As noted, saliva contains antibodies, salivary immunoglobulin A (IgA). IgA is synthesized by plasma cells in the connective tissue surrounding the secretory acini of the salivary glands, and both dimeric and monomeric forms are released into the connective tissue matrix (Fig. 16.28).
FIGURE 16.27 • Photomicrographs of the three major salivary glands. a. The parotid gland in the human is composed entirely of serous acini and their ducts. Typically, adipose cells are also distributed throughout the gland. The lower portion of the figure reveals an excretory duct within a connective tissue septum. 120. Inset. Higher magnification of the serous acinar cells. 320. b. The submandibular glands contain both serous and mucous acini. In humans, the serous components predominate. The mucus-secreting acini are readily discernible at this low magnification because of their light staining. The remainder of the field is composed largely of serous acini. Various ducts—excretory, striated, and intercalated—are evident in the field. 120. Left inset. Higher magnification of an acinus revealing a serous demilune surrounding mucus-secreting cells. 360. Right inset. Higher magnification of a striated duct. These ducts have columnar epithelium with visible basal striations. 320. c. The sublingual gland also contains both serous and mucous elements. Here, the mucous acini predominate. The mucous acini are conspicuous because of their light staining. Critical examination of the mucous acini at this relatively low magnification reveals that they are not spherical structures but, rather, elongate or tubular structures with branching outpockets. Thus, the acinus is rather large, and much of it is usually not seen within the plane of a single section. The ducts of the sublingual gland that are observed with the greatest frequency in a section are the interlobular ducts. 120. Inset. The serous component of the gland is composed largely of demilunes (asterisks), artifacts of conventional fixation. 320.
TABLE Composition of Unstimulated Saliva16.1 Organic constituents Mean (mg/mL) Protein 220.0 Amylase 38.0 Mucin 2.7 Muramidase (lysozyme) 22.0 Lactoferrin 0.03 ABO group markers 0.005 EGF 3.4 sIgA 19.0 IgG 1.4 IgM 0.2 Glucose 1.0 Urea 20.0 Uric acid 1.5 Creatinine 0.1 Cholesterol 8.0 cAMP 7.0 Inorganic constituents Sodium 15.0 Potassium 80.0 Thiocyanate Smokers 9.0 Nonsmokers 2.0 Calcium 5.8 Phosphate 16.8 Chloride 50.0 Fluoride Traces (according to intake) Modified from Jenkins GN. The Physiology and Biochemistry of the Mouth, 4th ed. Oxford: Blackwell Scientific Publications, 1978. sIgA, secretory IgA; EGF, epithelial growth factor.
A polymeric immunoglobulin receptor (pIgR) protein is synthesized by the salivary gland cells and inserted into the basal plasma membrane, where it serves as a receptor for dimeric IgA.
When the dimeric IgA binds to the receptor, the pIgR-dIgA complex is internalized by receptor-mediated endocytosis and carried through the acinar cell to the apical plasma membrane.