Human Defence Systems (AQA GCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
Non-Specific Defences
The human body has a number of mechanisms that are the first line of defence against an infection
The non-specific defence systems of the human body against pathogens include:
The skin
The nose
The trachea and bronchi
The stomach
The different mechanisms can be divided into biochemical and physical defences
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The Immune System
The immune system of the body is highly complex, with white blood cells being the main component
Once a pathogen has entered the body the role of the immune system is to prevent the infectious organism from reproducing and to destroy it
White blood cells help to defend against pathogens by:
Phagocytosis
Production of antibodies
Production of antitoxins (these are special types of antibody)
Phagocytosis
Phagocytes engulf and digest pathogens, this can be non-specific or helped by antibodies which cause agglutination (clumping) of pathogens
The phagocyte surrounds the pathogen and releases enzymes to digest and break it down to destroy it
Phagocytes engulf and digest all the pathogens that they come across, they can be enhanced by antibody production
Production of antibodies
Lymphocytes produce antibodies
Antibodies are Y-shaped proteins – each individual has the potential to make millions of different types of antibodies, each with a slightly different shape
The aim of antibody production is to produce the antibody that is specific (complementary) to the antigens on the surface of the pathogen
This is a specific type of immune response as the antibodies produced are specific to each pathogen's antigens
The lymphocytes produce antibodies that are specific to the antigen on the pathogen
It can take a few days to make the antibodies that are specific to a pathogen and this may give the pathogen causing the infection enough time to make you feel unwell (as its numbers in the body, and consequently the damage caused by them, increases)
Memory cells are lymphocytes that remain in the body after an initial infection with a particular pathogen; they produce the specific antibodies against its antigens so that if you get infected by the same pathogen again in the future (and the antigens are the same) you can produce antibodies much quicker against it before its numbers increase and it can cause damage to the tissues of the body
Production of antitoxins
Some pathogens (usually bacteria) can produce substances which act as toxins which make you feel unwell
Lymphocytes can produce antibodies against these substances – in this case, they are called antitoxins
The antitoxins neutralise the effects of the toxin
Lymphocytes produce specific antitoxins to specific pathogens
Examiner Tips and Tricks
Make sure you know the difference between antigen, antibody and antitoxin:
An antigen is a molecule found on the surface of a cell
An antibody is a protein made by lymphocytes that is complementary to an antigen and, when attached, clumps them together and signals the cells they are on for destruction
An antitoxin is a protein that neutralises the toxins produced by bacteria
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