An assay can be any test that is designed to show the composition of a substance or the proportions of the components within that substance. A cytokine assay is a test to determine the types or the concentrations of cytokine molecules in a particular biological sample. Cytokines are small proteins that are secreted from most cells in the body and act as signal carriers between cells. Some examples of categories of cytokines are growth factors, which either stimulate or inhibit cell growth; interleukins, which stimulate the body to produce T and B immune cells; and interferons, which interfere with virus replication along with other functions in the immune response. Many of these, such as interferons, originally were classified and named based on the first function that they were discovered to have but have since been found to have multiple roles in different parts of the body.
Most often, a cytokine assay is used in research as scientists work to understand how cells use cytokines to interact and communicate with each other. Monitoring the treatment of patients with certain diseases that are marked by the presence of certain cytokines or that require them to receive recombinant cytokines or anticytokine medication is made much more accurate with the use of a cytokine assay. In this case, the tests could show whether a disease process was active, whether the recombinant cytokines were biologically active or whether treatment with certain drugs was effective. Some diseases that are already treated by adjusting cytokine levels are rheumatoid arthritis, multiple sclerosis, psoriasis and some forms of cancer. Other conditions, such as fibromyalgia, chronic fatigue, asthma and chronic obstructive pulmonary disease (COPD), are known or suspected to involve cytokines, and scientists have worked to develop treatment strategies.
There are several common types of a cytokine assay, many of which are variations on the enzyme-linked immunosorbent assay (ELISA), which works by fixing the receptor for the cytokine is fixed to a surface. The cytokine is allowed to bond with it, and another receptor that is marked with either a fluorescent dye or radioactive isotope then bonds to the cytokine. The secretion assay is comparable to the ELISA but uses a technique called fluorescent-activated cell sorting that results in the collection of living cells rather than a blend of homogenized cell parts. The cells are labeled for sorting based on the type of cytokines they are secreting. Radioimmunoassays, radioreceptor assays, reverse hemolytic plaque assays, cell blot assays and kinase receptor activation assays can all function as a type of a cytokine assay.