Paragraphs on Vaccination

Paragraphs on Vaccination!

It was generally known for a long time that a person who survived an attack of small pox was not attacked by this disease for the second time. Edward Jenner observed in 1796 that a person who had suffered an attack of ‘cow-pox’, which was comparatively less harmful, became also immune to the more dreadful small pox. He utilized this knowledge to develop a process known as vaccination.

He deliber­ately infected a volunteer with cow-pox and a few months later infected the same person with a virulent material from true small pox. He found that the volunteer was not attacked by small pox. His method of vaccination was highly acclaimed and widely used. The last case of small pox occurred in Bangladesh in 1975 and in 1979 WHO declared that small pox has been completely eradicated through vaccination.

Although Jenner’s discovery was of great practical value which saved lives of thousands of people, the scientific principle of vaccination remained unknown for nearly a century. In 1879 Pasteur observed that the old cultures of bacteria causing chicken-cholera (Pasteurella multocida) lost their disease pro­ducing ability (pathogenicity) and became non-virulent.

But to his surprise he found that these non-virulent bacteria, when introduced into normal host, made it immune to the virulent pathogen. Thus Pasteur provided experimental evidence to prove that it was possible to induce immunity to a disease by artificial inoculation or vaccination of a host with a non-virulent derivative of the actual virulent patho­gen.

The process by which a pathogen lost its virulence was termed attenuation. Attenuation can be achieved by several means, one of which was accidentally discovered by Pasteur—keeping a pathogenic culture in artificial medium for a long time.

Pasteur’s finding aroused fresh interest in vaccination. Emil Roux and Alexander Yersin, working in Pasteur’s laboratory, discovered that diphtheria was caused by a poisonous toxin secreted by the caus­ative bacteria (Corynebacterium diphtheriae).

The organism was discovered earlier by Friedrich Loeffler in Koch’s laboratory. But the most important discovery which revealed the basis of induced immunity was made by von Behring and Kitasato (1896) who worked in Koch’s laboratory. They found that the blood of a vaccinated animal contained a substance which protected the animal against the disease. This protec­tive substance was designated as antibody.

They proved this by injecting repeatedly, at intervals, sub-­lethal (concentration which would not kill) doses of diphtheria toxin to some experimental animals and afterwards injected the serum (blood from which the blood cells have been removed) of one of these animals to susceptible animals together with a lethal (deadly) dose of diphtheria toxin.

The susceptible animals survived, whereas the animals which were injected with the diphtheria toxin alone died. The substance produced in the serum of an animal receiving small doses of toxin at intervals was called an antitoxin and the serum containing an antitoxin as antiserum.

The general term for the substance pro­duced in the blood of an immune host is antibody which is now known to be a special kind of globular proteins, the immunoglobulin’s. Behring and Kitasato also established that immunity against one disease gave no protection against another i.e. immunity is very specific.

For example, an individual immunized against diphtheria would not be immune to another pathogen, like tetanus bacteria. For making an individual resistant to a number of diseases, it has to be immunized separately for each of those diseases. For his historical achievements von Behring became the very first recipient of the Nobel Prize for Physi­ology and Medicine in 1901, even before Robert Koch who won it in 1905.

An important observation which also clarified another aspect of body’s defense against disease producing agents was made by Elie Metchnikoff, a Russian scientist working in Pasteur’s laboratory. He studied the process of ingestion of fungal spores by an aquatic protist called Daphnia (water flea) and observed that the spores were engulfed by amoeboid cells and digested.

The phenomenon of engulfing foreign materials by cells was designated as phagocytosis (Greek phagein = to eat + kytos = vessel or cell) and the cells were called phagocytes. Phagocytic cells are also present in the blood as natural compo­nents. The main phagocytes of blood are the granular leucocytes or polymorphonuclear leucocytes.

These leucocytes are again of three types, — eosinophil’s, basophils and neutrophils. The neutrophils show the maximum phagocytic activity. Metchnikoff demonstrated phagocytosis of foreign organisms by white blood cells and claimed that phagocytes protected the body by devouring invading microorgan­isms.