FIGURE 16.28 • Diagram of different forms of immunoglobulin A (IgA). This drawing shows the monomer of IgA (top). The dimer of IgA is a product of the plasma cell and contains a J chain (J ) connecting two monomers (middle). The secretory component (SC), a product of proteolytically cleaved pIgR is added to the dimer to form secretory IgA (sIgA) (sIgA, bottom).
Here, pIgR is proteolytically cleaved and the extracellular part of the receptor that is bound to dIgA is released into the lumen as secretory IgA (sIgA). This process of synthesis and secretion of IgA is essentially identical to that which occurs in the more distal parts of the  gastrointestinal tract, where sIgA is transported across the absorptive columnar epithelium of the small intestine and colon (see page 596).
Saliva contains water, various proteins, and electrolytes.
Saliva contains chiefly water, proteins and glycoproteins (enzymes and antibodies), and electrolytes. It has a high potassium concentration that is approximately seven times that of blood, a sodium concentration approximately one tenth that of blood, a bicarbonate concentration approximately three times that of blood, and significant amounts of calcium, phosphorus, chloride, thiocyanate, and urea. Lysozyme and -amylase are the principal enzymes present (see Table 16.1).