GCSE BIOLOGY REVISION: INFECTION AND RESPONSE
Please note: Text in bold is what the AQA GCSE biology specification requires an understanding of.
Human defence systems
Students should be able to describe the non-specific defence systems of the human body against pathogens, including the:
• trachea and bronchi
Students should be able to explain the role of the immune system in the defence against disease.
If a pathogen enters the body the immune system tries to destroy the pathogen.
White blood cells help to defend against pathogens by:
• antibody production
• antitoxin production
In a previous article, we have already discussed how the skin and nose act as barriers against pathogen entry as part of the non-specific defence system.
Trachea and bronchi
The trachea begins at the nose down towards the lungs. The tracheal lining consists of ciliated cells, smaller than those in the nose. The ciliated cells move their hairs to transport the mucus (carrying pathogens) over them towards the throat. Once the mucus reaches the throat it is swallowed and passed into the stomach. Goblet cells are the cells which create and secrete mucus. The production of mucus in the airways is a physical barrier.
You may be aware that we have acid in our stomachs, you may not be aware of what this acid is used for. Stomach acid is used as a non-specific first line of defence (it is not involved in the breakdown of food.) The hydrochloric acid does no harm to our bodies, but is strong enough to kill pathogens found in the mucus, water or food. This is an example of a chemical barrier against infection.
So, after all those first-line defence mechanisms against pathogen entry, pathogens can still enter the body! Once they enter the body, they cause infection. The body does however have a second line of defence to inhibit or minimise the infection, this is what we call the immune system. In the immune system there are two types of white blood cells, phagocytes and lymphocytes.
When a pathogen enters the body, it is surrounded by a phagocyte, and then engulfed. This process is non-specific (the same process happens for all pathogens and does not require specific phagocytes.) The phagocyte is attracted to the pathogen, it binds (with the help of specific proteins) and then the pathogen ends up inside the phagocyte by endocytosis. This process is called phagocytosis. Enzymes inside the phagocyte then fuse with the pathogen and cause it to break down. This ensures complete destruction of the pathogen.
Lymphocytes are white blood cells that recognise proteins found on the surface of pathogens. These proteins are called antigens. If a lymphocyte does not recognise the antigen on the surface of a cell, it is seen as foreign (meaning it does not naturally occur in the body.) This stimulates the production of antibodies. These antibodies can then help to fight against the pathogen. This process can take a few days, in the meantime, symptoms will show and we may feel a bit ill. The antibodies cause the pathogens to stick together (agglutination) and then can be engulfed by phagocytes. Certain pathogens produce toxins, these will make you feel ill. Luckily, lymphocytes can produce antitoxins which will neutralise toxins. This is a highly specific process; thus, different antibodies will work on different antigens. This is therefore a specific process.