Components and Organization Types
Mucosa-associated lymphoid tissue includes several key anatomical elements. These comprise Waldeyer's pharyngeal lymphoid ring (consisting of six tonsils), solitary follicles and aggregated lymphoid follicles (Peyer's patches) in the small intestine, lymphoid nodules in the appendix wall, and bronchus-associated lymphoid tissue (BALT) in the respiratory tract.
Morphologically, this tissue is organized in two ways:
- Aggregated (Organized): Presented as distinct lymphatic follicles.
- Diffuse: Scattered lymphocytes and cell clusters that do not form nodules.
Lymphatic Follicles and Inverted Polarity
Follicles (nodules) are the primary structural component of MALT. By cellular composition—dominated by B lymphocytes at various stages of antigen-dependent differentiation—and general architectural plan, they resemble follicles of lymph nodes. They reside in the lamina propria, occasionally extending into the submucosa.
The center of the follicle contains a germinal (reactive) center, consisting of a dark zone, light basal zone, and light apical zone. A mantle zone (corona) of small lymphocytes surrounds it.
Palisading of the mucosal follicles features a key physiological trait: inverted polarity. The light apical part of the germinal center always points toward the epithelium (organ lumen), while the dark zone lies deeper within the mucosa. This orientation occurs because antigens and initiating T helper cells arrive via the epithelial barrier rather than through afferent lymphatics.
Diffuse Lymphoid Tissue and T/B Zones
Diffuse tissue consists of cell clusters outside of nodules and divides into two functional groups:
- Intraepithelial lymphocytes (T and B cells): infiltrate directly into the epithelial layer, interacting with antigen-presenting cells (APCs) for primary threat recognition.
- Parafollicular clusters: concentrated around follicles. This is the T-dependent zone (analogous to the paracortical zone of a lymph node), where T- and B-cell interactions take place.
The B-dependent zone (diffuse portion) is located in the lamina propria and is rich in plasma cells, serving as an analog of lymph node medullary cords. An important difference from classic immune organs is that the MALT stroma is formed by ordinary loose connective tissue rather than reticular tissue.
The Dome Area and Antigen Transport
The mucosal region directly overlying a lymphatic follicle is called the dome. In the small intestine, villi disappear over follicles, and specialized intraepithelial APCs appear in the epithelium:
- Dendritic cells (characteristic of tonsils).
- Langerhans cells (in the respiratory tract and skin, distinguished by branching shapes and tennis-racket-shaped Birbeck granules).
- M cells (microfold cells — in the gut and respiratory tract).
M-cells are specialized columnar epithelial cells with a smooth apical surface and a deep basal pocket filled with macrophages and lymphocytes. They execute transcytosis, transporting intact antigens across the epithelium (without degradation into peptides) to deliver them to B lymphocytes.
Special Histology: Palatine Tonsils
Palatine tonsils lie between the palatine arches and are covered by non-keratinized stratified squamous epithelium. The mucosa forms deep folds and invaginations called crypts.
The tonsillar epithelium is actively infiltrated by lymphocytes and neutrophils. Due to leukocyte enzymes and microbial action, some epithelial cells undergo apoptosis and desquamation, forming characteristic debris plugs within the crypt lumens. The lamina propria beneath the epithelium contains lymphatic follicles (B-zone, with germinal centers oriented toward the crypt lumen) and parafollicular diffuse tissue (T-zone).
Plasma Cell Histochemistry
To visualize plasma cells (the terminal differentiation stage of B cells), methyl green-pyronin staining (Brachet's stain) is frequently used. This method highlights nucleic acids:
- Nuclei (DNA) stain green.
- Cytoplasm (RNA) stains red/pink.
This cytoplasmic staining is termed pyroninophilia and indicates active protein synthesis, specifically the production of immunoglobulins.