An Arkray Anti-Human Serum (Coomb's) with C3D is an in vitro diagnostic reagent used in blood banking to detect antibodies and complement components (specifically C3d) attached to red blood cells. It's a form of Anti-Human Globulin (AHG) and is used in both the direct antiglobulin test (DAT) and indirect antiglobulin test to determine if red blood cells are sensitized with these substances, a crucial step for compatibility testing and other blood-related diagnostics.