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Understanding how a vaccine works

By Dr Arthur Wellington on behalf of the Pig Veterinary Society
A vaccine is a preparation that contains an infectious agent, which has lost its ability to cause disease by attenuation (passaging through eggs or tissue culture) or being killed.
The vaccine thus brings about a resistance (immune response) to this agent. The body is hereby protected should the infectious agent invade the body at a later stage as it is prepared and ready for an attack should it occur.
What happens when a foreign agent enters the body? How does this immune response take place?

When an infectious agent enters the body it causes damage which sets off the process known as inflammation (swelling, pain, reddening). This reaction attracts white blood cells to the area. The agent (antigen e.g. E.coli, Parvovirus or lepto), is recognised as a foreign organism and is consumed by a phagocyte (macrophage, dendritic cell), processed (cut) and parts of this antigen are then attached to the surface of the cell and presented to another white blood cell (lymphocyte). The lymphocyte (T helper cell) is hereby activated. In this way the antigen is recognised as foreign to the body and the immune response triggered.
Once activated, these T helper cells multiply and then stimulate the multiplication of killer T cells and B cells. At this early stage memory T helper cells and memory B cells are generated and circulate in the blood and lymph streams for months or years (depending on the type of vaccine used) ready to respond more quickly to any subsequent infection. As the number of B cell increases, helper T cells signal them, by means of a chemical, to start producing antibodies.
Killer T cells specialise in killing cells of the body that have been invaded by this antigen. This action is done by puncturing the membrane of the body cell bringing about the release of the antigen into the blood stream. Here the antigen is neutralised by the antibody or tagged for later destruction by chemicals.
Once the antigen (foreign organism) has been contained suppressor T cells halt the activity of the B cells and other T cells.
As will be noted from the above there are four stages to each immune response (i.e. by the invasion of the antigen, be it naturally or from a vaccine) namely:
(a) recognition of the foreign agent,
(b) growth of the defences,
(c) contain and kill and
(d) slow down.
B cells form the humoral component and T cells form the cellular component of the immune response. These are circulating in the body via the bloodstream waiting for an opportunity to attack foreign bodies.
A vaccine is a preparation which is used to generate an action, which aids the body in the body’s objective to identify and destroy all substances, living or inert, that are not part of the body without succumbing to infection by the substance. This amazing cellular war is continuing all the time in all bodies and we do nor even consciously know anything about it.

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