Bacteria—anaerobes, lactobacilli, enterococci, and gram-positive aerobic organisms—colonize the gut after birth; in the upper small intestine there are 102-103 bacteria/ml and up to 109 bacteria/ml are present in the distal small intestine. The small intestine acts as an effective barrier, and together with the innate and acquired immune system, defends the body against foreign microorganisms. Bacteria, derived from ingested food that is rarely sterile, colonize the gastrointestinal tract soon after birth. After eating, more than 100 species of bacteria are found in the upper small intestine but ingested bacteria are greatly reduced in number by gastric juices in the hour following the meal. The bacteria in the distal small intestine are not affected by the gastric secretions. The human gut has mechanisms to protect itself against environmental agents that are foreign to the body (immunity). Immunity can be either innate or acquired. Innate immunity comprises those defense mechanisms that a person is born with that can be mobilized on very short notice. There are a multitude of innate mechanisms that include barriers to infection (thick mucus, ciliary action, and gastric acid), phagocytic cells (neutrophils and macrophages), defense due to complement and humoral (antibody-mediated) mechanisms, and antibacterici-dal peptides such as a-defensins, trefoil factor peptides, and cathepsin G secreted by intestinal epithelial cells and Paneth cells at the base of the intestinal crypts. Adaptive immunity is more specialized and produces both a humoral and a cellmediated immunity. It differs from innate immunity in that it remembers a previous infection and responds quickly to reexposure to a known foreign substance. Bacteria in the intestinal lumen are probed by dendritic cells, which reach out into the lumen to sample the bacteria and present them to lymphocytes in the lymphoid tissue present in the gut wall. Intestinal M cells achieve similar presentation by importing bacteria into the intestinal lymphoid tissue.
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