Mucosal immune system of the gastrointestinal tract: maintaining balance between the good and the bad

Abstract

The gastrointestinal tract (GI tract) is a unique organ inhabited by a range of commensal microbes, while also being exposed to an overwhelming load of antigens in the form of dietary antigens on a daily basis. The GI tract has dual roles in the body, in that it performs digestion and uptake of nutrients while also carrying out the complex and important task of maintaining immune homeostasis, i.e., keeping the balance between the good and the bad. It is equally important that we protect ourselves from reacting against the good, meaning that we stay tolerant to harmless food, commensal bacteria and self-antigens, as well as react with force against the bad, meaning induction of immune responses against harmful microorganisms. This complex task is achieved through the presence of a highly efficient mucosal barrier and a specialized multifaceted immune system, made up of a large population of scattered immune cells and organized lymphoid tissues termed the gut-associated lymphoid tissue (GALT). This review provides an overview of the primary components of the human mucosal immune system and how the immune responses in the GI tract are coordinated and induced.

Human Intestinal Mononuclear Phagocytes in Health and Inflammatory Bowel Disease

“…The small intestine has a single mucus layer while the colon has an inner mucus layer, lacking bacteria, and an outer mucus layer, which forms a habitat for numerous microorganisms (189) (Figure 2). Despite these systems, luminal antigens can cross the epithelial barrier using one or more routes, such as microfold cells in Peyer's patches, as recently reviewed (50,190,191) (Figure 2). Subsequently, antigens come in contact with immune cells, including MNPs, in secondary and tertiary lymphoid organs in the lamina propria (LP) (192,193).…”

“…Therefore, defining how MNPs control immune homeostasis in the healthy human gastrointestinal tract, and their contribution to the aberrant immunoregulation that results in disease, is critical to improving treatments for IBD patients (7,(48)(49)(50).…”

Modifications of abundances of some bacteria are associated with the severity of IgA nephropathy in Chinese population

“…Immune reaction, bacterial infection and in ammation have been reported as etiological factors of IgAN. More than one hundred trillion microbes exist in the intestinal tract, one of the biggest immune organs of the human body [12]. An increasing number of studies have revealed that the gut-kidney axis may contribute to the pathogenesis of numerous diseases [4,5].…”

“Candida AlbicansInteractions With The Host: Crossing The Intestinal Epithelial Barrier”

“…Among them, the γδ intraepithelial T lymphocytes (that represents the major T cell population within the intestine) respond directly to microbiota signals to promote intestinal homeostasis which (i) helps preserve the integrity of damaged epithelial surfaces 105 and (ii) produces innate antimicrobial factors and pro-inflammatory cytokines and chemokines in response to resident bacteria that penetrate the intestinal epithelium and to intestinal injury. 106,107 Another important line of immune cells for maintaining intestinal homeostasis are the CD4 regulatory T cells (Tregs). Indeed, Tregs are important response modulators to gut microbiota, notably through the secretion of anti-inflammatory cytokines such as IL-10, which trigger an immunosuppressive response and gut homeostasis.…”