Immunity is described as the ability of the body to resist or to recover from the invasion of disease-causing microbes (bacteria, viruses, and protozoas) and larger parasites (helminthes). Thus a person said to be immune to a particular disease would not contact it although others might do so.
Microbes and parasites cause several diseases in man. The disease-causing germs often release toxins (poisonous substances). Normally our body is able to defend itself against most disease-causing microbes. First the skin acts as a barrier to the entry of many microbes. And secondly the white blood cells destroy many microbes. But if the number of microbes exceeds the capacity of white blood cells they fail to protect the body against them.
Many persons are able to resist diseases to certain extent due to immunity mechanisms in their systems. The blood of a person produces substances called antibodies which fight the invading organisms. Each kind of antibody acts against only one type of microbe. Different antibodies have different characteristics. Some antibodies neutralize the toxins released by microbes. Certain others clump them together which can then be easily attacked by white blood cells. Some other antibodies dissolve the bacteria. Sometimes a certain amount of antibodies is permanently left in the blood plasma and this serves to protect the individual from future attacks. Such persons are said to be immune to that particular disease.
Immunological mechanisms are either specific or non-specific. Specific immunity, also called acquired immunity, which refers to mechanisms that are activated individually after a microbe or some other foreign material, invades the body. Non-specific immunity refers to general protective mechanisms that either kill or prevent the multiplication of microbes and other parasites.
Some persons are immune to certain diseases right from the birth. These people are said to have natural immunity or inborn immunity. In an epidemic of a particular disease, say cholera, people with natural immunity do not suffer from cholera. If someone gets smallpox or chickenpox once, he will not get it again during his whole lifetime. The body in such a case retains adequate levels of antibodies as a protection against future infection.
A person can also develop immunity by treatment with appropriate antigens. This is called artificial immunity. For instance, in the case of epidemics of smallpox, the health authorities vaccinate the people by which immunity is produced artificially. This vaccine was invented by Edward Jenner. It is produced by infecting a calf or horse with the smallpox virus. The virus in the calf or horse gets weakened. This weakened virus is collected as a vaccine and introduced into the human body. Since the virus is weak, it is not able to cause a severe attack of smallpox in man. But its presence induces the body, to produce antibodies and provides immunity against smallpox for several years. The introduction of weakened microbes in the body is called active immunity, the effect of which may last for even a lifetime. Immunity may also be passive. In this readymade antibodies are injected into the human body. An animal like a horse is infected with disease-producing microbes. The antibody is produced in the blood of the horse. The serum of the horse containing the antibody is extracted and introduced into the human body. The human body makes use of antibodies against the disease thus producing passive immunity. Passive immunity is produced almost instantly when the serum has been injected, but the effect lasts only for a short period.
Vaccines are now produced to provide immunity against diseases like whooping cough, diphtheria, measles, tetanus, typhoid, polio, rabies tuberculosis, mumps, scarlet fever, German measles, and chickenpox. All infants should be provided immunity by vaccination for different diseases.
In our country Haffkine Institute at Bombay and the Virus Institute at Poona produce several kinds of vaccines.