4.1 Natural barriers against pathogens
The physical and chemical barriers that prevent pathogens from getting into our body tissues in the first place are often overlooked. As Figure 1 shows, the most comprehensive barrier is the waterproof layer of skin that covers the body’s surface. Human skin keeps most pathogens out as long as it remains intact. The speed with which a cut or graze can become infected is a reminder of the protection we normally get from our skin.
Vector-borne infections (e.g. malaria) are transmitted by biting invertebrates (e.g. mosquitoes) penetrating the skin when taking a blood meal. Humans are also vulnerable to invasion by pathogens in the air, food, water and soil, or during physical contact with infected people, some animals (e.g. pigs, dogs, poultry) or their faeces. As Figure 1 illustrates, the inner surface of the respiratory system (nose, throat, airways and lungs), stomach, intestines, bladder and reproductive tract are lined with membranes that secrete jelly-like mucus, presenting a barrier against pathogens entering our tissues via these routes. Microscopic hairs called cilia [sill-ee-ah] line the respiratory system and ‘beat’ in unison to shunt mucus containing trapped pathogens towards the nose and mouth, where they can be expelled by coughing and sneezing, or swallowed into the stomach where acid destroys them.
Figure 1 refers to competition from commensal bacteria (sometimes referred to as ‘friendly bacteria’) in the gut and reproductive system. Commensal bacteria are non-pathogenic inhabitants of the gut and the reproductive system. They occupy space that could otherwise be colonised by pathogenic species, and they use nutrients for their own growth, thereby reducing the resources available for pathogens to multiply and cause disease.
Chemical barriers against infection include enzymes in tears, saliva and mucus that break down the surface of bacteria. The acid in sweat and in the stomach kills cellular pathogens and there are anti-bacterial proteins in semen (the fluid that contains male sperm).
The more complex mechanisms of the immune system are only needed if pathogens breach these physical and chemical barriers.