Defence against diseases
We are constantly surrounded by germs, but do no fall or remain ill continuously because our body has certain natural Defence against invasions by microorganisms.
The first line of Defence
This consists of barriers like hairs, mucous membranes and the skin. The nose, for example, has hairs and a sticky mucous membrane which trap dust and microorganisms. Mucus also contains an antimicrobial substance. The eyes are protected not only by eyelashes, but also tears, which have antimicrobial properties.
The skin plays an important role in protecting almost the entire body. It serves as a barrier against the entry of microorganisms.
The sweat secreted by the skin is acidic and discourages germs from passing through.
The second line of Defence
This becomes active when germs manage to break the first line of Defence and enter the body. It includes the cilia of the cells in the respiratory tract, as well as secretions like gastric juice. The hydrochloric acid in the latter kills germs. White blood is corpuscles from an important component of the second line of Defence. They have the ability to engulf germs and destroy them.
The third line of Defence
This line of Defence consists of the immune system, which gives the body its natural ability to resist infection. This ability is known as immunity.
When the first and the second lines of Defence fail to protect us against foreign bodies, we respond by producing special protein, called antibodies. These neutralize or destroy the germs. Specific antibodies act on specific foreign bodies, which are also known as antigens. Antibodies are produced by lymphocytes and reach the site of infection via the blood circulation.
Once antibodies have been produced, they remain in the circulation for some time. We are less susceptible to the disease for this period of time. The length of time varies in the case of different diseases in the case of diseases like chickenpox and measles, the body becomes completely immune to the infection it has recovered from it once. This is because the antibodies remain in the circulatory system in a dormant state and spring into action immediately if the infection strikes again.
Immunization
Children catch infection more easily than do adults because natural immunity develops with exposure to diseases. Such immunity is called acquire immunity. As children do not posses natural immunity, they are artificially immunized. This consists of taking vaccines, either orally or through injections.
A vaccine is a small quantity of dead or weakened microorganisms responsible for causing a particular disease. When there are made to enter the body, the immune system in induced to from the appropriate antibody. The body thus becomes equipped to fight off attacks by the particular microorganisms.
Immunization against diseases like smallpox, tuberculosis, polio and diphtheria is started soon after birth and continues up to the age of five years. Many vaccinations, such as that for tetanus, have to be taken several times during one’s lifetime to renew immunity. Some other diseases for which vaccines have been developed are rabies, whooping cough, typhoid and measles.
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