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Solitary lymphocytes and innate lymphoid cells are dispersed throughout the respiratory tract, forming a diffuse submucosal and intraepithelial network (Fig. 26.2, A). Each type of lymphoid cell and tissue (BALT, TELT, and solitary lymphocytes) plays a unique and crucial role in defending the lungs due to the broad dispersion of inhaled particles.

MALT, GALT, and BALT possess a distinctive antibody system. This system involves specialized features for the synthesis of immunoglobulin A (IgA). M cells located in the mucosal epithelium transport antigens from the lumen to submucosal pockets of lymphoid cells (TELT), predominantly composed of lymphocytes and macrophages (Fig. 26.2, A). IgA antibodies are secreted in response to antigens endocytosed by M cells. Activated B cells differentiate into IgA-producing plasma cells and migrate from the densely organized lymphoid follicle in BALT to the nearby submucosa (Fig. 26.2, B). There, they secrete IgA.

The unique features of IgA antibody, including non–complement-fixing properties, J chain for transport, and dimeric structure for stability in the airway lumen, are utilized. In submucosal areas, plasma cells synthesize and secrete IgA. The secreted IgA binds to a surface protein receptor called polymeric immunoglobulin (poly-Ig) on the submucosal surface of epithelial cells (Fig. 26.3). The poly-Ig receptor facilitates the pinocytosis of IgA into the epithelial cell, eventually leading to its secretion into the airway lumen. During exocytosis, the poly-Ig is enzymatically cleaved, with a portion of it, known as the secretory piece, remaining associated with the IgA complex. The secretory piece remains attached to the IgA complex in the airway, protecting it from proteolytic cleavage.

Unlike other immune complexes, the IgA-antigen immune complex does not bind complement in the classical manner. Hence, its proinflammatory properties are limited. The IgA-antibody system effectively binds particulates and viruses, forming large complexes. These complexes aid in their removal via the mucociliary clearance system before they can invade epithelial cells.

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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.