A molecular ribbon model of an antibody (immunoglobulin), a Y-shaped protein used in the immune system to identify and neutralize foreign antigens such as bacteria and viruses. Antibodies are secreted by white blood cells known as plasma cells, with five different antibody isotypes responsible for different roles during an immune response. Antibodies typically contain two large heavy chains (red and green) bonded to two light chains (yellow and blue) to create the immunoglobulin monomer.