Indirect (Passive) Hemagglutination Assay (IHA)
The indirect hemagglutination assay (also known as passive hemagglutination) is based on the use of red blood cells. In this case, erythrocytes do not act as independent antigens, but rather as inert carriers that make the reaction visible.
The underlying mechanism is as follows: soluble antigens are pre-adsorbed onto the surface of erythrocytes. When specific antibodies (such as large IgM class molecules) are added to this system, they actively interact with the bound antigens. Due to their structure, antibodies link antigens on different erythrocytes, forming bridges. This cross-linking results in visibly distinct cell clumping—agglutination.
This technique is widely used in medical practice to detect hypersensitivity (sensitization) to various drugs and hormones. Additionally, IHA serves as an essential tool for the accurate diagnosis of diverse infectious diseases.
Co-Agglutination: Rapid Diagnostics
The co-agglutination reaction is an efficient rapid diagnostic method that allows for the prompt identification of specific infection agents.
Instead of erythrocytes, bacterial cells—specifically Staphylococcus aureus—are used as the solid phase (carrier) in this assay. Before the analysis, these cells are pre-treated with immune diagnostic serum.
A key factor for the success of this method is the presence of specific Protein A on the staphylococcal surface. The molecular mechanism relies on the exceptionally high affinity of this protein for the Fc fragment of immunoglobulins.
Non-specific adsorption occurs: antibodies from the diagnostic serum firmly attach to the staphylococcus with their "tail" (Fc fragment) pointing downward. Meanwhile, the active antibody centers, known as Fab fragments, remain entirely free and directed outward.
The reaction proceeds as follows:
- "Sensitized" staphylococci are gently mixed with the biological specimen obtained from the patient.
- If the target microbe is present in the sample, the free Fab fragments of the antibodies specifically bind to its antigens.
- The result is the formation of easily visible clumps. These clumps represent a complex structural aggregate: "staphylococcus – diagnostic antibody – target microbe".
Hemagglutination Inhibition Assay (HI / HIQ)
The hemagglutination inhibition (HI) assay is a classic tool for diagnosing viral infections. The method relies on a unique baseline property of many viruses. Pathogens such as influenza, measles, and rubella viruses possess specific surface spikes called hemagglutinins. These surface structures are capable of spontaneously clumping erythrocytes.
The core of the reaction is the targeted blockade (neutralization) of these viral hemagglutinins using specific antibodies.
Performing the assay requires three mandatory components:
- The virus, acting as the antigen.
- The immune serum, suspected to contain antibodies.
- Erythrocytes, functioning as the indicator system.
The mechanism is straightforward: antibodies from the serum bind tightly to the hemagglutinins on the viral surface. As a result, the virus completely loses its ability to interact with erythrocytes.
Interpreting HI results requires attention because it is counterintuitive:
- Positive result (antibodies are present in the serum): inhibition has successfully occurred, the virus is neutralized, and therefore there is no agglutination. Erythrocytes sediment freely.
- Negative result (no antibodies): the virus remains active and uses its spikes to clump erythrocytes—visible, pronounced agglutination is observed.