Antibody. Computer graphic of a molecule of immunoglobulin G1, a type of antibody protein that the immune system uses to fight infections. The molecule's atoms have been colour-coded: carbon is grey, sulphur is yellow, nitrogen is blue and oxygen is red. Immunoglobulin G is the most common antibody found in blood. They are produced by lymphocytes (a type of white blood cell) in response to the proteins (antigens) found on foreign cells such as bacteria and viruses. Each antibody recognises a specific antigen, either destroying or neutralising it. Immunoglobulin G can cross a mother's placenta, making it important in the immune response of a developing foetus.