Immunity is the ability of the human body to protect itself from infectious diseases. This involves complex defence mechanisms, which can be broadly categorised in two ways.
- Innate (non-specific) immunity – present from birth and includes physical barriers (e.g., intact skin), chemicals in the blood and immune system cells that attack foreign cells in the body (e.g., phagocytic cells).
- Acquired (specific, adaptive) immunity – generally specific to a single organism or to a group of closely related organisms such as immunity against Staphylococcus aureus (a bacteria which can cause skin infections).
There are two ways of acquiring immunity – active and passive.
1 Active immunity
This is protection produced by an individual’s own immune system and is usually long-lasting. Active immunity can be acquired by natural disease or by vaccination.
2 Passive immunity
This is protection provided from the transfer of antibodies from immune individuals. The most common example of this is the transfer of antibodies from mother to child through the placenta. This protection is temporary, usually lasting for a few weeks or months.
How vaccines work
Vaccines work by inducing active immunity and enabling the immune system to recognise and respond rapidly to exposure to natural infection in the future, thereby preventing or modifying the disease.
Vaccines can be made from inactivated (killed) viruses or bacteria (e.g., polio vaccine) or weakened live viruses or bacteria (e.g., the measles, mumps and rubella (MMR) vaccine). Although these antigens do not cause an infection, they do induce an immune response within the body that stimulate the production of antibodies. These antibodies are stored in the memory of the immune system and are there to protect the body if met with the same antigens at a later date.