Press the start button then select the most appropriate word to fill each gap from the drop-down list. Each word in the list is used only once. Hint: read ahead and think about grammar to narrow down the choice of possible answers. When you think that you have it right, press the check sequence button. There is a 5 second penalty if you are wrong!
Don't forget to login if you want your time to be recorded.
Pathogens are that cause disease. Fortunately the body has several defences against pathogens so we do not fall ill with the diseases they cause.
The skin covers the whole body. It protects the body from physical damage, infection by microorganisms and dehydration. Its dry, outer cells are difficult for microbes to penetrate, and the glands produce oils which help kill microbes.
If you cut yourself you bleed because blood vessels that pass through the skin are broken, allowing liquid blood to escape. Before long, however, the flow of blood stops. This process is known as blood clotting or , and is very important in making sure that we can survive injury. There are two main reasons why blood clotting is so important.
If our blood did not clot, even a small cut would mean that we lost a lot of blood. Eventually, we would not have enough blood to transport to our cells for and we would die.
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 can enter to cause .
Clotting needs special proteins like that are in the blood . It also requires , that help to trigger the clotting process when a blood vessel is damaged. Platelets are small fragments of larger cells. A is basically platelets and red blood cells stuck in a mesh of formed from fibrinogen. A is clotted blood on the surface of the skin that stops blood escaping and prevents microorganisms from entering the body.
Some people carry a genetic disease called that stops their blood from clotting properly. Even a small cut can kill them and they need regular blood to provide the plasma proteins that they cannot make.
The respiratory system is protected in several ways. Nasal keep out dust and larger microorganisms. Sticky traps dust and microbes, which are then carried away by - tiny hairs on the cells that line the respiratory system.
acid in the stomach kills harmful microorganisms that might be in the food or drink that we swallow.
Despite these defences, some bacteria and can still get through. Once inside the body, reproduce quickly. Viruses reproduce inside cells and damage them, while escaping to infect more cells. Bacteria produce - poisons. Cell damage and toxins cause the of infectious diseases.
Once pathogens enter the body, however, the immune system destroys them. blood cells are important components of the immune system that destroy pathogens in two main ways.
Some white blood cells called can pathogens and destroy them. Other white blood cells called make - these are special proteins that help to destroy pathogens and neutralise the toxins they release.
Pathogens contain chemicals that are foreign to the body, called . White blood cells - lymphocytes - carry antibodies - proteins that have a chemical 'fit' to a certain antigen. When a white blood cell with the appropriate antibody meets the antigen, it reproduces quickly and makes many copies of the antibody that the pathogen and speed up its clearance from the body.
Active and passive immunity
Once you have been infected with a particular pathogen and produced antibodies against it, some of the white blood cells remain so that the body remembers the pathogen. This is called immune . If you become infected again with the same pathogen, these white blood cells reproduce very rapidly and the pathogen is destroyed before you become ill. This is immunity and explains why works to protect you from a disease like measles without you having to catch it. Vaccines contain a killed or form of the pathogen that effectively trains the white blood cells how to respond without making you ill.
Sometimes you may be treated for infection by an injection of certain antibodies from someone else. This is immunity. A foetus gets antibodies from its in the womb, and a baby gets antibodies in its mother's . That is one reason why breast-feeding is important.