**Anti-Hepatitis B Core Antibody (Anti-HBc) IgM** is a vital serological marker that indicates a clinical diagnosis of acute or recent Hepatitis B virus (HBV) infection. It is the earliest antibody to appear during an acute outbreak, often becoming detectable in the blood shortly after symptoms surface.
This specific IgM antibody is especially critical during the "window period"—the clinical phase when Hepatitis B Surface Antigen (HBsAg) has cleared, but Hepatitis B Surface Antibody (HBsAb) is not yet detectable. Its presence confirms an active, sharp immune response to a new infection rather than a chronic condition.