CFT Full Form

The complement fixation test is an immunology diagnostic assessment that could predict the existence of a certain antibody or antigen in a clinical specimen depending on whether or not complements fixation occurs. Antibodies against MAP generated in response to pathogens are detected in serum (the fluid, non-cellular part of blood). Antibodies bind to proteins and subsequently bind (fix) complement in this approach, which is also used to diagnose various infectious illnesses. An indication system that includes anti-red blood cell antibody-coated red blood cells permits assessment about whether addition was established, implying that anti-MAP antibody was present in the data serum sample. 

Principles of CFT

It is possible to determine how much antibody to MAP is present in a sample by evaluating each serum sample’s serial dilutions. The greatest serum dilution is given, not just the “fixing” component. Although, in professional diagnostics labs, alternative serological approaches like ELISA and DNA-based pathogen detection methods, particularly PCR, have essentially supplanted it. It was commonly used to diagnose infections, especially caused by bacteria that were difficult to detect using culture methods and rheumatic illnesses. The CF test is based on two fundamental principles:

1. Many types of antibody-antigen interactions bind (fix) complement (C) in an irreversible manner (certain classes of antibodies do not fix the complement). The relative concentration of antibody or antigen depends on the degree of attachment.
2. The availability of an unbound response is required for the lysis of SRBC sensitised with hemolysin. The following is how the CF test is interpreted:

• Minimal hemolysis if the antibody is expressed
• Hemolysis occurs when an antibody is missing

Because there is some crossover in antigenicity across the several fungi and the symptoms of the infections are extremely similar, patient sera should indeed be examined for each of the antigens. Higher CF titers are frequently seen when patient sera are tested against the other antigen as the etiologic agent.

Antigen detection

While antibody detection is the most common test, antigen detection is also feasible. In this case, a particular antibody is added to the patient’s serum to encourage the formation of compounds; complementing and the marker sRBC are added as before.

Semi-quantitative Testing

Setting up a series of dilutions of patient serum and establishing the largest dilution factor that still yields a positive CF test can make the test quantitative. The titer relates to this dilution factor.

Process of CFT

The complement structure combines serum proteins that bind to antigen-antibody complexes and react with them. If such a reaction happens on a cell’s surface, this should result in the creation of trans-membrane holes, and, as a consequence, the cell will be destroyed. The following are the basic steps in a complement fixation test: 

1. The patient’s serum is extracted.
2. Complement proteins are found in varying amounts in the blood of patients. The complement proteins in the clinical specimen must be eliminated and replaced with a predetermined amount of standardised complement proteins to negate any impact on the test.
3. The serum is heated to the point where all complement proteins are eliminated, and none of the antibodies is. (This is because complement proteins are far more vulnerable to heat than antibodies.)
4. The serum is spiked with a known amount of standard complement proteins. (Guinea pig serum is usually used to obtain these proteins.)
5. The serum is infused with the protein of interest.
6. The serum is supplemented with sheep red blood cells (RBCs) that have been pre-bound to anti-sRBC antibodies. If the solution turns pink at this stage, the test is false; alternatively, it is affirmative.
7. Antibodies against the antigen of interest in the clinical specimen would attach to the antigen in step 3 to produce antigen-antibody combinations. Complementary protein will be diminished due to their reactions with these complexes. There will be no complement in the serum whenever the sRBC-antibody complexes are introduced in step 4. Suppose no antibodies are available against the protein of interest. In that case, the complement will be not be depleted. It will react with the RBC-antibody complexes supplied in step 4, data the sRBCs, and spill their components into the solution, colouring the solution pink.

Conclusion

The CF test is frequently used to screen for antibodies against several potentially harmful microorganisms (particularly viruses); a pool of 15–20 antigens can be utilised for this purpose. The majority of antigens are available on the market and are inexpensive. The test may largely be done with relatively crude antigens (infected cell lysates). Because an overabundance of antibodies in high-titer sera may fail to form complement-activating immune complexes (the “prozone’ effect), test sera must be tested in at least two dilutions. If the patient serum specimen is ‘anticomplementary,’ that is, it consumes complement from the test mixture without adding antigen, the CF test cannot be used. 

The presence of pre-existing complement-activating elements in the patient’s sera is usually the cause of anti-complementarity. Immunological complexes and other immune aggregates, cryoglobulins, contaminating microorganisms, and bacterial products like endotoxin are examples of these. Acute sera from patients infected with parvovirus B19, for example, frequently include immune complexes and are anticomplementary.