The complement fixation test is an

immunological Immunology is a branch of medicineImmunology for Medical Students, Roderick Nairn, Matthew Helbert, Mosby, 2007 and biology that covers the medical study of immune systems in humans, animals, plants and sapient species. In such we can see the ...

medical Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care practic ...

test that can be used to detect the presence of either specific

antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of t ...

or specific

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...

in a patient's

serum Serum may refer to: * Serum (blood), plasma from which the clotting proteins have been removed **Antiserum, blood serum with specific antibodies for passive immunity * Serous fluid, any clear bodily fluid *Truth serum, a drug that is likely to mak ...

, based on whether complement fixation occurs. It was widely used to diagnose infections, particularly with microbes that are not easily detected by culture methods, and in rheumatic diseases. However, in clinical diagnostics labs it has been largely superseded by other serological methods such as

ELISA The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...

and by DNA-based methods of pathogen detection, particularly PCR.

Process

The

complement system The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and ...

is a system of serum proteins that react with antigen-antibody complexes. If this reaction occurs on a cell surface, it will result in the formation of trans-membrane pores and therefore destruction of the cell. The basic steps of a complement fixation test are as follows: # Serum is separated from the patient. # Patients naturally have different levels of complement proteins in their serum. To negate any effects this might have on the test, the complement proteins in the patient's serum must be destroyed and replaced by a known amount of standardized complement proteins. ## The serum is heated in such a way that all of the complement proteins—but none of the antibodies—within it are destroyed. (This is possible because complement proteins are much more susceptible to destruction by heat than antibodies.) ## A known amount of standard complement proteins are added to the serum. (These proteins are frequently obtained from guinea pig serum.) # The antigen of interest is added to the serum. # Sheep

red blood cells Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "hol ...

(sRBCs) which have been pre-bound to anti-sRBC antibodies are added to the serum. The test is considered negative if the solution turns pink at this point and positive otherwise. If the patient's serum contains antibodies against the antigen of interest, they will bind to the antigen in step 3 to form antigen-antibody complexes. The complement proteins will react with these complexes and be depleted. Thus when the sRBC-antibody complexes are added in step 4, there will be no complement left in the serum. However, if no antibodies against the antigen of interest are present, the complement will not be depleted and it will react with the sRBC-antibody complexes added in step 4, lysing the sRBCs and spilling their contents into the solution, thereby turning the solution pink.

Testing for antigen

While detection of antibodies is the most common test format, it is equally possible to test for the presence of antigen. In this case, the patient's serum is supplemented with specific antibody to induce formation of complexes; addition of complement and indicator sRBC is performed as before.

Semi-quantitative testing

The test can be made quantitative by setting up a series of dilutions of patient serum and determining the highest dilution factor that will still yield a positive CF test. This dilution factor corresponds to the

titer Titer (American English) or titre (British English) is a way of expressing concentration. Titer testing employs serial dilution to obtain approximate quantitative information from an analytical procedure that inherently only evaluates as positiv ...

References

External links

* {{Immunologic techniques and tests Blood tests Immunologic tests