Knops System

Knops system antigens (ISBT 022) were discovered in 1964, but their function was not known until 1992 when they were found to be located on complement receptor 1 (CR1). CR1 is a membrane-bound glycoprotein found on most blood cells, with the exception of platelets. CR1 mediates phagocytosis by neutrophils and monocytes and has an inhibitory effect on complement activation, which aids in the prevention of autohemolysis.

Antibodies against Knops system antigens are generally not considered clinically significant. However, CR1 depression has been linked to several disease states, including SLE, Insulin Dependent Diabetes (IDD), Cold Agglutinin Disease (CAD), PNH, hemolytic anemia, and HIV infection.

Knops system antigens are resistant to enzymes and sensitive to trypsin, alpha-chymotrypsin, and disulfide bond-reducing agents, which can help aid in antibody identification studies.

The Knops system consists of 9 antigens—Kn^a and Kn^b, McC^a and McC^b, Sl^a, Sl3, Yk^a, KCAM, and Vil. Kn^a, McC^a, Sl^a, Yk^a, and KCAM are found in more than 90% of the population and are the most common antibodies in the system to interfere with serological testing.