Cigarette smoking can cause changes in the respiratory tract, such as increased inflammation, increased mucosal permeability, and paralysis of the respiratory tract's cilia.
T-cells are another type of white blood cell found in the human body. T-cells are not considered a physical barrier because they are produced or originated from bone marrow and mature in the thymus. The skin, mucous membranes, tears, earwax, mucus, and stomach acid are examples of natural barriers. Additionally, normal urine flow washes out microorganisms that enter the urinary tract.
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