The body has many ways to protect itself from foreign agents in the external environment. Firstly the skin covers the entire body from head to toe. The skin is the largest organ in our body and protects the inner organ systems from exposure and attack by foreign agents of disease. Furthermore, the hair follicle is kept oiled and moist by the sebum produced by the sebaceous gland. Combined with sweat, sebum produces antibacterial and antifungal properties keeping the skin moist and protected from infection. Imagine what we would do without our skin?
When we inhale air, the hairs within the nasal cavity and the mucous secreted by glands in the nasal cavity help to filter the dirt and dust that enter the lungs. The layer of epithelial cells lining the trachea and the bronchi have cilia that help to capture and sweep foreign particles like a broom out of the respiratory tract. Thus air free of dust, particles and chemicals enters the air sacs or alveoli. Our eyes are also washed daily by secretions from the lacrymal glands, keeping the eyes free of infection. Ear wax too acts as a barrier to infections in the ear. Even the urinary and reproductive tracts produce secretions to ward off harmful disease causing agents. God has equipped us with protective weapons, in this way, against foreign invasion to lengthen lifespan of organs and systems.
Next when we ingest food, there is a very high chance of contamination through food that has been exposed to the surrounding environment. The food that enters is moistened with saliva that has antiseptic properties. As food travels down the digestive tract, gastric juice contains a potent acid, HCl acid, responsible for further destruction of germs and fungus in the ingested food.
Last but not least are the actions of leucocytes or white blood cells that are present in blood that circulates thoughout the body through arteries, veins and minute capillaries. These white blood cells travel everywhere and anywhere, even squeezing out of blood vessels to attack foreign agents within the body. They are like the air force, the army and the navy of a nation’s defence system because they have different modes of attack. Some ingests and digest the enemy; some ambush the enemy and then kill it; some mark the enemy which is then identified and killed and digested by other white blood cells. Some white blood cells produce antibodies to create immunity against a known enemy, introduced through natural or artificial active immunity.
However there is also another form of immunity called passive immunity which can be either natural or artificial. But this type of immunity is obtained from an outside source like from mother’s milk or antiserum obtained from animals. Passive immunity is only a temporary measure until the body itself can develop its own immunity.
Finally when there is an injury to the skin, a blood clot plugs the outflow of blood. Then white blood cells rush to accumulate themselves over the clot forming serum, preventing infection or entry of foreign agents through the wound. Here again the body ensures its safety from infections of any kind.