Probiotics and the Human Immune Defense System

The intestinal microbiota is a vital part of the body's defense system. Many scientific experimental data suggest that probiotic microorganisms may positively contribute to intestinal activities, in particular to the intestinal mucosal barrier.

The defense system of the intestinal mucosa consists of three main components:

  • 1) Pre-ephitelial: mucus, peptides, lipids, forming a continuos gel in which neutral pH is maintained by a flux of carbonates.
  • 2) Epithelial: cells bound together by tight junctions (small intercellular spaces). They can prevent the passage of material between cells. The functionality of these tight junctions can be modulated by proteins such as occludins and claudins.
  • 3) Post-epithelial: components of lamina propria.

Several researches, conducted also on germ-free mice, have shown that microbial antigens, derived from the intestinal microbiota as well as from the environment, may play a crucial role in the maturation of the GALT (gut-associated lymphoid tissue) (ILSI Europe, 2013). The GALT is formed by different compartments such as: lymph nodes, lymph follicles, and Peyers patches. The main function of the GALT is to constitute a barrier for bacteria and food antigens from GI lumen through intestinal mucosa. The GALT allows the passage of antigens by specialized cells, such as the M-cells that cover the Peyer's patches and the dendritic cells, and have a role of sentinels along the mucosa. The integrity of the epithelial lining of the GI tract is very important to avoid the disruption of the intestinal barrier, since when damaged there may be a crucial risk of intestinal disorders and/or diseases. Generally, microorganisms predominate in the lumen of GI tract, but they are also associated with the mucous layer and may adhere to the cell lining of the small intestine if the mucous layer is damaged. At this level, the beneficial microbes may positively compete with pathogens (ILSI Europe, 2013).

The immune system of the host is generally composed of two parts:

  • 1) The innate (or nonspecific immediate) immune response
  • 2) The acquired (or specific adaptive) immune response

These two arms of the complex immune system involve cells (cellular immunity) and other substances secreted in the blood stream such as antibodies and cytokines. The innate and acquired immune system work together to contrast any pathogen microorganism and foreign materials (antigens).

 
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