Vaccines and their functioning

A vaccine is a biological substance designed to boost immunity against an infection caused by bacterias and viruses. These vaccines function by training the immune system to a particular infection. To dwell further we should know the basics of how the immune system functions (for more details refer to the previous article on "The production of antibodies and their roles"). The immune system relies on immune cells called the lymphocytes; they produce the chemical substance known as antibodies along with a memory cell. These secreted antibodies are used to fight against the foreign invaders by attaching to the antigen of the pathogen. This antigen is the active part of the pathogen causing infections. The binding of the antigens and the antibodies suggest how the body fights the infection.  

With the case of vaccines the same antigen is injected into the body to create defences against specific diseases before you even get them. The simple concept of the working of the vaccines is to stimulate antibodies and memory cells without causing an actual illness. Thereby advantaged to gain immunity without being sick. Hence suggesting that the vaccine must contain at least one antigen from the bacteria or virus in order to get a response. Some of the ways in which the antigens present in the vaccine helps produce memory cells to boost immunity are by injecting different forms of live and dead viruses. 

For instance the different forms of viral and bacterial antigen used in vaccines are listed below. 

1. Attenuated live viruses - This is the weakened forms of a live virus. They do not cause illness, but will create an immune response. 
2. Inactivated viruses - A version of the virus that has been killed is injected into the human system. Although the virus is dead, antibodies will still be produced as the structure of the inactive antigen remains the same causing the production of antibodies. 
3. Recombinant - These viruses are made in a lab through genetic engineering. This way, only a specific gene is expressed. Thus causing the infectious function of viruses to be nullified when interacting with the human body.  
4. Conjugate - Bacteria and virus antigens may have a polysaccharide coating, a sugar-like substance to protect it. Conjugate vaccines work around the disguise to recognise the bacteria. 
5. Subunit - These types of vaccines use only the antigens that stimulate an immune response. Which means in this case the antigen is directly being injected into the system.
6. Toxoid - This is an inactivated version of bacterial toxins used to make the immunity, as the inactive toxins stimulate allergic responses to which the body learns to counter. 
7. Viral vector vaccines - In this case a viral vector (extra mobile genetic material) is used to deliver the genetic material coding for the production of the virus's antigen inside a living human cell. This causes the human cell to produce the antigen of the virus triggering the production of immune cells and antibodies.
8. mRNA vaccine -  These vaccines teach our cells on how to make a protein (antigen) that triggers an immune response inside our bodies against a particular virus. This is done as the mRNA is injected into a functional human cell to make the viruses antigen protein. In this case the function of mRNA is used as an advantage as it carries the genetic material coding for making the particular antigen protein. 
9. Adenovirus vaccine - This vaccine is a modified version of the chimpanzee's adenovirus. The function of this vaccine is to make the antigen within the adenovirus to enter into the human cells but without causing a replication (multiplication), it trains the body to produce immune cells.

The presence of a vaccine in the body causes the immune system to produce antibodies against the invading antigens. Usually, it takes more than one vaccine shot to attain a full response as the memory cells need to be boosted into the system after a particular time interval.

The below is a figure suggesting the different types of vaccines classification chart. 

(Source : About Vaccines – COVID19 Vaccine Tracker,

covid19.trackvaccines.org/how-do-vaccines-work)