1. What is an antigen? Test to find blood type Type of Leukocyte Protein that attacks foreign threats Marker that identifies a cell

antigen is a **marker that identifies a cell**. More specifically, it's any substance that can trigger an immune response, often by being recognized as foreign by the body's immune system. While some antigens are proteins, they can also be carbohydrates, lipids, or nucleic acids. Blood types are determined by the presence or absence of specific antigens on the surface of red blood cells.