5.1 Defence mechanisms

Question 1

Define pathogen

Answer 1

Any microorganism that causes disease

Question 2

Define immunity

Answer 2

The means by which the body protects itself from infection

Question 3

What 2 forms of defence mechanisms are there? What are they?

Answer 3

1. non-specific (response is immediate and the same for all pathogens)
   - physical barrier e.g skin
   - phagocytosis
2. specific (response is slower and specific to each pathogen)(involves a WBC called a lymphocyte)
   - cell-mediated response , T lymphocytes
   - humoral response , B lymphocytes

Question 4

Why are specific lymphocytes not described as ‘produced’ in response to an infection?

Answer 4

All 10 million different types of lymphocytes already exist in the human body. Hence there is a high possibility that when a pathogen enters the body, one of these lymphocytes will have a protein on its surface that is complementary to one of the proteins of the pathogen.
The lymphocyte will ‘recognise’ the pathogen, and as there are very few of each type, the one type already present is stimulated to divide and build up its numbers to a level where it can be effective in destroying it. (known as clonal selection)