Mucosa-associated lymphoid tissue (MALT)

Mucosa-associated lymphoid tissue (MALT) is a diffuse collection of non-encapsulated lymphoid tissue found throughout mucosal surfaces of the body. It plays a critical role in the immune surveillance and defense of mucosal membranes — which are common entry points for pathogens. MALT includes organized structures like Peyer’s patches as well as scattered immune cells embedded in mucosa.

Distribution

MALT is widely distributed in the following systems:

Gastrointestinal tract: Most abundant (gut-associated lymphoid tissue, GALT)
Respiratory tract: (bronchus-associated lymphoid tissue, BALT)
Urogenital tract
Oropharynx: Including tonsils
Conjunctiva: Eye-associated lymphoid tissue (CALT)

Types of MALT

Type	Location	Examples
GALT (Gut-associated lymphoid tissue)	Intestinal mucosa	Peyer’s patches, appendix, isolated lymphoid follicles
BALT (Bronchus-associated lymphoid tissue)	Bronchial walls and lungs	Subepithelial lymphoid clusters in bronchioles
CALT (Conjunctival-associated lymphoid tissue)	Eye conjunctiva	Lymphoid follicles along the inner eyelid
SALT (Skin-associated lymphoid tissue)	Dermis and epidermis	Langerhans cells, dermal dendritic cells

Structure

May be diffuse (scattered lymphocytes) or organized (e.g., Peyer’s patches)
Composed of:
- Lymphoid follicles (B cells)
- Interfollicular T cells
- Antigen-presenting cells (dendritic cells, macrophages)
- M cells (microfold cells) in intestinal epithelium that transport antigens to underlying lymphoid tissue

Function

Immune Surveillance: Constantly monitors and responds to antigens at mucosal surfaces
Antigen Sampling: M cells capture antigens and deliver them to lymphoid cells beneath the epithelium
IgA Production: Stimulates plasma cells to secrete IgA, which neutralizes pathogens at mucosal surfaces
Memory Cell Generation: Develops long-lasting mucosal immune memory

Blood and Lymphatic Supply

Supplied by local mucosal capillaries and arterioles
Drains into nearby lymphatic vessels and regional lymph nodes

Clinical Significance

Appendicitis: Inflammation of the appendix (a GALT-rich structure)
Peyer’s patches infection: Entry site for pathogens like Salmonella or poliovirus
MALT lymphoma: A type of extranodal non-Hodgkin lymphoma; commonly arises in stomach, often associated with chronic H. pylori infection
Autoimmunity and chronic inflammation: Excess MALT activity may contribute to diseases like IBD or Sjögren’s syndrome

Comparative Anatomy

MALT is present in many vertebrates. In mammals, particularly humans, it has evolved to provide strong mucosal immunity due to high antigen exposure from the environment, diet, and airborne particles.