Innate Barriers
- Respiratory system
Once a microbe is inhaled, it has to survive and penetrate the air filtration system of the URT and LRT. Since the airflow is turbulent in the passage, microbes are deposited on the mucosal surface of the RT. Microbes larger than 10 µm are trapped by cilia and hair in the nasal cavity. The cilia beat towards the pharynx and the mucus with entrapped microbes is expelled from the mouth. Humidification of air in the nasal cavity causes some microbes to swell up that aids in phagocytosis. Microbes that are smaller than 10 µm, pass through the nasal cavity and are trapped by the mucociliary blanket of the lower respiratory system. The cilia here, move the organism away from the lungs. Those that reach the alveoli encounter a number of phagocytic cells called as alveolar macrophages. Saliva also washes away microorganisms from nasopharynx and mouth into the stomach.
- GIT (Gastrointestinal System)
Most microorganisms that reach the stomach are killed by the gastric juice made up of HCl, proteolytic enzymes and mucus. The very high acidity (pH 2-3) is sufficient to destroy most microorganisms and their toxins. However, some microorganisms that are embedded in food particles are protected from the gastric juice and enter the small intestine. In the small intestine, damage to the organisms is done by various pancreatic enzymes, bile, enzymes in the intestinal secretions and the GALT (Gut Associated Lymphoid Tissues) system. Peristalsis in conjugation with shedding of columnar epithelial cells of the intestinal tract purge intestinal microbes. There are special cells in the intestine called as Paneth cells which secrete lysozyme and a set of peptides called cryptins. The normal microbiota of the large intestine also plays a vital role by preventing the establishment of pathogenic organisms. This is accomplished by either producing toxic end products like fatty acids or they outcompete the potential microorganisms for attachment sites and nutrition.
- Genitourinary tract
Under normal circumstances the kidneys, ureter, urinary bladder are sterile. The urine in the urinary bladder also remains sterile. However, few bacteria are harboured in the distal portion of the urethra. Besides the flushing action of urine, it kills bacteria by its low pH and presence of urea and other metabolic end products like uric acid, hippuric acid, FA, enzymes, mucin etc. The kidney medulla is so hypertonic that very few organisms can survive there. Moreover, the long urethra (20 cm) in males provides an anatomical barrier excluding microorganisms. Conversely, the short urethra (5 cm) in females is greatly traversed by microorganisms which provides the explanation for why females are more susceptible to UTI than males.
The vagina has its unique defense. The vaginal epithelium produces a large amount of glycogen that is degraded into lactic acid by LAB like Lactobacillus acidophilus. This creates an acidic environment which is unfavourable to most microorganisms. Cervical mucus also has some antibacterial activity.