Mucosa-associated lymphoid system

Components and organization types

Mucosa-associated lymphoid tissue includes several important anatomical elements. These include the pharyngeal lymphoid ring (consisting of six tonsils), solitary follicles and their clusters (Peyer's patches) in the small intestine, nodules in the wall of the vermiform appendix, and bronchus-associated lymphoid tissue (BALT) in the respiratory tract.

Morphologically, this tissue is organized in two ways:

• Aggregated: represented by distinct lymphoid follicles.
• Diffuse: scattered lymphocytes and their clusters not forming nodules.

Lymphoid follicles and inverted polarity

Follicles (nodules) are the main structural component of MALT. In terms of cellular composition (dominated here by B lymphocytes at various stages of antigen-dependent differentiation) and general structural plan, they are analogous to lymph node follicles. They are located in the lamina propria (lamina propria), sometimes penetrating into the submucosa.

In the center of the follicle is the reactive (germinal) center, consisting of dark, light basal, and light apical zones. A corona (mantle zone) of small lymphocytes forms around it.

The main physiological feature of mucosal follicles is inverted polarity. The light apical part of the reactive center is always directed toward the epithelium (lumen of the organ), while the dark zone is hidden deep within the mucosa. This is because antigens and T helpers initiating the reaction come not from the lymph, but from the epithelial barrier side.

Diffuse lymphoid tissue and T/B zones

Diffuse tissue represents clusters of cells outside nodules. It is divided into two functional groups:

1. Intraepithelial lymphocytes (T and B cells): penetrate directly into the epithelial layer, where they contact antigen-presenting cells (APCs) for primary threat recognition.
2. Parafollicular accumulations: concentrated around the follicles. This is the T-dependent zone (analogous to the paracortical zone of a lymph node), where T- and B-cell interaction occurs.

The B-dependent zone itself (diffuse part) is located in the lamina propria of the mucosa and is rich in plasmacytes, serving as an analogue of the medullary cords of the lymph node. An important difference from classic immune organs: the MALT stroma is formed not by reticular tissue, but by ordinary loose fibrous connective tissue.

Dome region and antigen transport

The area of the mucous membrane directly above the lymphoid follicle is called the dome. In the small intestine, villi disappear above the follicles, and specialized intraepithelial APCs appear in the epithelium:

• Dendritic cells (characteristic of the tonsils).
• Langerhans cells (in the respiratory tract and skin, distinguished by their dendritic shape and specific tennis racket-like Birbeck granules).
• M cells (microfold cells — in the intestine and respiratory tract).

M cells are a type of columnar epitheliocytes with a smooth apical surface and a deep basal "pocket" filled with macrophages and lymphocytes. They perform transcytosis — transporting the antigen through the epithelium whole (without cleavage into peptides) and transferring it to B lymphocytes.

Special histology: palatine tonsils

The palatine tonsils lie between the palatine arches and are covered with stratified squamous non-keratinizing epithelium. The mucosa forms deep folds and invaginations — crypts.

The tonsil epithelium is actively infiltrated by lymphocytes and neutrophils. Under the action of leukocyte enzymes and microorganisms, some epithelial cells die and desquamate, forming characteristic "plugs" in the crypt lumen. In the lamina propria beneath the epithelium lie lymphoid follicles (B zone, with germinal centers directed toward the crypt lumen) and parafollicular diffuse tissue (T zone).

Histochemistry of plasma cells

To visualize plasmacytes (the final stage of B cell differentiation), methyl green and pyronin staining according to Brachet is often used. This method reveals nucleic acids:

• Nuclei (DNA) stain green.
• Cytoplasm (RNA) stains red/pink.

This cytoplasmic glow is called pyroninophilia and indicates active protein synthesis, in this case, immunoglobulins.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cellular composition of the lymphoid follicle reactive center
• Comparative morphology of Peyer's patches and the appendix
• Origin and functions of Langerhans cells
• Mechanisms of transcytosis in mucosal epithelium

Other topics in the section "Hematopoietic and lymphoid organs"

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