Chapter Capsule

Biomedical Applications of Specific Immunity

I. Immunization: Producing immunity by medical intervention

A. Passive immunotherapy includes administering immune serum globulin and specific immune globulins pooled from donated serum to prevent infection and disease in those at risk; antisera and antitoxins from animals are occasionally used.
B. Active immunization is synonymous with vaccination; provides an antigenic stimulus that does not cause disease but can produce long-lasting, protective immunity. Vaccines are made with:
1. Killed whole cells or inactivated viruses that do not reproduce but are antigenic.
2. Live, attenuated cells or viruses that are able to reproduce but have lost virulence.
3. Acellular or subunit components of microbes such as surface antigen or neutralized toxins (toxoids).
4. Genetic engineering techniques, including cloning of antigens, recombinant attenuated microbes, and DNA-based vaccines.
C. Boosters (additional doses) are often required.
D. Vaccination increases herd immunity, protection provided by mass immunity in a population.

II. Serological/Immune Testing: Serology is a science that attempts to detect signs of infection in a patient’s serum such as antibodies specific for a microbe.

A. The basis of serological tests is that Abs specifically bind to Ag in vitro. An Ag of known identity will react with antibodies in an unknown serum sample. The reverse is also true; known antibodies can be used to detect and type antigens.
B. These Ag-Ab reactions are visible in the form of obvious clumps and precipitates, color changes, or the release of radioactivity. Test results are read as positive or negative.
C. Desirable properties of tests are specificity and sensitivity.
D. Types of Tests
1. In agglutination tests, antibody cross-links whole-cell antigens, forming complexes that settle out and form visible clumps in the test chamber; examples are tests for blood type, some bacterial diseases, and viral diseases.
2. Double diffusion precipitation tests involve the diffusion of Ags and Abs in a soft agar gel, forming zones of precipitation where they meet.
3. In immunoelectrophoresis, migration of serum proteins in gel is combined with precipitation by antibodies.
4. The Western blot test separates antigen into bands. After the gel is affixed to a blotter, it is reacted with a test specimen and developed by radioactivity or with dyes.
5. Complement fixation tests detect lysins—antibodies that fix complement and can lyse target cells. It involves first mixing test Ag and Ab with complement and then with sensitized sheep RBCs. If the complement is fixed by the Ag-Ab, the RBCs remain intact, and the test is positive. If RBCs are hemolyzed, specific antibodies are lacking.
6. In direct assays, known marked Ab is used to detect unknown Ag (microbe).
a. In indirect testing, known Ag reacts with unknown Ab, and the reaction is made visible by a second Ab that can affix to and identify the unknown Ab.
b. Immunofluorescence testing uses fluorescent antibodies (FABs tagged with fluorescent dye) either directly or indirectly to visualize cells or cell aggregates that have reacted with the FABs.
7. Immunoassays are highly sensitive tests for Ag and Ab.
a. In radioimmunoassay, Ags or Abs are labeled with radioactive isotopes and traced.
b. The enzyme-linked immunosorbent assay (ELISA) can detect unknown Ag or Ab by direct or indirect means. A positive result is visualized when a colored product is released by an enzyme-substrate reaction.
c. Tests are also available to differentiate B cells from T cells and their subtypes.
8. With in vivo testing, Ags are introduced into the body directly to determine the patient’s immunologic history.

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