A virus antigen is a toxin or other substance given off by a virus which causes an immune response in its host. The antigen is what causes common symptoms related to a viral infection, such as fever. Although the responses caused by a virus antigen can be annoying, they are often the first alert that something is wrong within the body that needs to be taken care of.
When a virus enters the body of a person or animal, it releases proteins, toxins, or enzymes into the bloodstream. These can cause symptoms such as sore throat or cough, but they also alert white blood cells of the virus’s presence. The cells recognize the virus antigen as a foreign body and send signals to the brain to release more white blood cells. Once the white blood cells, including killer cells, are released in higher numbers; they hunt down and destroy viral cells.
This process is how the immune system recognizes disease and acts to destroy it. Symptoms like fever, swelling, or pus in certain areas mean that the immune system is doing its job. The immune response caused by a virus antigen can also result in swollen lymph nodes, runny nose, inflammation, and other symptoms like mucus buildup in the nose, throat, or chest.
The impact a virus antigen has on the body varies based on the type of virus it is. Some viral infections are more deadly and illicit a much harsher response than others. Many childhood illnesses are caused by viruses, and these are generally uncomfortable but easily wiped out. Others, like influenza, may produce antigens that cause more severe reactions. These are often treated with anti-viral medications to alleviate symptoms until the immune system can destroy the virus.
In some cases, a virus antigen may not be released or may not be detected. This results in infections that do not cause symptoms, or those that lie dormant for weeks, months, or even years. The human immunodeficiency virus (HIV) often does not cause any discomfort or illness until many years after exposure. This does not mean that HIV does not produce an antigen, but that it does not result in a strong immune response.
Many times the immune response caused by a virus antigen is the most dangerous part of being infected with a virus. Though some viral infections do damage on their own, the immune system itself causes symptoms so severe that they become lethal to the infected person. For example, the flu can result in extreme inflammation and congestion in the lungs. This can be fatal for those who have pre-existing lung conditions or suppressed immune systems.