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Immunodiagnostic reactions are the cornerstone of serological diagnosis of infectious and non-infectious diseases. They are based on the specific interaction of antigens with antibodies in vitro, allowing the identification of the pathogen or the evaluation of the patient's immune response.
This section covers both classical and modern methods. You will study the mechanisms of agglutination reactions (including indirect hemagglutination) and precipitation, examine complex complement-mediated reactions, and the neutralization test. Special attention is paid to routine clinical tests—blood typing and Rh factor determination—as well as highly sensitive methods: enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assay (IFA).
In practical classes and tests, you are most often asked to explain the visualization of the antigen-antibody complex, so focus on understanding the principles of ELISA, the mechanism of indirect hemagglutination, and complement-mediated reactions. This knowledge will be useful to you not only for your microbiology exam, but also in your future clinical practice.
Why is the serum heated to 56 °C when performing the complement fixation test (CFT)?
This is a critical step to inactivate the patient's own complement. Failure to do so will result in an uncontrolled amount of endogenous complement leading to false hemolysis in the second phase.
What is a serum titer?
This is the highest dilution of the patient's blood serum at which a precipitate is still visually determined in the tube agglutination test.
Why are erythrocytes used in the indirect hemagglutination test (IHA)?
They play the role of a visible solid phase—inert carriers on whose surface soluble antigens are adsorbed. This is necessary for clear visualization of the antigen-antibody complex.
Why are two different batches of anti-Rh sera used when determining the Rh factor?
This strict methodical requirement is necessary to confirm the reliability of the test result and completely exclude reagent error.
What is the zone of equivalence in a precipitation reaction?
This is the strict equivalence of antigen and antibody concentrations upon mixing. Only in this case is a large immune complex and visible precipitate formed. An excess of either component blocks the reaction.
Under what circumstances does the complement system begin to function in serological reactions?
Activation occurs exclusively at the moment when the specific antigen-antibody complex is formed.
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