A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. The agent stimulates the body’s immune system to recognize the agent as foreign, destroy it, and keep a record of it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.
There is natural immune system in the body of man. When an antigen enters into our system specific antibodies are formed in response to a specific antigen and fight against the antigen. In most of the cases the ill effect of the antigens are neutralized by the antibodies and we get relief from the infection or diseases caused by the parasites. However, in some cases, natural immunity fails to encounter the effect of toxin released by the intruder. To combat the adverse effects of the antigen, a system has been developed in which attenuated antigen is introduced inside the body of a healthy mammal at a small dose, which can induce the lymphocytes to produce huge quantity of antibodies. These antibodies are called vaccines. This vaccine is then introduced inside the body of a healthy man to develop passive immunity in their body. Process of introducing of vaccine is therefore, termed as vaccination. Vaccination is a type of immunization.
For the production of vaccines, usually attenuated antigen is collected from any pathogen. This is then injected into the body of horse or sheep where huge quantity of antibodies is produced due to induction of the antigen. These antibodies are applied in the body of a human- being where it acts as antitoxin. Usually vaccines are prepared from the toxin of bacteria and viruses. Bacterial toxin can be modified chemically into anti toxin substances, called toxoid. This toxoid can be injected in the body of healthy man to develop antibodies against bacteria. Anti tetanus serum (ATS) is such a type of toxoid.
Types of immunity:
Usually there are two types of immunity, (1) Innate and, (ii) Acquired. When a child gets immunity from birth, this immunity is called innate. On the other hand if the immunity is developed after birth this is called acquired immunity.
Acquired immunity is mainly of two types:
(i) Naturally acquired active immunity.
(ii) Artificial acquired active immunity.
(i) Naturally acquired active immunity: This type of immunity is developed when an antigen enters into the body. Antibodies are developed against the antigen, which may enter into the body accidentally. When antibodies are formed against the pathogen which enters into the body accidentally, the immunity thus developed is called naturally acquired active immunity. Immunity against mumps virus is an example of naturally acquired active immunity.
The antibodies produced by the memory cells of the lymphatic tissues make immunity against these antigens. These memory cells remain active for many years and produce IgG as and when required. This type of antibody response is called secondary anamnestic response.
(ii) Artificially acquired active immunity: This type of immunity is developed by introducing known antigen in the form of vaccine and toxoid in the body. Therefore, this immunity is known as artificially acquired active immunity.
Types of Vaccines:
Vaccines are of various types. They may be killed vaccines or live attenuated vaccines. Both killed and live attenuated vaccines are obtained from Bacteria or Viruses.
Some commonly used killed Bacteria and viral vaccines are as follows:
|Name of the diseases
|Sources of vaccines (Bacterial)|
|Cholera||Body parts of Vibrio cholerae|
|Plague||Dead Yersinia pastis|
|Pneumonia||Dead Streptococcus pneumoniae|
|Rickette||Dead Rickettsia rickettsii|
Common killed viral vaccines are poliomyelitis, rabies, hepatitis B, Influenza etc. Viral vaccines can be used as attenuated form. A few common viral antigens and their sources are as follows.
|Name of the diseases||Sources of vaccines (Viral)|
|Hepatitis B||Attenuated virus|
|Small Pox||Attenuated virus|
|Yellow fever||Attenuated virus|
Vaccines prepared from viruses, bacteria and other micro-organisms are known as first generation vaccines.
What is Toxoid vaccine?
Toxoids are produced by chemical changes of the toxin. Toxoid is introduced to raise immunity against toxin. At present the toxoids of diphtheria, tetanus and pertussis, are added together to make a toxoid called DTP. Similarly a common toxoid called MMR is applied against measles, mumps and rubella. A trivalent oral polio (TOP) vaccine is used as toxoid against polio.
Second generation vaccines: Utilizing all the toxin producing microorganisms and chemically detoxifying their toxin a kind of toxoid is prepared. This is called sub unit vaccine or Second generation vaccine.
What is Conjugate vaccine?
Certain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g., toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen. This approach is used in the Haemophilus influenzae type B vaccine.
What is Subunit vaccine?
Protein subunit – rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a “whole-agent” vaccine), a fragment of it can create an immune response. Examples include the subunit vaccine against Hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients, but now produced by recombination of the viral genes into yeast), the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein, and the hemagglutinin and neuraminidase subunits of the influenza virus. Subunit vaccine is being used for plague immunization.
What is Genetically engineered vaccine?
Recently various vaccines are being producing using recombinant DNA technology. These vaccines are known as third generation vaccines.
Vaccines against hepatitis B, foot and mouth disease of cattle has been prepared by gene manipulation.