Viral antigens are specific molecules found on the surface of viruses that trigger an immune response in the host. These antigens can be proteins or carbohydrates and are recognized by the immune system, particularly by antibodies produced by B cells. When the body detects these antigens, it activates various immune responses to fight off the viral infection. The presence of viral antigens is crucial for diagnosing viral infections. Medical tests, such as ELISA (Enzyme-Linked Immunosorbent Assay) or PCR (Polymerase Chain Reaction), can detect these antigens in blood or tissue samples. Identifying viral antigens helps healthcare professionals determine the type of virus and guide appropriate treatment.