Each secretory IgA molecule has a molecular weight of 400,000 and consists of two H2L2 units and one molecule each of J chain and secretory component. Some IgA exists in serum as a monomer H2L2 with a molecular weight of 160,000. Some bacteria, such as Neisseria, can destroy IgA-1 by producing protease. It is the major immunoglobulin in milk, saliva, tears, mucus, sweat, gastric fluid, and colostrum. IgA does not fix complement, so one would anticipate that a complement fixation test would not be useful for IgA antibody. It is important to remember that of the five classes of immunoglobulins, only IgG and IgM are complement-fixing/activating anti bodies.