Chapter 18: Practical Applications of Immunology – Vaccines and Diagnostic Immunology

Practical Applications of Immunology

Introduction

This chapter explores the use of immunological principles in the prevention, diagnosis, and treatment of infectious diseases. It focuses on vaccines and diagnostic immunology, providing foundational knowledge for understanding how immunity can be harnessed in clinical and laboratory settings.

Vaccines

Definition and Historical Context

• Vaccine: A suspension of organisms or fractions of organisms that induce immunity in the host.

• Variolation: Early method of inoculating smallpox (Variola virus) into the skin.

• Edward Jenner: Developed the first vaccine by inoculating cowpox (Vaccinia) to prevent smallpox in 1796.

• Modern definition: Vaccination refers to the artificial induction of immunity against any infectious disease.

Principles and Effects of Vaccination

• Primary immune response: Initial exposure to antigen leads to antibody and memory cell formation.

• Secondary immune response: Subsequent exposure produces a rapid and intense response due to memory cells.

• Herd immunity: When most of the population is immune, outbreaks are sporadic due to lack of susceptible individuals.

Types of Vaccines

• Live attenuated vaccines: Contain weakened pathogens that closely mimic natural infection. Usually require only 1–2 doses for lifelong immunity. Not recommended for immunocompromised individuals; require refrigeration.

• Inactivated (killed) vaccines: Contain killed pathogens. Safer than live vaccines but require repeated booster doses. Induce mainly humoral immunity.

• Subunit vaccines: Use antigenic fragments to stimulate an immune response. Includes recombinant vaccines (produced by genetic modification), virus-like particle (VLP) vaccines (resemble viruses but lack genetic material), and conjugated vaccines (used for children with poor response to polysaccharides).

• Nucleic acid (DNA) vaccines: Injected naked DNA produces the protein antigen encoded in the DNA, stimulating both humoral and cellular immunity.

• Toxoids: Inactivated toxins that target harmful substances produced by bacteria. Antitoxins are serums containing antibodies against the toxin (e.g., tetanus toxoid, diphtheria toxoid).

Adjuvants

• Adjuvants: Chemical additives added to vaccines to improve effectiveness by enhancing the innate immune response.

• Common adjuvants: Aluminium hydroxide and aluminium phosphate have replaced alum in commercial vaccines.

Safety of Vaccines

• On rare occasions, vaccines can cause the disease they are meant to prevent.

• No scientific evidence links MMR vaccines to autism.

• Vaccines are the safest and most effective means of preventing infectious diseases in children.

Principal Vaccines Used in the United States

Bacterial Diseases

Disease	Vaccine	Recommendation	Booster
Hib	Haemophilus influenzae type b conjugate	Children at 2, 4, 6, and 12–15 months	None recommended
MenACWY	Meningococcal conjugate	Adolescents at 11–12 years, booster at 16 years	Booster at 16 years
PPSV/PCV	Pneumococcal polysaccharide/conjugate	Adults ≥65 years, children Recommended for high-risk groups
DTaP	Diphtheria, tetanus, acellular pertussis	Children at 2, 4, 6, and 15–18 months, 4–6 years	Booster every 10 years (Td)

Viral Diseases

Disease	Vaccine	Recommendation	Booster
Hepatitis A	Inactivated hepatitis A	Children at 12–23 months, travelers	Booster 6–18 months after first dose
Hepatitis B	Recombinant hepatitis B	Infants at birth, health care workers	Booster for high-risk groups
Influenza	Inactivated or live attenuated influenza	Annual for all ≥6 months	Annual
Measles	Attenuated measles	Children at 12–15 months, 4–6 years	Booster at 4–6 years
Mumps	Attenuated mumps	Children at 12–15 months, 4–6 years	Booster at 4–6 years
Polio	Inactivated polio	Children at 2, 4, 6–18 months, 4–6 years	Booster at 4–6 years
Rabies	Inactivated rabies	For high-risk groups, post-exposure	Booster every 2 years
Rubella	Attenuated rubella	Children at 12–15 months, 4–6 years	Booster at 4–6 years

Diagnostic Immunology

Key Concepts

• Sensitivity (true positive rate): Probability that a test is reactive if the specimen is a true positive. High sensitivity means few false negatives.

• Specificity (true negative rate): Probability that a test is not reactive if the specimen is a true negative. High specificity means few false positives.

Monoclonal Antibodies (Mabs)

• Hybridoma: Fusion of an "immortal" myeloma cell with an antibody-producing B cell, resulting in a cell line that produces monoclonal antibodies.

• Monoclonal antibodies: Uniform, highly specific antibodies produced in large quantities. Used in diagnostics and therapy (e.g., neutralizing TNF in rheumatoid arthritis, treating allergic asthma).

• Types of Mabs:

  • Murine: Made from mouse proteins (-omab)

  • Chimeric: Mouse variable region, human constant region (-ximab)

  • Humanized: Mostly human, mouse antigen-binding sites (-zumab)

  • Fully human: Produced from human gene in mouse (-umab)

Comparison of Solution and Suspension

Solution	Suspension
Clear, homogeneous; solute particles do not settle; pass through filter paper	Cloudy, heterogeneous; solid particles settle; do not pass through filter paper

Precipitation Reactions

• Soluble antigens react with antibodies to form large aggregates (lattices) that precipitate from solution.

• Precipitin ring test: Cloudy line forms at optimal antigen-antibody ratio.

• Immunodiffusion tests (Ouchterlony): Precipitation in agar gel at optimal ratio.

• Immunoelectrophoresis: Combines electrophoresis with immunodiffusion to separate serum proteins.

Agglutination Reactions

• Particulate antigens bind to antibodies, forming visible aggregates.

• Direct agglutination tests: Detect antibodies against large cellular antigens; measure antibody concentration (titer); seroconversion indicates disease progression.

• Indirect (passive) agglutination tests: Antibody reacts with soluble antigen adhering to particles (e.g., latex beads).

Hemagglutination

• Agglutination of RBC surface antigens and antibodies; used in blood typing.

• Viral hemagglutination: Viruses agglutinate RBCs without antigen-antibody reaction (e.g., mumps, measles, influenza).

Neutralization Reactions

• Antigen-antibody reaction blocks harmful effects of exotoxins or viruses.

• Viral hemagglutination inhibition test: Used for subtyping viruses; serum antibodies neutralize virus and inhibit hemagglutination.

Fluorescent-Antibody Techniques

• Combine fluorescent dyes with antibodies for detection.

• Direct FA tests: Identify microorganisms in clinical specimens.

• Indirect FA tests: Detect specific antibodies in serum using anti-human immune serum globulin (anti-HISG).

• Fluorescence-activated cell sorter (FACS): Laser-based device separates cells based on size and fluorescence.

Enzyme-Linked Immunosorbent Assay (ELISA)

• Direct ELISA: Detects antigens using one antibody; color change indicates positive result.

• Indirect ELISA: Detects antibodies using two antibodies; color change indicates positive result.

Protein Analytical Techniques

• Western blotting: Analytical technique to identify and locate specific proteins via electrophoresis and antibody binding.

• Application: Used in diagnostics (e.g., HIV testing) and research.

Example: Home Pregnancy Test

• Uses monoclonal antibodies specific for human chorionic gonadotropin (hCG).

• Sandwich ELISA format: Color change indicates presence of hCG.