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A surface antigen is a type of protein on a cell membrane that can signal various biological processes, including immune reactions. There are various antigens which often include receptors, adhesion molecules, and enzymes. Some varieties are released into the environment between cells and, depending on the type of protein, certain diseases or the rejection of an organ transplant can be detected. Surface antigens are often released by viruses such as Hepatitis B (HBV). Able to signal viruses to replicate, these compounds in a blood sample are typically used to diagnose the disease.
Antigens for HBV are generally released by replicating viruses in the liver, and then excess proteins coalesce in the blood. A laboratory test can identify the surface antigen in a sample. One type associated with HBV is usually found in liver cells, while others can be detected in the bloodstream. If the virus is found in conjunction with antibodies, this typically means the immune system is attempting to fight off the infection. A negative test for antibodies generally means that there is no immune response to the virus.
In general, the kind of surface antigen present helps determine the type of immune cells that are most prevalent, as well as the stage of their lifecycle. Several types of antigens can be on the surface of an immune cell called a lymphocyte. Since they are often used to help identify the cell, antigen markers are also called Clusters of Differentiation (CD). The different types of antigens on a cell surface can include an adhesion molecule called immunoglobulin, and Beta 2 integrin that is associated with lymphocyte function. Others include antigens such as selectin and cadherin.
Some CD antigens act as markers for helper and suppressor cells in the immune system. Viruses can dock to certain ones in order to trigger an infection, as when Human Immunodeficiency Virus (HIV) binds to the receptor to enter a cell. Another kind of surface antigen serves as a marker for stem cells, or those that can be activated and form into any other tissue in the body.
There are many surface antigen proteins that transmit signals across the membrane of cells. These signals can be sent if an immune cell is activated and is communicating to others to respond. Researchers generally know what antigens are related to what kinds of cells, as well as those which are associated with particular illnesses. An antigen test is often enough to diagnose some diseases.