Immunization and Immune Testing: Principles and Applications

Immunization

Artificial Methods of Immunity

Immunization refers to the process by which an individual's immune system is fortified against an agent. There are two main artificial methods:

• Active Immunization: Administration of antigens so the patient actively mounts a protective immune response.

• Passive Immunization: Transfer of antibodies formed by an immune individual or animal to another individual, providing immediate but temporary protection.

Brief History of Immunization

• Early observations in China noted that children who recovered from smallpox did not contract the disease again. This led to variolation, the practice of infecting children with material from smallpox scabs to induce immunity.

• Variolation spread to England and America but was discontinued due to the risk of death.

• In 1796, Edward Jenner discovered the process of vaccination using cowpox to protect against smallpox.

• In 1879, Louis Pasteur developed a vaccine against Pasteurella multocida.

• Antibody transfer was developed after it was discovered that vaccines protect through the action of antibodies.

Impact of Immunization on Disease Prevalence

Immunization has dramatically reduced the prevalence of many infectious diseases, as illustrated by the decline in polio and measles cases following the introduction of vaccines.

Challenges in Immunization

• Many developing nations do not receive vaccines.

• Effective vaccines have not been developed for some pathogens.

• Vaccine-associated risks can discourage investment in new vaccine development.

Types of Vaccines (Active Immunization)

• Attenuated (Live) Vaccines: Use pathogens with reduced virulence. They can result in mild infections and stimulate a strong immune response, sometimes providing contact immunity. However, they may retain enough residual virulence to cause disease in some individuals.

• Inactivated (Killed) Vaccines: Include whole agent and subunit vaccines. These are safer than live vaccines but are antigenically weaker and often require adjuvants to increase their effectiveness.

• Toxoid Vaccines: Contain chemically or thermally modified toxins to stimulate active immunity, especially useful for bacterial diseases. They require multiple doses due to few antigenic determinants.

• Combination Vaccines: Simultaneous administration of antigens from several pathogens.

• Vaccines Using Recombinant Gene Technology: Employ recombinant DNA techniques to improve vaccine effectiveness, safety, and cost.

Vaccine Manufacture

• Vaccines are mass-produced by growing microbes in culture vessels.

• Viruses are often cultured inside chicken eggs, which can be problematic for individuals with egg allergies.

CDC Recommended Immunization Schedule

The CDC provides guidelines for the timing and administration of vaccines throughout childhood, adolescence, and adulthood.

Vaccine Safety

• Mild toxicity is the most common problem associated with immunization.

• There is a risk of anaphylactic shock and residual virulence from attenuated viruses.

• Allegations that vaccines cause autism, diabetes, and asthma have not been substantiated by research.

Passive Immunotherapy

Passive immunotherapy involves the administration of antiserum containing preformed antibodies, providing immediate protection against recent or ongoing infections. However, antisera can trigger allergic reactions (serum sickness), may be contaminated with viral pathogens, and are degraded quickly. These limitations are addressed by the development of hybridomas, which produce monoclonal antibodies.

Comparison of Active Immunization and Passive Immunotherapy

Active immunization leads to long-term immunity by stimulating the body's own immune response, while passive immunotherapy provides immediate but short-lived protection.

Immune Testing

Principles of Immune Testing

Immune testing, or serology, involves the study and diagnostic use of antigen-antibody interactions in blood serum. Tests are chosen based on the suspected diagnosis, cost, and speed of obtaining results. There are two main categories:

• Direct Testing: Detects the presence of antigens.

• Indirect Testing: Detects antibodies formed against antigens.

Precipitation Tests

Precipitation tests are among the simplest serological tests. When antigens and antibodies are mixed in optimal proportions, they form large complexes called precipitates. Immunodiffusion is used to determine optimal concentrations of antibody and antigen.

Radial Immunodiffusion

This technique measures specific antibodies in a person's serum by using anti-antibodies. Human antibodies act as the "antigen" in the test, and the antibody is anti-human antibody.

Agglutination Tests

Agglutination occurs when antibodies cross-link particulate antigens, causing clumping of insoluble particles. This is distinct from precipitation, which involves soluble molecules. Agglutination reactions are easy to interpret visually. Hemagglutination is the agglutination of red blood cells and is used to determine blood type.

Neutralization Tests

• Viral Neutralization: Viruses introduced into cell cultures kill the cells (cytopathic effect). If the virus is neutralized by antibodies, the cells survive, indicating the presence of antibodies against the virus.

• Viral Hemagglutination Inhibition Test: Used for viruses that are not cytopathic. If a person's serum contains antibodies against a virus, it will prevent the virus from clumping red blood cells.

Complement Fixation Test

This test detects the presence of specific antibodies by measuring the generation of membrane attack complexes during complement activation. It is sensitive enough to detect small amounts of antibody that may not be detected by agglutination.

Labeled Antibody Tests

Labeled antibody tests use antibodies linked to a detectable label, such as a fluorescent dye or enzyme, to identify antigens or antibodies.

• Fluorescent Antibody Tests: Use fluorescent dyes (e.g., fluorescein) to label antibodies. There are direct and indirect versions of these tests.

• ELISA (Enzyme-Linked Immunosorbent Assay): Uses an enzyme as the label. The enzyme reacts with its substrate to produce a colored product, indicating a positive test. ELISA can detect and quantify either antigen or antibody and is widely used due to its sensitivity and ability to process many samples quickly.

• Antibody Sandwich ELISA: A modification of ELISA used to detect antigens by "sandwiching" the antigen between two antibody molecules.

• Western Blot Test: Detects antibodies against multiple antigens and is less subject to misinterpretation than other tests.

Recent Developments in Immune Testing

• Immunofiltration: A rapid ELISA that uses antibodies bound to membrane filters, allowing for quicker assays due to the large surface area.

• Immunochromatography: A very rapid and easy-to-read ELISA where antigen solution flows through a porous strip and encounters labeled antibody. A visible line is produced when antigen-antibody complexes are formed. This method is used in pregnancy tests to detect human chorionic growth hormone.