What is vaccine?

                            Vaccine refers to a biological
preparation that stimulates immune response by formation of antibodies in an
organism and provides future protection against a given pathogen. First of all,
vaccine was developed by a British doctor, Edward Jenner against smallpox in
1796.

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Types of vaccines:                             

                               Followings are
the major types of vaccines:

1.     
Live
attenuated vaccines:

                                           Such
a vaccine contains weakened organisms which cannot harm the body. However, they
possess the ability of division or replication just like a living pathogen. When
such vaccine is introduced in a body it stimulates the immune system of that
organism. As a result, the body obtains immunity against that microbe.

Advantages:

1)     
 It gives better
immunity by providing both humoral and cellular immune responses.

2)     
Durable immunity is given by it which proves
inexpensive.

Disadvantage:

1)     
Secondary mutation may takes place and may cause
virulence.

2)     
It may lead to different problems in immuno-compromised
people.

3)     
Transportation problems as specific conditions are
required like temperature.

2.     
Inactivated
killed vaccines:

                                              To
prepare inactivated killed vaccines, pathogens are grown in culture under
controlled conditions and are killed by heat, formalin or phenol to reduce
virulence. Such microbes cannot replicate. So booster shocks are frequently
required to obtain a strong level of immunity.

Advantages:

1)     
It lessens the possibilities of spreading infection.

2)     
It is more stable to store.

3)     
Easy in transportation due to lack of refrigeration.

Disadvantage:

1)     
Repeated booster injections are given to develop
immunity.

2)     
It does not stimulate mucosal immune response.

3.     
Toxoids:

          
   There are some diseases
which are not caused by the bacteria. Instead these diseases are caused by the
toxins made by bacteria. In this case, modification (inactivation) of toxins is
done to use them as a vaccine e.g. in case of tetanus, Toxoid is being used.
These vaccines inhibit the attachment of toxin with the body of a host living
organism.

Advantage:

1)     
It gives strong production of antibody.

2)     
It makes long-term memory cells.

3)     
Diseases caused by toxins of bacteria are eliminated.

Disadvantage:

1)     
They are limited and effective against only those
bacteria which cause infection by producing toxins in the body of an organism.

2)     
They require regular booster after every ten years.

4.     
Conjugated
vaccines:

                                    These
vaccines are developed when antigens obtained by a microbe are poor and alone
cannot develop enough immunity. In such a case, poor antigen is covalently
attached to a strong antigen (protein) in order to create enough immunological
response. For example Haemophilus
influenzae is a conjugate vaccine.

Advantages:

1)     
It may create a mucosal immune response and reduces the
mucosal carriage.

2)     
It can elicit immunogenic memory response.

Disadvantage:

1)     
It requires several booster shots.

5.     
DNA vaccines:

                         In few cases, certain
proteins are able to produce immune response and their genes can be used as
vaccines. In this method, genetically engineered DNA is directly injected to
the host in which antigen is formed by the DNA (by transcription and
translation) and as a result immunological responses are produced. West Nile
virus is an example of such vaccine.

Advantages:

1)     
There is no risk of infection.

2)     
MHC class 1 and MHC class ll both can present it.

3)     
Antigen based immune response are developed.

4)     
It is relatively easy to produce.

5)     
It is economical.

6)     
It provides long term immunity to every cell of body.

             Disadvantages:

1)     
It can stimulate immune response to protein immunogens.

2)     
It may affect the genes regulating cell growth.

3)     
Antibody against DNA may be induced.

6.     
Subunit
vaccines:

                             These vaccines are also known as
recombinant vaccines. Such vaccines are composed of some portions of bacterial
or viral proteins that stimulate immunity. It is somehow like DNA vaccines.
Viral and bacterial genomes contain antigenic epitopes which are responsible
for creating a disease. To prepare a subunit vaccine, these parts are bound to
an expression vector which makes the specified subunit proteins. After it,
these subunits are collected and applied as vaccine to stimulate immunological
responses. They behave like an inactivated killed vaccine. For example, vaccine
for hepatitis B and Foot and Mouth diseases has been developed by this
mechanism.

                             Now efforts are
being made to stimulate immune responses in animals by incorporating genes in
the plants eaten by them. These genes will behave like expression vector to
stimulate immune responses by M cells of intestine. 

Advantages:

1)     
Such vaccines can be administered to persons of poor
immune system.

2)     
They provide long term immunity.

3)     
The risk of infection is low as small parts of viruses
are used.

4)     
There are very low chances of adverse reactions with
this vaccine.

Disadvantage:

1)     
To get life-long immunity, many doses are required.

2)     
Research requires time and is difficult.

Mode of action of a vaccine:

                                                        A
vaccine is capable of developing immunity in          our body. It consists of a pathogen or
such a part of its body that can initiate immune response in our body.

          When a vaccine enters the blood,
white blood cells are stimulated. As a result antibodies and lymphocytes are
produced which remain in the blood as a memory. B-Lymphocytes and T-Lymphocytes
remember that how they have to fight against this pathogen if it makes a
contact in future with our body. In this way, immunity against a pathogen is
provided for future infection. Sometimes, fever or mild infection may be appeared
after application of a vaccine (but it is not of much danger). A vaccine is
mostly administered to a patient by an injection. However, some vaccines are
given through nasal cavity or oral cavity.

The End