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")
Bilinear form form on a normed vector space is elliptic, or coercive, if there is a constant such that for all , Given a ring "R" and a right "R"-module "M" and its dual module "M", a mapping is called a bilinear form if for all , , . The mapping is known as the "natural pairing", also called the "canonical bilinear form" on . A linear map induces the bilinear form , and a linear map induces the bilinear form . Conversely, a bilinear form induces the "R"-linear maps and . Here, "M" denotes the double dual of "M". Bilinear form