First exposure: T cells become sensitized and proliferate.

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

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

Example: The Mantoux tuberculin skin test for Mycobacterium tuberculosis infection is a classic example of Type IV hypersensitivity.

Contact Dermatitis

• Caused by contact with allergens such as nickel, poison ivy, or latex.

• Results in localized skin inflammation, redness, and blistering.

Tuberculin Skin Test Procedure

• Inject 0.1 mL of 5 TU PPD tuberculin intradermally.

• Read reaction 48–72 hours after injection.

• Measure induration (not redness) in millimeters.

• Positive test indicates prior sensitization to M. tuberculosis antigens.

Organ Transplant Rejection

• Primarily mediated by T cells recognizing foreign antigens on transplanted tissue.

• Results in inflammation and destruction of the graft.

Summary Table: Hypersensitivity Types

Type	Immune Component	Mechanism	Examples
I (Anaphylactic)	IgE, mast cells	Immediate degranulation	Allergies, anaphylaxis
II (Cytotoxic)	IgG/IgM, complement	Cell lysis	Transfusion reaction, hemolytic disease of newborn
III (Immune Complex)	IgG/IgM, complement	Immune complex deposition	SLE, rheumatoid arthritis
IV (Cell-Mediated)	T cells	Delayed inflammation	Contact dermatitis, TB skin test, transplant rejection

Key Equations and Concepts

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

• Vaccine Efficacy: $\text{Efficacy} = \frac{\text{Attack rate in unvaccinated} - \text{Attack rate in vaccinated}}{\text{Attack rate in unvaccinated}} \times 100\%$

Additional info: The notes include clinical images and CDC guidelines for the tuberculin skin test, which are standard in microbiology immunology chapters.