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Any disease is, in effect, an interaction between the pathogen and the body’s various defence mechanisms. Sometimes the pathogen overwhelms the defences and the individual dies. Sometimes the body’s defence mechanisms overwhelm the pathogen and the individual recovers from the disease. Having overwhelmed the pathogen, however, the body’s defences seem to be better prepared for a second infection from the same pathogen and can repel it before it can cause any harm. This is known as immunity and is the main reason why some people are unaffected by certain pathogens. Much depends on the overall state of health of an individual. A fit, healthy adult will rarely succumb to an infection. Those in ill health, the young and the elderly are usually more vulnerable.

Defence mechanisms

The human body has a range of defences to protect itself from pathogens. They are of two main types:

·         Non-specific mechanisms that do not distinguish between one type of pathogen and another, but respond to all of them in the same way. These mechanisms act immediately and take two forms:

a)      A barrier to the entry of pathogens

b)      Phagocytosis

·         Specific mechanisms that do distinguish between different pathogens. The responses are less rapid but provide long-lasting immunity. The responses involve a type of white blood cell called a lymphocyte and again take two forms:

a)      Cell-mediated responses involving T lymphocytes

b)      Humoral responses involving B lymphocytes


Recognising your own cells

To defend the body from invasion by foreign material, lymphocytes must be able to distinguish the body’s own cells and chemicals (self) from those that are foreign (non-self). If they could not do this, the lymphocytes would destroy the organism’s own tissues.

It is important to remember that specific lymphocytes are not produces in response to an infection, but that they already exist – all 10 million different types. Given that there are so many different types of lymphocytes, there is a probability that, when a pathogen gets into the body, one of these lymphocytes will have a protein on its surface that is complementary to one of the proteins on the pathogen. In other words, the lymphocyte will “recognise” the pathogen. Not surprisingly with so many different lymphocytes, there are very few of each type. When an infection occurs, the one type already present that has the complementary proteins to those of the pathogen is stimulated to build up its numbers to a level where it can be effective in destroying it. This explains why there is a time lag between exposure to the pathogen and the body’s defences bringing it under control.


How lymphocytes recognise their own cells

·         There are probably around 10 million different lymphocytes, each capable of recognising a different chemical shape.

·         In the foetus, the lymphocytes are constantly colliding with other cells.

·         Infection in the foetus is rare because it is protected from the outside world by the mother passing on her


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