Chapter 17: Immunization and Immune Testing

Immunization and Immune Testing

Overview of Immunization

Immunization is a critical strategy in microbiology and public health, designed to protect individuals and populations from infectious diseases. It involves the induction of immunity by artificial means, either by stimulating the body's own immune response (active immunization) or by providing preformed antibodies (passive immunotherapy).

Types of Immunity

• Active Immunity: Immunity produced by the individual's own immune system in response to antigen exposure. Can be natural (infection) or artificial (vaccination).

• Passive Immunity: Immunity acquired by receiving antibodies from another individual. Can be natural (maternal antibodies) or artificial (injection of antibodies).

Brief History of Immunization

• Early observations in China: Children who recovered from smallpox did not contract the disease again; led to variolation (inoculation with smallpox scabs).

• 1796: Edward Jenner developed the first vaccine using cowpox to protect against smallpox.

• 1879: Louis Pasteur developed vaccines against other pathogens, such as Pasteurella multocida.

• Discovery that vaccines work by stimulating antibody production.

Global Impact and Challenges

• Vaccines have drastically reduced the prevalence of many infectious diseases.

• Many developing nations still lack access to vaccines, and some pathogens lack effective vaccines.

• Vaccine hesitancy and safety concerns can hinder immunization efforts.

Types of Vaccines

Vaccines are classified based on their composition and method of preparation. Each type has specific advantages and limitations.

• Attenuated (Live, Modified) Vaccines: Contain live pathogens with reduced virulence. Induce strong, long-lasting immunity and may provide contact immunity, but can cause disease in immunocompromised individuals.

• Inactivated (Killed) Vaccines: Contain whole microbes or antigenic fragments that have been killed or inactivated. Safer but often require multiple doses and adjuvants to enhance immune response.

• Toxoid Vaccines: Contain inactivated toxins (toxoids) to stimulate immunity against toxin-mediated diseases. Require multiple doses due to low antigenicity.

• Combination Vaccines: Contain antigens from multiple pathogens, allowing simultaneous immunization.

• Recombinant Gene Technology Vaccines: Use genetic engineering to produce antigens or mRNA that codes for antigens, improving safety and efficacy.

CDC-Recommended Immunization Schedule

The CDC provides guidelines for the timing and administration of vaccines throughout life, from infancy to adulthood.

Principal Vaccines and Their Characteristics

Vaccine	Disease Agent	Disease	Vaccine Type	Method of Administration
Hepatitis B	Hepatitis B virus	Hepatitis B	Inactive subunit (recombinant yeast)	Intramuscular
Rotavirus	Rotavirus	Gastroenteritis	Attenuated, recombinant	Oral
Diphtheria/tetanus/pertussis (DTaP)	Diphtheria toxin, Tetanus toxin, Bordetella pertussis	Diphtheria, Tetanus, Whooping cough	Toxoid, Inactivated subunit	Intramuscular
Haemophilus influenzae type b (Hib)	Haemophilus influenzae	Meningitis, pneumonia, epiglottitis	Inactivated subunit	Intramuscular
Pneumococcal (PCV)	Streptococcus pneumoniae	Pneumonia	Inactivated subunit	Intramuscular
Polio	Poliovirus	Poliomyelitis	Inactivated (attenuated also available)	Subcutaneous or intramuscular/oral
Influenza	Influenza viruses	Flu	Inactivated subunit	Intramuscular or oral

Vaccines Not Routinely Recommended in the U.S.

Vaccine	Disease Agent	Disease	Vaccine Type	Method of Administration
Anthrax	Bacillus anthracis	Anthrax	Inactivated whole	Subcutaneous
BCG	Mycobacterium tuberculosis, M. bovis	Tuberculosis, leprosy	Attenuated	Intradermal
Japanese encephalitis vaccine	Japanese encephalitis virus	Encephalitis	Inactive	Subcutaneous
Rabies vaccine	Rabies virus	Rabies	Inactivated whole	Intramuscular or intradermal
Typhoid fever vaccine	Salmonella enterica	Typhoid fever	Attenuated (inactive also available)	Subcutaneous, intramuscular, or oral
Vaccinia (cowpox)	Smallpox, monkeypox virus	Smallpox, monkeypox	Attenuated	Subcutaneous
Yellow fever vaccine	Yellow fever virus	Yellow fever	Attenuated	Subcutaneous

Vaccine Safety and Limitations

• Most vaccines are safe, but mild toxicity, risk of anaphylactic shock, and residual virulence are possible.

