Infection: Immunological Barriers

Abstract

Microorganisms can infect a host through various portals of entry. The host attempts to counter microbial infection and dissemination using many physical, chemical and immunological strategical barriers. Among them, epithelial cell layer comprises the outer barrier defence of the body being in constant contact with microorganisms and full‐time communication with adjacent immune cells. In this matter, host immune system needs to develop the ability to distinguish members of the microbiota from pathogens in order to provide protection against harmful invaders without inciting unnecessary immune responses. In this article we will discuss the mechanisms by which innate and adaptive immune system can sense the external environment and through the crosstalk with epithelial cells can coordinate effective immune response in skin and mucosal surfaces.

Key Concepts

  • The skin and the mucosal surfaces comprise the major portals of entrance for microbes' colonisation and infection.
  • From birth, the skin and mucosal surfaces are naturally populated by microorganisms (bacteria, fungi, viruses) called microbiota, which helps to educate the host immune system to distinguish infectious agents from harmless particles.
  • The immune system encompasses nonspecific immune responses nominated innate immunity and highly‐specialised deference mechanisms called adaptive immunity.
  • Lamina propria consists the connective tissue immediately adjacent to epithelial cell layer rich of cells including fibroblasts, innate and adaptive immune cells.
  • Along with the epithelium and the basement membrane, lamina propria constitutes the mucosa which is present in the several tubes of the body such as respiratory, genitourinary and gastrointestinal tract.
  • The epithelial cell layer acts as a physical barrier and releases chemical substances that play a key role in innate immune responses.
  • Mucosal surfaces harbour a great number of T cells which due to the presence of T‐cell receptor (TCR) can recognise specific microorganisms and coordinate a specific response against the invader.
  • Cytokines and chemokines are chemical substances that signal between cells to communicate and activate cells and perform the immune responses.

Keywords: skin; mucosa; epithelium cells; innate immunity; adaptive immunity; T cells; antibodies; cytokines and chemokines

Figure 1. Mechanisms of defence and regulation of intestinal homeostasis. AG, antigen; PRRs, pattern‐recognition receptors; AMP, antimicrobial peptides; SCFAs, short‐chain fatty acids.
Figure 2. Short‐chain fatty acids (SCFAs) are products of the activity of commensal bacteria present in our gut as of dietary fibre intake. These metabolites generate several benefits for intestinal homeostasis and the body as a whole, mainly to avoid the colonisation of pathogenic microorganisms (they induce the expansion of commensal bacteria), induction of peripheral tolerance (increase of Treg differentiation and secretion of IgA by B lymphocytes) and improvement of the mechanism of physical barrier (mucus production and increased epithelial integrity).
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Cipelli, Marcella, Ignacio, Aline, and Saraiva Camara, Niels Olsen(Sep 2019) Infection: Immunological Barriers. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000940.pub3]