26.3: Agglutination Assays
-
- Last updated
- Save as PDF
- Ying Liu
- City College of San Francisco
Learning Objectives
- Compare direct and indirect agglutination
- Identify various uses of hemagglutination in the diagnosis of disease
- Explain how blood types are determined
- Explain the steps used to cross-match blood to be used in a transfusion
In addition to causing precipitation of soluble molecules and flocculation of molecules in suspension, antibodies can also clump together cells or particles (e.g., antigen-coated latex beads) in a process called agglutination ( Figure 18.1.8 ). Agglutination can be used as an indicator of the presence of antibodies against bacteria or red blood cells. Agglutination assays are usually quick and easy to perform on a glass slide or microtiter plate (Figure \(\PageIndex{1}\)). Microtiter plates have an array of wells to hold small volumes of reagents and to observe reactions (e.g., agglutination) either visually or using a specially designed spectrophotometer. The wells come in many different sizes for assays involving different volumes of reagents.
Agglutination of Bacteria and Viruses
The use of agglutination tests to identify streptococcal bacteria was developed in the 1920s by Rebecca Lancefieldworking with her colleagues A.R. Dochez and Oswald Avery. 1 She used antibodies to identify M protein, a virulence factor on streptococci that is necessary for the bacteria’s ability to cause strep throat. Production of antibodies against M protein is crucial in mounting a protective response against the bacteria.
Lancefield used antisera to show that different strains of the same species of streptococci express different versions of M protein, which explains why children can come down with strep throat repeatedly. Lancefield classified beta-hemolytic streptococci into many groups based on antigenic differences in group-specific polysaccharides located in the bacterial cell wall. The strains are called serovars because they are differentiated using antisera. Identifying the serovars present in a disease outbreak is important because some serovars may cause more severe disease than others.
The method developed by Lancefield is a direct agglutination assay , since the bacterial cells themselves agglutinate. A similar strategy is more commonly used today when identifying serovars of bacteria and viruses; however, to improve visualization of the agglutination, the antibodies may be attached to inert latex beads. This technique is called an indirect agglutination assay (or latex fixation assay), because the agglutination of the beads is a marker for antibody binding to some other antigen (Figure \(\PageIndex{2}\)). Indirect assays can be used to detect the presence of either antibodies or specific antigens.
To identify antibodies in a patient’s serum, the antigen of interest is attached to latex beads. When mixed with patient serum, the antibodies will bind the antigen, cross-linking the latex beads and causing the beads to agglutinate indirectly; this indicates the presence of the antibody (Figure \(\PageIndex{3}\)). This technique is most often used when looking for IgM antibodies, because their structure provides maximum cross-linking. One widely used example of this assay is a test for rheumatoid factor (RF) to confirm a diagnosis of rheumatoid arthritis. RF is, in fact, the presence of IgM antibodies that bind to the patient’s own IgG. RF will agglutinate IgG-coated latex beads.
In the reverse test, soluble antigens can be detected in a patient’s serum by attaching specific antibodies (commonly mAbs) to the latex beads and mixing this complex with the serum (Figure \(\PageIndex{3}\)).
Agglutination tests are widely used in underdeveloped countries that may lack appropriate facilities for culturing bacteria. For example, the Widal test, used for the diagnosis of typhoid fever, looks for agglutination of Salmonella enterica subspecies typhi in patient sera. The Widal test is rapid, inexpensive, and useful for monitoring the extent of an outbreak; however, it is not as accurate as tests that involve culturing of the bacteria. The Widal test frequently produces false positives in patients with previous infections with other subspecies of Salmonella , as well as false negatives in patients with hyperproteinemia or immune deficiencies.
In addition, agglutination tests are limited by the fact that patients generally do not produce detectable levels of antibody during the first week (or longer) of an infection. A patient is said to have undergone seroconversion when antibody levels reach the threshold for detection. Typically, seroconversion coincides with the onset of signs and symptoms of disease. However, in an HIV infection, for example, it generally takes 3 weeks for seroconversion to take place, and in some instances, it may take much longer.
Similar to techniques for the precipitin ring test and plaque assays, it is routine to prepare serial two-fold dilutions of the patient’s serum and determine the titer of agglutinating antibody present. Since antibody levels change over time in both primary and secondary immune responses, by checking samples over time, changes in antibody titer can be detected. For example, a comparison of the titer during the acute phase of an infection versus the titer from the convalescent phase will distinguish whether an infection is current or has occurred in the past. It is also possible to monitor how well the patient’s immune system is responding to the pathogen.
