Of the five immunoglobulin classes, IgA is the main immunoglobulin of secretions from the genital, respiratory, and intestinal tracts. As a result, IgA antibody is the first line of defense against infections at the mucous membrane. It is usually an early specific antibody. Which of the following statements most accurately describes IgA?

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.