Figure 1625

Electron micrograph of an enteroendocrine cell of the colon. Arrowheads mark the boundary between the enteroendocrine cell and the adjacent epithelial cells. At its base, the enteroendocrine cell rests on the basal lamina (BL). This cell does not extend to the epithelial or luminal surface. Numerous secretory granules (G) in the base of the cell are secreted in the direction of the arrows across the basal lamina and into the connective tissue (CT). C, capillary; M, mitochondria; rER, rough endoplasmic reticulum; and sER, smooth endoplasmic reticulum.

dergo activation (see page 358), leading to antibody secretion by newly differentiated plasma cells.

Most of the plasma cells in the lamina propria of the intestine secrete dimeric IgA rather than the more common IgG; other plasma cells produce IgM and IgE (see page 461). IgA is transported across the epithelium, linked to a secretory (75 kDa) glycoprotein component that is synthesized by enterocytes and inserted in the basal plasma membrane as a receptor for IgA. The complex of IgA and secretory component enters into the epithelial cell by en-docytosis at the basal plasma membrane and is subsequently released from the cell into the gut lumen by exo-cytosis at the apical plasma membrane (Fig. 16.26). In the lumen IgA binds to antigens, toxins, and microorganisms. Secretory IgA (slgA) is the principal molecule of mucosal immunity and is the only immunoglobulin isotype that can be selectively passed across the mucosal wall to reach the lumen of the gut. Some of the IgE binds to the plasma membranes of mast cells in the lamina propria (see page 144), selectively sensitizing them to specific antigens derived from the lumen.

Submucosa

A distinguishing characteristic of the duodenum is the presence of submucosal glands

The submucosa consists of a dense connective tissue and localized sites that contain aggregates of adipose cells. A conspicuous feature in the duodenum is the presence of submucosal glands, also called Brunner's glands.

The branched, tubular submucosal glands of the duodenum have secretory cells with characteristics of both zy-mogen-secreting and mucus-secreting cells (Fig. 16.27). The secretion of these glands has a pH of 8.1 to 9.3 and contains neutral and alkaline glycoproteins and bicarbon-

dimers

LAMINA PROPRIA plasma cell

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