Immunoglobulin A only weakly. IgA exists in two isotypes, IgA1 and IgA2. They are both heavily glycosylated proteins. While IgA1 predominates in serum (~80%), IgA2 percentages are higher in secretions than in serum (~35% in secretions); the ratio of IgA1 and IgA2 secreting cells varies in the different lymphoid tissues of the human body: Both IgA1 and IgA2 have been found in external secretions like colostrum, maternal milk, tears and saliva, where IgA2 is more prominent than in the blood. Polysaccharide antigens tend to induce more IgA2 than protein antigens. Both IgA1 and IgA2 can be in membrane-bound form. ("see B-cell receptor")
Hasse invariant of a quadratic form Hasse invariant of a quadratic form In mathematics, the Hasse invariant (or Hasse–Witt invariant) of a quadratic form "Q" over a field "K" takes values in the Brauer group Br("K"). The name "Hasse–Witt" comes from Helmut Hasse and Ernst Witt. The quadratic form "Q" may be taken as a diagonal form Its invariant is then defined as the product of the classes in the Brauer group of all the quaternion algebras This is independent of the diagonal form chosen to compute it. It may also be viewed as the second Stiefel–Whitney class of "Q". The invariant may be computed for