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Metabolic component: The third line of defense is slow, best measured by the base excess, [9] and mostly depends on the renal system which can add or remove bicarbonate ions (HCO − 3) to or from the ECF. [5] Bicarbonate ions are derived from metabolic carbon dioxide which is enzymatically converted to carbonic acid in the renal tubular cells.
In the first line of defense, inhaled bacteria are trapped by mucus and are swept toward the pharynx and are swallowed. [1] Bacteria which penetrate the mucous layer are dealt with a second line of defense which includes antimicrobial peptides that are secreted by the surface epithelium of the respiratory tract which kill many strains of ...
A large number of immune system cells in the intestines are found in dome-like structures called Peyer’s patches and in small mucosal lymphoid aggregates called cryptopatches. [14] Above the Peyer’s patches is a layer of epithelial cells , which together with the mucus form a barrier against microbial invasion into the underlying tissue.
The other cells involved in the innate response include innate lymphoid cells, mast cells, eosinophils, basophils, and natural killer cells. [ 28 ] Phagocytosis is an important feature of cellular innate immunity performed by cells called phagocytes that engulf pathogens or particles.
M cells lack microvilli but, like other epithelial cells, they are characterized by strong cell junctions. This provides a physical barrier that constitutes an important line of defense between the gut contents and the immune system of the host.
When host cells die, either by apoptosis or by cell injury due to an infection, phagocytic cells are responsible for their removal from the affected site. [9] By helping to remove dead cells preceding growth and development of new healthy cells, phagocytosis is an important part of the healing process following tissue injury.
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These M cells then alert the B cells and T cells in the tonsil that a pathogen is present and an immune response is stimulated. [6] B cells are activated and proliferate in areas called germinal centers in the tonsil. These germinal centers are places where B memory cells are created and secretory antibody (IgA) is produced.