Serum is neither an antigen nor an antibody. Serum contains various antibodies because serum is the liquid part left after the plasma has clotted. Compared to plasma, serum contains fewer components of the various clotting factors involved in the clotting pathway and the same other components as plasma, including various ions, blood glucose, blood lipids, and various antibodies. Therefore, in clinical practice, serum antibodies are applied when checking a patient’s antibodies, such as hepatitis B virus antibodies, hepatitis C antibodies, syphilis antibodies, AIDS antibodies, epidemic hemorrhagic fever antibodies, mycoplasma antibodies, chlamydia antibodies, etc. By checking the antibodies in the serum, it is possible to know whether the body is infected with various viruses or pathogenic microorganisms and thus make a clear diagnosis of the disease. Therefore, the serum contains various antibodies, and by testing the antibodies in the serum, it is possible to know whether the body is infected with a disease or not.