Mechanism of the Reaction
The core of the reaction is the formation of a unified network (lattice) of antigens and antibodies. Antibodies act as specific "bridges" connecting particulate antigens together.
The scale of aggregation depends on the class of participating immunoglobulins:
- IgM have a pentameric structure, allowing them to capture multiple bacteria or cells at once, forming large clusters.
- IgG are monomers, so they link only adjacent cells while also building a lattice.
Types of Diagnostic Reagents
Depending on the antigen to be detected, different types of diagnostic reagents (microbial suspensions) are used:
- O-diagnostic reagent: Prepared from heat-killed bacteria, preserving the heat-stable somatic O-antigen. The reaction yields a fine-granular agglutination.
- H-diagnostic reagent: Prepared from formalin-killed bacteria, preserving the heat-labile flagellar H-antigen. Agglutination is large-flocculated and occurs faster than O-agglutination.
Clinical Significance
The agglutination reaction is widely used in medical practice:
- Serodiagnosis. Allows quantification of specific antimicrobial antibody titers in the patient's blood serum.
- Pathogen Identification (Serotyping). Using known diagnostic sera, the exact species of bacteria isolated from a patient can be identified.
- Immunohematology. Used for blood typing using monoclonal antibodies against erythrocyte alloantigens.
Quantitative Tube Agglutination
This tube-based test method is used for precise antibody quantification.
Steps:
- Prepare a series of two-fold serial dilutions of the patient's blood serum.
- Add an equal amount of the diagnostic reagent (killed microbial suspension) to all tubes.
- Incubate the mixture for several hours at 37 °C.
The result determines the serum titer — the maximum dilution at which sediment formation (clumping) is still visually noticeable.
Cross-Reactions
During serotyping, a complication may arise: closely related microorganisms often share common antigens. This leads to false-positive results, where the same serum clumps different bacteria.
To solve this, the Castellani method (1902) uses absorbed agglutinating sera. All cross-reacting antibodies are removed from standard serum by precipitation with related bacteria, leaving only specific antibodies that react strictly against the target pathogen.