Q) Discuss the various ways by which the human body battles against germs?
Q) How does vaccination protect us from diseases?
Skin and mucous membranes keep out the germs. Glands in the skin produce oily substances to provide a protective cover to the surface of the skin. Perspiration contains a chemical known as lyzozyme which destroys germs. It is also found in saliva, tears, nasal secretions and tissue fluids. Germs which enter our stomach are destroyed by strongly acidic stomach juices. Germs enter our body, reach our organs or survive in the stomach, take nourishment from the body and multiply. Then they start destroying our cells and secrete toxic substances and makes us sick. Soon the immune system, the body's defense mechanism kicks in.
Defense is in the form of special cells called WBCs which circulate throughout the body along with blood. WBCs are of different types and fight germs in different ways. During infection, the no.of WBCs are doubled, tripled or quadrupled depending on the severity of infection. When germs attack, WBC migrate to the infected site and destroy the germs by engulfing them. These cells are called engulfing cells.
When the fight is over, another type of WBCs move to the site to remove dead germs and dead WBC. The pus thus contains a large no.of dead cells and germs. Another type of WBC produce antibodies which attack poisons to make them ineffective. These antibodies also tag the invader so that it is easily recognised by the engulfing cells. Another type of WBC works as killer cells and directly destroy the invader or the infected body cell. Some WBCs keep the memory of the invader as 'trained cells', which act quickly once the same kind of invader comes back.
Certain WBC develop memory of an invader and is trained to ward off future attacks. In this way, the body becomes "immune" to that infection and the process is called immunisation. Thus our body regularly develops natural immunity. Artificial immunisation is done through vaccination. A weak infection is artificially introduced into the body which triggers a defense mechanism and produces WBCs trained to combat that particular infection.