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Biology AS Best summary notes
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Cell structure_ A level
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AS & A Level Biology 9700 : Respiration, Immunity and Infectious Diseases
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AS Bio notes Raida 12DImmunity
11.1 Defence against disease
> External defence system
Humans have a variety of mechanisms to protect them against infectious diseases
Many pathogens do not harm us, because if we are healthy, we have physical, chemical
and cellular defences that prevent them from entering
For example, the epithelia that cover the airways are an effective barrier to the entry of
pathogens; hydrochloric acid in the stomach kills many bacteria that we ingest with food
and drink; blood clotting is a defence mechanism that stops the loss of blood and
prevents the entry of pathogens through wounds in the skin
> Internal defence system
If pathogens do successfully enter the body, white blood cells can recognise them as
something foreign and destroy them
White blood cells are part of the immune system and they recognise pathogens by the
distinctive, large molecules that cover the pathogen surfaces
These molecules include proteins, glycoproteins, glycolipids and polysaccharides, the
toxins and waste materials which some pathogens produce
Any molecule which the body recognises as foreign is an antigen
There are two types of white blood cell: phagocytes and lymphocytes
Important features of the immune system is the ability to distinguish between Self and
Non-self, and the production of antibodies
Antibodies are glycoprotein molecules that act against specific antigens
Although cell surface antigens do not stimulate production of antibodies in their own
body, they may do if they enter someone else
The response of lymphocytes to the presence of a foreign antigen is known as the
immune response
In some cases, lymphocytes respond by producing antibodies; in others, they respond
by killing cells that have become infected by pathogens
EXAMPLE:
The cell surface antigens of the human ABO blood group system are a good example
If you are blood group A, then you have a specific carbohydrate chain on the glycolipids
and glycoproteins on your red blood cells that is not on the red blood cells of people who
are blood group B
If blood of type A is given to someone who has blood of type B during a transfusion, the
recipient’s immune systems will recognise the blood cells as foreign and start to produce
antibodies
This happens because the recipient’s immune system has recognised the antigens on
blood cells of type A as non-self
If blood of type B is used during the transfusion, as it should be, the recipient’s immune
system recognises the antigens on the red blood cells as self and no antibodies are
produced
11.2 Cells of the immune system
The cells of the immune system are produced from stem cells in bone marrow
There are two groups of these cells involved in defence:
, 1. Phagocytes (neutrophils and macrophages)
2. Lymphocytes
Phagocytes
Phagocytes are produced throughout life in the bone marrow
They are stored there before being distributed around the body in the blood
They remove any dead cells as well as invasive microorganisms
Neutrophils are a kind of phagocyte and form about 60% of the white cells in the blood
They travel throughout the body, often leaving the blood by squeezing through the walls
of capillaries to move through the tissues engulfing any pathogens that they find
During an infection, neutrophils are released in large numbers from their stores, but they
are short-lived cells
Macrophages are also phagocytes but are larger than neutrophils and tend to be found
in organs such as the lungs, liver, spleen, kidney and lymph nodes, rather than
remaining in the blood
After they are made in the bone marrow, macrophages travel in the blood as monocytes
which develop into macrophages once they leave the blood and settle in the organs,
removing any foreign matter found there
Phagocytosis
If pathogens invade the body and cause an infection, some of the cells under attack
respond by releasing chemicals such as Histamine
These, together with any chemicals released by the pathogens themselves, attract
neutrophils to the site
Movement towards a chemical stimulus is called Chemotaxis
The neutrophils destroy the pathogens by phagocytosis
They move towards the pathogens, which may be clustered together and covered in
antibodies
The antibodies further stimulate the neutrophils to attack the pathogens
This is because neutrophils have receptor proteins on their surfaces that recognise
antibody molecules and attach to them
When the neutrophil attaches to the pathogens, the neutrophil’s cell surface membrane
engulfs the pathogens, and traps them within a phagocytic vacuole in a process called
Endocytosis
Lysosomes fuse with the phagocytic vacuoles releasing enzymes that breakdown the
pathogens
Neutrophils have a short life: after killing and digesting some pathogens, they die
Dead neutrophils often collect at a site of infection to form pus
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