GALT Flashcards
Describe GALT
Lymphoid tissue associated with the lining of the intestinal tract
Located immediately below epithelium of the mucosal tissue
Primary lymphoid organ in ruminants (B cell proliferation)
Secondary lymphoid organ in other species
What does GALT contain?
Contains organised structures:
- Peyer’s patches
- isolated lymphoid follicles
- cryptopatches (aggregates of lymphoid cells)
Scattered lymphocytes in intestinal villi
Where are Peyer’s patches found?
Jejunum:
- multiple discrete accumulation of follicles
- B and T cells
Ileum:
- form a single continuous structure that extends towards ileocaecal junction
- densely packed lymphoid follicles
- Only B-cells
Describe the histological features of Peyer’s patches
Extend from lamina propria to submucosa
Morphologically:
- follicle associated epithelium (FAE)
- subepithelial dome (SED)
-interfollicular area
- lymph follicle with germinal centre (GC)
Label the histology of the Peyer’s Patches
Describe the immunocytological content of intestinal villi
Intraepithelial lymphocytes
Lymphocytes in lamina propria
Describe the immunocytological content of Peyer’s Patches
Follicle associated epithelium:
- Microfold (M) cells - transport antigens to immune cells under epithelium (transcytosis) => activates T cells
Sub-epithelial dome:
- dendritic cells
- macrophages, neutrophils, B and T cells
Interfollicle areas:
- T cells
Follicles areas with germinal centres:
- B cells
Lamina propria:
- Tregs and T helper cells
What are the pathways of luminal intestinal antigen uptake in the GIT?
M cells
Intestinal epithelial cells
Dendritic cells that interdigitise the lamina propria
How do M cells cause an immunological response?
M cells and dendritic cells sample antigens and present to T cells
Activates T cells
Activated Th cells secrete cytokines => promote differentiation and proliferation of:
- B cells => plasma cells
- T cells => cytotoxic T lymphocytes
How do lymphocytes remain at site of infection in an intestinal infection?
Mucosal adhesion molecules upregulated on local endothelial cells
Activated T cells express mucosal adhesion receptor
Antigen specific lymphocytes re-circulate through local lymph node and site of infection, adhere and remain at site = homing
Describe the action of mucosal B-lymphocytes in GALT
B cells activated by cytokines from T helper cells => plasma cells
IgM secreted first
Cytokines stimulates plasma cells to switch antibody isotype from IgM => IgA
how is IgA protected from enzymatic digestion in the intestinal lumen
Secretory component added to IgA antibody in lamina propria as it passes through mucosal epithelial cells
How does IgA enter the intestinal lumen?
Binds to receptors on basolateral aspect of epithelial cells
IgA-receptor complex translocated to apical aspect where secretory component is applied
Secretory IgA either released into lumen or captures antigen directly
what is the action of secretory IgA in the lumen?
interferes with binding of pathogens to receptors on epithelial cells to prevent invasion
Immune exclusion
What is the role of IgG in GALT?
secreted from plasma cells
mostly produced in serum and lamina propria, some transported to lumen
Neutralises bacterial toxins
Opsonises invading pathogens to enhance phagocytosis or complement mediated lysis
Immune elimination
