brief overview of some immune tests and their applicability in clinical microbiology, intended as a support for dentistry students

1. Immune tests in the laboratory diagnosis
of infections
- brief overview for dentistry students -

2. Definition of terms
Antigen = foreign substance that, when introduced into the
body, is capable of stimulating an immune reaction e.g.
foreign molecules in bacteria, viruses, protozoa, serum
components, etc.
Antibody aka immunoglobulin = large protein produced by
plasmocytes which identifies and neutralises antigens
Immune reaction = reversible binding of antigen to
homologous antibody (high specificity)

3. Immune reaction: Antigen-Antibody reaction
(Ag-Ab)
High specificity:
Antigen-binding site of the
antibody molecule perfectly
matches the antigen

4. Ag-Ab Reactions: applications in laboratory
diagnosis of infections
• Same basic principle:
– specific detection and binding of antigen by
antibody → Ag-Ab complex
• Differences:
– the methods used to reveal the Ag-Ab
complex

5. Ag-Ab Reactions: applications in laboratory
diagnosis of infections (examples)
• Agglutination
• Immunofluorescence
• Enzyme-linked Immunosorbent Assay (ELISA)
• Immunoblotting, Western blot

6. Agglutination: on slide/in tubes
• clumping together by
antibodies of microscopic
foreign particles:
– red blood cells
– bacteria
– inert particles (latex)
• agglutinated particles are
visible with the naked eye
(pellet-like agglutination
product)

7. Agglutination: applications in microbiology
1. Serological diagnosis: detection (and quantification) of
unknown antibodies by use of known antigens
• Principle:
• serum from patient is put in contact with serial dilutions of Ag;
• if Ab present in serum →agglutination
• Titre of Ab = dilution of the tube with agglutination
1. Bacteriological diagnosis: identification of unknown
antigens (bacteria) by use of known antibodies
• Principle:
• Sera containing known Ab are put in contact with bacterial
suspension
• Identification of bacteria – indicated by the type of serum which
agglutinates the bacterial suspension

8. Antigen: isolated bacteria e.g. Salmonella
Antibody: kit with antibodies (antisera) against Salmonella
types

9. Passive agglutination (inert particles)
Ag on latex (latex-agglutination)/RBC (hemagglutination)
• If Ab are present in sample →agglutination of latex/RBC
Ab on inert particles: bacterial identification kits e.g.
Streptococcus pneumoniae, Neisseria gonorrhoeae,
Neisseria meningitidis, E.coli

10. Immunofluorescence
• Immunofluorescence:
– Bacterial culture (Ag) incubated with specific
antibody coupled with fluorescent dye
– if Ab matches Ag (bacterial culture) →Ag-Ab
complex + fluorescent dye
– under UV light bacteria covered with
antibodies coupled with fluorescent dye will
produce fluorescence

11. Treponema denticola – wet mount, dark field
microscopy + fluorescent dye staining

12. ELISA
immune reaction (Ag-Ab)
linked to
enzymatic reaction (Enzyme-Substrate)
• Solid support coated with Ag - Add Patient serum
• if specific Ab present→ Ag-Ab complex
• Add Ab anti-human Ab conjugated to Enzyme
• Complex: Ag-Ab-(Ab anti-human Ab)-Enzyme
• Add substrate of enzyme → COLOUR develops:
– action of enzyme on substrate AND
– presence of specific Ab in patient serum

13. Western blot