White Blood Cells
White blood cells do 3 things:
- 1. Engulf the microbe
- 2. Produce antibodies to neutralise the microbe
- 3 Produce antitoxins to neutralise the toxins released by the microbe
Two types of white blood cell:
1. Lymphocytes – Pathogens contain certain chemicals that are foreign to the body and are called antigens. Each lymphocyte carries a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly, and makes many copies of the antibody that neutralises the pathogen.
Antibodies neutralise pathogens in a number of ways:
- they bind to pathogens and damage or destroy them
- they coat pathogens, clumping them together so that they are easily ingested by phagocytes
- they bind to the pathogens and release chemical signals to attract more phagocytes
Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging the body.
2. Phagocytes – Phagocytes can easily pass through blood vessel walls into the surrounding tissue and move towards pathogens or toxins. They then either:
- ingest and absorb the pathogens or toxins
- release an enzyme to destroy them
Having absorbed a pathogen, the phagocytes may also send out chemical messages that help nearby lymphocytes to identify the type of antibody needed to neutralise them.
This video looks at Phagocytes and how they fight pathogens
This video looks at Natural Killer Cells are the most aggressive white cells in the immune system. They make up about 5% to 15% of the total lymphocyte circulating population. They target tumor cell and protect against a wide variety of infectious microbes. Natural Killer Cells are a very important factor in the fight against cancer. Immune Stimulation is the key to keeping the white blood cell count high and giving the Natural Killer Cells a chance to fight cancer and other diseases.
People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens.
Vaccination involves putting a small amount of an inactive or weakened form of a pathogen into the body. Vaccines can contain:
- dead pathogens
- live pathogens treated to make them harmless
- harmless fragments of the pathogen
- toxins produced by pathogens.
These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen.
Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated person is not in danger of developing disease (although some people may suffer a mild reaction). If the person does get infected by the pathogen later, the required lymphocytes are able to reproduce rapidly and destroy it.
This video looks at how a vaccine works
Vaccines and boosters
Vaccines in early childhood can give protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.
Sometimes vaccine boosters are needed, because the immune response “memory” weakens over time. Anti-tetanus injections may need to be repeated every ten years.
Mutations of bacteria and viruses
Some bacteria and viruses mutate very quickly. This means that vaccines developed to protect against these pathogens no longer work so effectively. When this happens, an epidemic occurs where lots of people become infected and ill - and often die. An recent example is bird flu. A new vaccine has to be developed to protect against these new forms of pathogen.
Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses: it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.
Here are some common antibiotics, with descriptions of how they work.
|Antibiotic||How it works|
|Penicillin||Breaks down cell walls|
|Erythromycin||Stops protein synthesis|
|Neomycin||Stops protein synthesis|
|Vancomycin||Stops protein synthesis|
|Ciprofloxacin||Stops DNA replication|
The first antibiotic - penicillin - was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould.
Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most antibiotics used in medicine have been altered chemically to make them more effective and safer for humans.
How bacteria develop resistance to antibiotics
Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.
MRSA (Methicillin-Resistant Staphylococcus Aureus) is very dangerous strain of bacterium, because it is resistant to most antibiotics. It is important to avoid over-use of antibiotics, so we can slow down, or stop, the development of other strains of resistant bacteria.
One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, ideas about the importance of cleanliness in hospitals were ignored people did not know that diseases were caused by pathogens that could be killed with the approprate cleaning agents.
The Human Body Defence
The body has several defences against pathogens so that we do not fall ill with the diseases they cause.
The skin covers the whole body. It protects the body from physical damage, microbe infection and dehydration. Its dry, dead outer cells are difficult for microbes to penetrate, and the sebaceous glands produce oils that help kill microbes.
If microorganisms get into the body through a cut in the skin, the most important thing to do is close the wound quickly so that no more microorganisms can enter. A scab does just that. The blood contains tiny structures called platelets, and a protein called fibrin. A scab is basically platelets stuck in a fibrin mesh. The animation shows how this works.
The respiratory system is protected in several ways. Nasal hairs keep out dust and larger microorganisms. Sticky mucus traps dust and microbes, which are then carried away by cilia. These are tiny hairs on the cells that line the respiratory system.
Hydrochloric acid in the stomach kills harmful microorganisms that might be in the food or drink that we swallow.