Introduction to MALT
Mucosa-associated lymphoid tissue (MALT) is a critical component of the immune system that is found in various mucosal linings of the body. It plays a fundamental role in protecting the body from pathogens that enter through mucosal surfaces. MALT includes structures like the tonsils, Peyer's patches, and lymphoid tissues in the respiratory and genitourinary tracts.What is MALT?
MALT is a diffuse system of small concentrations of lymphoid tissue located in the submucosal membrane sites of the body. These tissues contain a variety of immune cells including lymphocytes, plasma cells, and macrophages. MALT can be categorized based on its location, such as gut-associated lymphoid tissue (GALT) and bronchus-associated lymphoid tissue (BALT).
Histological Features of MALT
MALT is characterized by a dense aggregation of lymphoid cells, which often form follicles. These follicles may contain germinal centers where B cells proliferate, differentiate, and undergo somatic hypermutation. Surrounding these follicles are T cells and antigen-presenting cells (APCs) such as dendritic cells and macrophages.- Tonsils: Located in the pharyngeal region, they contain multiple lymphoid follicles with germinal centers. The surface is lined by stratified squamous epithelium.
- Peyer's Patches: Found in the ileum of the small intestine, these are large aggregates of lymphoid tissue. They have a specialized epithelium known as follicle-associated epithelium (FAE), which includes M cells.
- BALT: Present in the bronchi, BALT contains lymphoid follicles similar to those in Peyer's patches but is associated with the respiratory epithelium.
- B Lymphocytes: These are the primary cells in the follicles and germinal centers.
- T Lymphocytes: Found mainly in the interfollicular areas and play a crucial role in cell-mediated immunity.
- Macrophages and Dendritic Cells: Function as APCs to present antigens to T cells.
- Plasma Cells: Derived from B cells, they secrete antibodies.
Function of MALT
The primary function of MALT is to generate an immune response to pathogens that enter the body via mucosal surfaces. This is achieved through:- Antigen Sampling: Specialized M cells in the epithelium transport antigens from the lumen to underlying immune cells.
- Activation of Immune Cells: B and T cells are activated upon encountering their specific antigens.
- Production of Secretory IgA: Plasma cells produce IgA, which is transported across the mucosal epithelium to neutralize pathogens.
Clinical Relevance of MALT
MALT is involved in various clinical conditions, including:- MALT Lymphoma: A type of cancer originating from the B cells in MALT. It is often associated with chronic infections like Helicobacter pylori in the stomach.
- Autoimmune Diseases: Conditions such as celiac disease and Crohn's disease involve dysregulation of MALT.
- Infections: Pathogens targeting MALT can lead to conditions like tonsillitis and bronchitis.
Conclusion
MALT is an essential component of the immune system, providing localized immune responses at mucosal surfaces. Its unique organization and cellular composition enable it to effectively combat pathogens and maintain mucosal immunity. Understanding the histology and function of MALT is crucial for diagnosing and treating various immunological and infectious diseases.
