Gut associated lymphoid tissue

The gut is heavily invested with immune cells in organized regions, including gut associated lymphoid tissue (GALT).

The GALT is populated with the same cell types in systemic immunity (lymphocytes, DCs and macrophages).

The environment of the GALT influences the behaviour and differentiation of immune cells in response to Ag.

The GALT is primarily proactive and avoids inflammation.

Organization
In the small intestine, the surface is folded into finger-like projections called '''villi. '''

Goblet cells interspersed between enterocytes (epithelial cells) secrete mucus.

Paneth cells in crypts secrete anti-microbial factors, e.g. defensins, lysozyme.

M cells, "microfold" cells, are specialized to transport micro-organisms and Ags to the GALT.

Peyer's patch
Peyer's patches (PPs) are found throughout the small intestine.

Similar

 * Organization of T cell and B cell zones
 * Cell types found (Ts, Bs, DCs)
 * Lymphocytes can leave via efferent lymphatic
 * Germinal centres can form for B cell class switching and somatic hypermutation

Different

 * Cell entry
 * LN = via blood (HEV) or afferent lymphatic
 * PP = via blood (HEV)
 * Ag entry
 * LN = via afferent lymphatic, sometimes carried by DC
 * PP = via M cell to be delivered to waiting DC
 * Cell adhesion molecules
 * To restrict entry to cells previously activated in the structure

Isolated lymphoid follicle
Isolated lymphoid follicles (ILFs) are frequently found in the large intestine and, to a lesser extent, in the small intestine.

Intestinal macrophages
In the LP, intestinal Mφs phagocytose bacteria and apoptotic cells. They constitutively secrete IL-10 to limit inflammation.

Unlike non-mucosal tissue Mφs, they are not instigators of inflammation. Intestinal macrophages do not...
 * Produce pro-inflammatory cytokines
 * Produce reactive oxygen or nitrogen species
 * Express B7 co-stimulatory molecules
 * Make cytokines needed for T cell activation and proliferation (not APCs)

Intestinal DCs
Under resting conditions, DCs in the PP produce IL-10 in response to Ag uptake.

Everyday some mucosal DCs migrate to mLNs to present Ags to T cells. In the absence of infection/inflammation, naive CD4 T cells are activated to become FoxP3+ T regulatory cells. These T regs go to the wall of the small intestine to suppress inflammatory responses. This is important in the maintenance of tolerance to food Ags.