General structure of a local MALT segment
Mucosal-associated lymphoid tissue is constantly present in the digestive tract and less consistently in other barrier systems. The local segment of mucosal immunity is a unified complex permeating the organ wall.
Its structure includes:
- Wall layers: epithelium (containing specialized cells), lamina propria (L. propria), and submucosa.
- Lymphoid follicle: structurally integrates these layers.
- Outflow pathways: lymph flows from the follicle directly into regional lymph nodes.
Functionally, the MALT system is clearly divided into two compartments:
- Afferent compartment — receives and primarily processes immune information. Represented by organized structures.
- Efferent (effector) compartment — carries out the immune response. Represented by diffuse elements.
M-cells: customs of barrier tissues
Normally, antigens and microbial bodies continuously penetrate even intact, undamaged mucosal barriers. This is facilitated by specialized M cells (microfold cells, from the English microfold).
They are bell-shaped, with the concave part facing the underlying follicles, and are part of the follicular epithelium (occupying about 10% of the surface of Peyer's patches).
M-cell features:
- The apical surface has microfolds that drastically increase the absorption area.
- They completely lack a mucus layer, which distinguishes them from neighboring epithelial cells.
- They actively transport antigenic material into the tissues. The transport mechanism is independent of MHC-dependent processing.
- The associated space (directly beneath the M-cell) contains T and B lymphocytes (predominantly memory cells), and slightly deeper — macrophages and dendritic cells.
Organized structures and Peyer's patches
The afferent compartment is represented by solitary follicles and their more complex aggregations.
The most developed organized structures of the GI tract are Peyer's patches in the small intestine (zones of maximal antigenic load). These are functional analogs of lymph nodes, including thymus-dependent zones and areas of T and B cell co-localization.
Their two main differences from regular lymph nodes:
- Absence of a connective tissue capsule.
- Presence of a follicular dome (dome).
The dome is located immediately beneath the M-cell layer. This is where dendritic cells capture material brought by M-cells and present it to T lymphocytes, initiating humoral and cellular responses.
In addition to the intestine (where lymphoid tissue peaks in the appendix), organized structures are represented by tonsils. Six of them (lingual, pharyngeal, a pair of palatine, and a pair of tubal) form Waldeyer's pharyngeal ring.
Diffuse lymphoid tissue (efferent compartment)
The cellular composition of the effector compartment is strictly distributed across layers:
Epithelial layer immunocytes Represented by intraepithelial T lymphocytes (mostly memory cells and activated lymphocytes), dendritic cells, and NKT cells. In the small intestine, CD8+ T cells predominate over CD4+. There are many γδ T cells here (significantly more than in the blood, up to 10–15% in humans). Unique subpopulations are identified, such as CD8αα+ γδ T cells and "double-negative" lymphocytes lacking co-receptors (up to 5%), which presumably develop extrathymically.
Lamina propria immunocytes (L. propria) The composition here is closer to peripheral blood: macrophages, αβ T cells, B1 cells, natural killer (NK) cells, and mast cells are present. The main feature of B lymphocytes in this layer is the total predominance of IgA+ cells, which is atypical for the spleen or lymph nodes.
Recirculation, homing, and tract-specific features
Mucosal immunity possesses autonomy. Tissues of different mucous membranes are connected by their own recirculation pathways.
The intestine holds a special position in this regard. To enter the intestinal mucosa, a lymphocyte requires additional homing molecules — for example, integrin α₄β₇, which binds to the MadCAM molecule on the intestinal vascular endothelium. A lymphocyte that has received a "pass" into the intestine can migrate to any other mucous membranes, but the reverse rule does not apply.
Features of barrier tissues:
- Bronchopulmonary tract: Organized follicles are present in less than half of people (depending on the level of antigenic load in early postnatal periods). Temporary (transient) follicles can form during infection. Practical conclusion: the absence of follicles makes inhalation vaccination ineffective.
- Urogenital tract: Lymphoid tissue is critically sparse. In men, follicles are completely absent; in women, they are found only in the fallopian tubes.
Briefly: answers to topical questions
What is the mechanism of secretory IgA transport across the epithelial barrier?
The transport of secretory IgA across the epithelial barrier is carried out via transcytosis involving specific receptors.
- Binding — dimeric IgA interacts with the polymeric immunoglobulin receptor (pIgR) on the basal membrane of the epithelial cell.
- Internalization and transport — the resulting complex is endocytosed and transported within a vesicle to the apical pole.
- Modification — proteolysis of the complex occurs at the apical membrane, where the cleaved part of the receptor becomes the secretory component (SC) and integrates into the molecule, forming secretory IgA (sIgA).
- Secretion — the mature sIgA is released into the organ lumen via exocytosis.
Which cytokines stimulate the isotype switching of B lymphocytes to IgA synthesis in MALT?
The switching of B lymphocytes to IgA synthesis in mucosal lymphoid tissue is stimulated by several specific cytokines.
- Transforming growth factor beta (TGFβ) — promotes local differentiation of IgM-bearing B cells into IgA-expressing cells.
- Interleukin-5 (IL-5) — acts synergistically with TGFβ as a local cytokine, stimulating IgA production at the post-transcriptional level.
- Tumor necrosis factor beta (TNFβ) — induces switching to α1 and α2 isotypes, ensuring the synthesis of human IgA1 and IgA2.
MALT compartments: Afferent — Actively Accumulates (captures antigens in organized structures). Efferent — Effectorily Eliminates (diffuse cells work to destroy).
Class and exam questions
How do Peyer's patches differ from lymph nodes?
Unlike regular lymph nodes, Peyer's patches lack a capsule. In addition, they have a specific zone — the follicular dome, which is located directly beneath M-cells and serves as the site for primary antigen processing.
Why might an intranasal or inhaled vaccine fail to work?
Organized lymphoid follicles are necessary to perceive the antigenic signal of a vaccine, but they are present in the bronchial mucosa in only 40–45% of people (their development depends on childhood antigenic load).
What is the difference between dome dendritic cells and epithelial layer dendritic cells?
Dome dendritic cells receive antigens from M-cells. Epithelial layer dendritic cells can perceive antigens independently, after which they migrate to mesenteric lymph nodes to trigger a systemic response.
What else to study on this topic
- Mechanisms of independent antigen capture by epithelial layer dendritic cells.
- Extrathymic development of specialized T lymphocytes in barrier tissues.
- Features and clinical significance of homing molecules (integrin α₄β₇ and MadCAM).
- Causes of transient lymphoid follicle formation in the lungs.
- Organization of skin-associated lymphoid tissue (SALT).
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