Query \(\PageIndex{1}\)
Query \(\PageIndex{1}\)
Blood Typing and Cross-Matching
In addition to antibodies against bacteria and viruses to which they have previously been exposed, most individuals also carry antibodies against blood types other than their own. There are presently 33 immunologically important blood-type systems, many of which are restricted within various ethnic groups or rarely result in the production of antibodies. The most important and perhaps best known are the ABO and Rh blood groups (see Figure 19.1.3 ).
When units of blood are being considered for transfusion, pretransfusion blood testing must be performed. For the blood unit, commercially prepared antibodies against the A, B, and Rh antigens are mixed with red blood cells from the units to initially confirm that the blood type on the unit is accurate. Once a unit of blood has been requested for transfusion, it is vitally important to make sure the donor (unit of blood) and recipient (patient) are compatible for these crucial antigens. In addition to confirming the blood type of the unit, the patient’s blood type is also confirmed using the same commercially prepared antibodies to A, B, and Rh. For example, as shown in Figure \(\PageIndex{7}\), if the donor blood is A-positive, it will agglutinate with the anti-A antiserum and with the anti-Rh antiserum. If no agglutination is observed with any of the sera, then the blood type would be O-negative.
Following determination of the blood type, immediately prior to releasing the blood for transfusion, a cross-match is performed in which a small aliquot of the donor red blood cells are mixed with serum from the patient awaiting transfusion. If the patient does have antibodies against the donor red blood cells, hemagglutination will occur. To confirm any negative test results and check for sensitized red blood cells, Coombs’ reagent may be added to the mix to facilitate visualization of the antibody-red blood cell interaction.
Under some circumstances, a minor cross-match may be performed as well. In this assay, a small aliquot of donor serum is mixed with patient red blood cells. This allows the detection of agglutinizing antibodies in the donor serum. This test is rarely necessary because transfusions generally use packed red blood cells with most of the plasma removed by centrifugation.
Red blood cells have many other antigens in addition to ABO and Rh. While most people are unlikely to have antibodies against these antigens, women who have had multiple pregnancies or patients who have had multiple transfusions may have them because of repeated exposure. For this reason, an antibody screen test is used to determine if such antibodies are present. Patient serum is checked against commercially prepared, pooled, type O red blood cells that express these antigens. If agglutination occurs, the antigen to which the patient is responding must be identified and determined not to be present in the donor unit.
Query \(\PageIndex{1}\)
Table \(\PageIndex{1}\) summarizes the various kinds of agglutination assays discussed in this section.
| Type of Assay | Mechanism | Example |
|---|---|---|
| Agglutination | Direct: Antibody is used to clump bacterial cells or other large structures | Serotyping bacteria |
| Indirect: Latex beads are coupled with antigen or antibody to look for antibody or antigen, respectively, in patient serum | Confirming the presence of rheumatoid factor (IgM-binding Ig) in patient serum | |
| Hemagglutination | Direct: Some bacteria and viruses cross-link red blood cells and clump them together | Diagnosing influenza, mumps, and measles |
| Direct Coombs’ test (DAT): Detects nonagglutinating antibodies or complement proteins on red blood cells in vivo | Checking for maternal antibodies binding to neonatal red blood cells | |
| Indirect Coombs’ test (IAT): Screens an individual for antibodies against red blood cell antigens (other than the A and B antigens) that are unbound in a patient’s serum in vitro | Performing pretransfusion blood testing | |
| Viral hemagglutination inhibition: Uses antibodies from a patient to inhibit viral agglutination | Diagnosing various viral diseases by the presence of patient antibodies against the virus | |
| Blood typing and cross-matching: Detects ABO, Rh, and minor antigens in the blood | Matches donor blood to recipient immune requirements |
Key Concepts and Summary
- Antibodies can agglutinate cells or large particles into a visible matrix. Agglutination tests are often done on cards or in microtiter plates that allow multiple reactions to take place side by side using small volumes of reagents.
- Using antisera against certain proteins allows identification of serovars within species of bacteria.
- Detecting antibodies against a pathogen can be a powerful tool for diagnosing disease, but there is a period of time before patients go through seroconversion and the level of antibodies becomes detectable.
- Agglutination of latex beads in indirect agglutination assays can be used to detect the presence of specific antigens or specific antibodies in patient serum.
- Neutralization assays quantify the level of virus-specific antibody by measuring the decrease in hemagglutination observed after mixing patient serum with a standardized amount of virus.
- Hemagglutination assays are also used to screen and cross-match donor and recipient blood to ensure that the transfusion recipient does not have antibodies to antigens in the donated blood.
Footnotes
- 1 Lancefield, Rebecca C., “The Antigenic Complex of Streptococcus haemoliticus . I. Demonstration of a Type-Specific Substance in Extracts of Streptococcus haemolyticus ,” The Journal of Experimental Medicine 47, no. 1 (1928): 91-103.