• Allegations of vaccines causing autism, diabetes, or asthma are not supported by scientific evidence.

Passive Immunotherapy

Passive immunotherapy involves the administration of preformed antibodies (antiserum) to provide immediate protection. It is useful for recent infections or ongoing diseases but does not confer long-term immunity.

• Can trigger allergic reactions (serum sickness).

• Antibodies are degraded quickly; no memory response is generated.

• Hybridomas (antibody-producing cell lines) are used to overcome some limitations.

Immune Testing (Serological Tests)

Principles of Serological Testing

Serology is the study of antigen-antibody interactions in vitro. Serological tests are used to detect the presence of specific antigens or antibodies in blood serum, aiding in diagnosis, monitoring, and epidemiological studies.

Precipitation Tests

• Antigen and antibody mixed in optimal proportions form visible precipitates (immune complexes).

• Immunodiffusion is a common technique for precipitation reactions.

Turbidimetric and Nephelometric Tests

• Measure the cloudiness (turbidity) of a solution to quantify proteins or immune complexes.

• Turbidimetry measures transmitted light; nephelometry measures reflected light.

Agglutination Tests

• Antibodies cross-link particulate antigens, causing visible clumping (agglutination).

• Used for blood typing (hemagglutination) and diagnosis of various infections.

Titration

• Measures the concentration of antibodies in serum by serial dilution and observation of agglutination.

• The highest dilution showing a positive reaction is the titer.

Neutralization Tests

• Determine the ability of antibodies to neutralize the cytopathic effects of viruses in cell cultures.

• Viral hemagglutination inhibition tests are used for viruses that agglutinate red blood cells.

Complement Fixation Test

• Detects small amounts of antibody by measuring complement activation and formation of membrane attack complexes.

• Has largely been replaced by more modern serological methods.

Labeled Antibody Tests

• Use antibodies linked to detectable labels (fluorescent dyes, enzymes) to identify antigens or antibodies.

• Types include fluorescent immunoassays, ELISA, and immunoblot (Western blot).

ELISA (Enzyme-Linked Immunosorbent Assay)

• Uses enzyme-labeled antibodies to detect and quantify antigens or antibodies.

• Advantages: high sensitivity, quantification, easy automation, and ability to test many samples quickly.

• Antibody sandwich ELISA is used to detect antigens by "sandwiching" them between two antibodies.

Immunoblot (Western Blot)

• Detects specific proteins in a mixture using electrophoresis, transfer to a membrane, and labeled antibodies.

• Used to confirm HIV infection and diagnose Lyme disease.

Point-of-Care Testing

• Rapid immunoassays (immunofiltration, immunochromatography) provide quick results for diagnosis.

• Commonly used for pregnancy testing and rapid infection identification.

Summary Table: Serological Tests and Their Uses

Test	Use
Immunodiffusion (precipitation)	Diagnosis of syphilis, pneumococcal pneumonia
Agglutination	Blood typing, pregnancy testing, diagnosis of various infections
Viral neutralization	Diagnosis of infections by specific virus strains
Viral hemagglutination inhibition	Diagnosis of viral infections (influenza, measles, mumps, rubella, etc.)
Complement fixation	Diagnosis of measles, influenza, syphilis, and others (historical)
Direct fluorescent antibody	Diagnosis of rabies, group A Streptococcus, lymphocyte subsets
Indirect fluorescent antibody	Diagnosis of syphilis, mononucleosis
ELISA	Pregnancy testing, diagnosis of hepatitis, HIV, rubella, etc.
Immunoblot (western blot)	Confirmation of HIV infection, diagnosis of Lyme disease

Key Concepts and Applications

• Immunization is essential for disease prevention and control.

• Serological tests are vital tools for diagnosis, epidemiology, and monitoring immune status.

• Understanding the principles and limitations of each test is crucial for interpreting results accurately.