Lymphatics: MALT

Common Histological Features of MALT

Despite appearing in different organs and carrying different regional names, all MALT shares a recognizable set of histological features:

Lymphoid follicles (primary and secondary) are embedded directly in the lamina propria or submucosa — the connective tissue layers just beneath the epithelium — without a surrounding fibrous capsuleThe outer protective layer of the kidney.. This is an important distinction from lymph nodes: MALT is not encapsulated.

A specialized epithelium overlies the follicles in many MALT sites. This epithelium contains M cells (microfold cells) — modified epithelial cells with fewer microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption., a thinner mucous coat, and deepAway from the surface of the body. pockets in their basal surface that cradle antigen-presenting cells. M cells sample antigens from the lumenThe inside space of a hollow organ or structure. and transcytose them across the epithelial barrier, delivering them directly to underlying lymphocytes and dendritic cellsImmune cells in the epidermis that help fight infections..

Diffuse lymphoid tissue surrounds and extends beyond the organized follicles, filling the lamina propria with scattered T cells, plasma cellsImmune cells that develop from B cells and produce antibodies., macrophages, and dendritic cells. This diffuse component is present even in areas without discrete follicles.

Section through MALT (e.g., Peyer’s patch or tonsillar crypt) at low power — identify overlying epithelium, lymphoid follicle, and surrounding diffuse lymphoid tissue in the lamina propria

GALT — Gut-Associated Lymphoid Tissue

GALT is the largest subdivision of MALT in the body, which makes sense given that the GI tract has the most extensive mucosal surface and the highest antigen load of any body system. GALT is distributed along the entire length of the GI tract and takes several organizational forms:

Peyer’s patches are the most structurally organized component of GALT. They are large, permanent aggregations of lymphoid follicles found primarily in the submucosa of the ileumThe last section of the small intestine, responsible for absorbing vitamin B12 and bile salts. (the final segment of the small intestine). On a low-power slide, Peyer’s patches appear as prominent bulges of lymphoid tissue pushing up into and deforming the overlying mucosaThe innermost lining of the digestive tract that contains mucus-secreting cells for protection and a. The follicle-associated epithelium above each patch is flattened and contains abundant M cells. Peyer’s patches are where many oral vaccines — including the polio vaccine — do their work. Antigens sampled through M cells activate B cells in the follicles, which differentiate into IgA-secreting plasmaThe liquid component of blood. cells. The IgA is then exported across the epithelium into the gut lumen, where it neutralizes pathogens before they can attach to the mucosal surface.

Isolated lymphoid follicles are single, smaller follicles distributed throughout the length of the intestine, particularly abundant in the colon. They lack the organized, permanent architecture of Peyer’s patches.

The lamina propria throughout the GI tract contains a massive diffuse component: thousands of plasma cells secreting IgA, T cells, macrophages, dendritic cells, and eosinophils embedded in loose connective tissue beneath the epithelium. This diffuse GALT is always present and does not require antigen stimulation to be maintained.

The appendixA small, finger-like pouch attached to the cecum, thought to play a role in immune function., despite its reputation, contains substantial organized GALT — dense lymphoid follicles packed into its wall — and is considered a functional component of gut immunity, not simply a vestigial structure.

Low- or medium-power section A cut or slice of the body or an organ for study. through the ileum showing a Peyer’s patch — identify the submucosa location, multiple follicles, overlying modified epithelium (FAE), and M cells if resolution permits

BALT — Bronchus-Associated Lymphoid Tissue

BALT is found in the walls of the bronchiThe large airways that branch from the trachea into the lungs, dividing into smaller bronchioles. and bronchiolesSmall airways branching from the bronchi that lead to alveoli; lack cartilage and control airflow wi. Unlike Peyer’s patches, which are permanent structures present from birth, BALT is largely inducible — meaning it forms in response to infection or chronic inflammation and may not be present in histologically normal lung tissue from a healthy individual. This makes it what researchers call tertiary lymphoid tissue in its induced form.

When present, BALT appears as clusters of lymphoid follicles in the bronchial submucosa, often with a visible germinal center indicating active immune response. B cell zones are adjacent to T cell zones, mirroring the architecture of a lymph node, but without the surrounding capsule or defined sinus system.

BALT plays a significant role in respiratory immunity — particularly in chronic infections (tuberculosis, influenza), autoimmune lung disease, and in the context of lung transplantation, where its presence has been associated with both protection and rejection, depending on the context.

Bronchial wall section showing BALT — identify the bronchial epithelium, submucosa, and lymphoid follicle aggregate (with or without germinal center)

NALT — Nasal-Associated Lymphoid Tissue (and the Waldeyer’s Ring)

NALT refers to the lymphoid tissue associated with the nasalTwo small rectangular bones forming the bridge of the nose. passages and nasopharynx. In humans, its most prominent and clinically familiar manifestation is Waldeyer’s tonsillar ring — the circular arrangement of tonsillar tissue surrounding the entrance to the pharynxThe muscular passageway connecting the mouth to the esophagus and larynx..

Waldeyer’s ring includes the pharyngeal tonsil (adenoid) on the posterior wall of the nasopharynx, the two palatineL-shaped bones forming posterior part of hard palate and floor of nasal cavity. tonsils on either side of the oropharynx, the lingual tonsil at the baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). of the tongue, and smaller tubal tonsils near the openings of the auditory tubes. Together they form a first line of immune surveillance for anything entering through the nose or mouthThe opening of the digestive tract where food enters and mastication begins..

All tonsils share a distinctive histological appearance: they are not fully encapsulated (only a partial capsule is present on the deep surface), and they contain large secondary follicles with prominent germinal centers invaginating deep into the lymphoid tissue. The surface epithelium dips inward to form deep pits called crypts. Crypts dramatically increase the surface area available for antigen sampling, but they also trap debris and bacteria — the reason tonsils can become chronically infected.

The palatine tonsil is covered by stratified squamous epithelium (continuous with the oral cavity), while the pharyngeal tonsil (adenoid) is covered primarily by pseudostratified ciliated columnar epithelium, consistent with its nasopharyngeal location.

— [IMAGE PLACEHOLDER] —
Suggested label: Palatine tonsil section — identify the stratified squamous epithelium, crypts (epithelial infoldings), lymphoid follicles with germinal centers, and the partial capsule on the deep surface

MALT in Other Locations

MALT is not restricted to the gut, lungs, and pharynx. Organized or diffuse lymphoid tissue is found in the mucosae of virtually every system that communicates with the external environment:

Conjunctiva-associated lymphoid tissue (CALT) is found in the conjunctiva of the eye and is part of the immune defense of the ocular surface.

Lacrimal drainage-associated lymphoid tissue (LDALT) is present in the nasolacrimal duct system.

Salivary gland-associated lymphoid tissue occurs in and around the major salivary glands. In Sjögren’s syndrome — an autoimmune condition — pathological MALT accumulations in the salivary and lacrimalSmall, thin bones on the medial wall of each orbit; contain a groove for tear drainage. glands lead to the destruction of secretory tissue, causing dry mouth and dry eyes.

Urogenital tract MALT is present in the mucosae of the urethraThe tube that carries urine from the bladder to the outside of the body., vaginaThe muscular canal connecting the uterus to the external genitalia., and cervix, providing local IgA-mediated defense.

The significance of MALT across all these sites is the same: IgA. Plasma cells in MALT predominantly produce IgA, which is then transcytosed across the epithelium and released as secretory IgA (sIgA) into the lumen. sIgA is the dominant antibody of mucosal immunity — it does not fix complement or cause inflammation, but neutralizes pathogens and prevents them from adhering to the epithelial surface in the first place.