Fig. 26.2, A ). Another prominent feature is a diffuse submucosal and intraepithelial network of solitary lymphocytes and innate lymphoid cells scattered throughout the respiratory tract. Because inhaled particles are broadly dispersed throughout the respiratory tract, each type of lymphoid cell and tissue (BALT, TELT, and solitary lymphocytes) play important and unique roles in the overall defense of the lungs.
One of the specialized features of MALT, GALT, and BALT is a unique antibody system in which specialized features • Fig. 26.2 Representation of Bronchus-Associated Lymphoid Tissue (BALT)/Tertiary Ectopic Lymphoid Tissue (TELT), M Cells, and Immunoglobulin a (IgA) Synthesis.A, M cells located in mucosal epithelium endocytose antigen in the lumen and transport it for processing to loosely organized submucosal pockets of lymphoid cells, predominantly lymphocytes and macrophages (TELT). B, Diagram of a mucous membrane, showing secretion of IgA antibodies in response to antigen endocytosed by M cells. Activated B cells differentiate into IgA-producing plasma cells (lymphocytes) and migrate from the densely organized lymphoid follicle (in bronchus-associated lymphoid tissue [BALT]) to the nearby submucosa, where they secret IgA.
of the IgA antibody (non–complement-fixing, J chain for transport, and dimeric structure for stability in the airway lumen) are used. In submucosal areas, plasma cells synthesize and secrete IgA, which migrates to the submucosal surface of epithelial cells, where it binds to a surface protein •Fig. 26.3 Structure and Formation of Secretory Immunoglobulin a (IgA). A, SecretoryIgAconsistsofatleasttwoIgAmoleculesthatarecovalentlylinkedviaJchainandcovalentlyassociatedwiththesecretorycomponent.Thesecretorycomponentcontainsfiveimmunoglobulin-likedomainsandislinkedtodimericIgAthroughbindingtoanIgAheavychain.B, SecretoryIgAisformedduringtransportthroughepithelialcells.Poly-Ig,polymericimmunoglobulin.
receptor, polymeric immunoglobulin (poly-Ig;
Fig. 26.3 ). The poly-Ig receptor aids in the pinocytosis of IgA into the epithelial cell and its eventual secretion into the airway lumen. During exocytosis of the IgA complex, the poly-Ig is enzymatically cleaved, and a portion of it, the secretory piece, remains associated with the complex. The secretory piece stays attached to the IgA complex in the airway and helps protect it from proteolytic cleavage. The IgA-antigen immune complex does not bind complement in the same classical manner as do other immune complexes, and thus its proinflammatory properties are limited. The IgA-antibody system is very effective in binding particulates and viruses to form a large complex, which promotes its removal via the mucociliary clearance system, before they invade epithelial cells.