Kissing the Epstein-Barr virus goodbye?
Box 1 | Acquired immunity: Antibodies and T lymphocytes
The two potent weapons of acquired immunity are the antibody and the T-cell. They operate at different levels: antibodies at the molecular level and T-cells at the cellular level.
Antibodies
Antibodies are protein molecules that move freely in the bloodstream. They are manufactured by white blood cells in response to the presence of molecules on the surface of the disease-causing microorganism. These molecules are known as antigens. A particular antibody will combine best with the antigen that caused the production of that antibody: thus, the antibody is said to be specific for its antigen. Antibodies react chemically with these antigens in such a way that the microorganism or toxin is neutralised and destroyed before a cell is infected.
T-cells
At the cellular level, the body manufactures what are known as cytotoxic T lymphocytes – T-cells. These white blood cells recognise certain antigens produced by microorganisms and kill cells that harbour them.
If an invading microorganism evades the antibodies and infects a cell, the T-cells will recognise the infected cell and kill it. Thus the two weapons of acquired immunity, the antibody and the T-cell, complement each other.
Thanks for the memory
Once an infection has been overcome, antibodies and T-cells remain in the body, often for years and sometimes for life, acting as a kind of ‘memory’. If the microorganism attacks again, these antibodies ‘remember’ it. They combine with the antigens on the surface of the microorganism and immobilise it so it is no longer able to damage cells in the body. If, despite the efforts of the antibodies, some cells in the body become infected, the memory T-cells are mobilised rapidly to kill the infected cells. Thus, the body has acquired immunity against that particular disease.
Active immunisation
Vaccination works by introducing harmless versions of a disease-causing microorganism or certain antigens of the microorganism to the immune systems of uninfected individuals. This ‘tricks’ the immune system into producing antibodies and, when required, T-cells specific to the microorganism. The body is thus equipped with the right armory should infection by the real microorganism occur. The vaccines used in vaccination are of several types:
- live, attenuated microorganisms;
- killed microorganisms; and
- only one, or a few, antigens of the microorganisms, such as a toxin (toxic molecule) which is first rendered safe (detoxified). This type is called a subunit vaccine and is a type of acellular vaccine.
Passive immunisation
Protection against some infections may be achieved by passive immunisation. With this type of immunisation, antibodies produced in one person are introduced into another. These antibodies are injected into the host shortly before the expected exposure to a disease-causing microorganism. The antibodies are obtained from the blood plasma of people who have had the disease (or have been immunised against it). Injection of the antibodies confers immediate protection against infection for a short time (weeks). It may also be effective if given shortly after such exposure; for example, after being bitten by a snake or rabid dog.
Posted November 1997.