disease and restinance
pathogens - pathogens are microorganism that can cause infectious diseases. the main types of pathogens are bacteria and viruses.
bacteria - they tend to come in all sorts of shapes and sizes. they are living cells that can multiply rapidly in certain conditions. once they get inside the body they release poisons and toxins that can make us feel very ill.
viruses - these are a lot smaller than bacteria and can only reproduce in host cells and damage them as they do so. once inside they will take over the cell and make hundreds of copies of themselves. eventually the virus copies will fill the whole cell and burst open and letting the virus out into the bloodstream and airways.
population growth of microorganisms
- in the correct condtions bacteria can multiply rapidly nd the human body can supply these conditions. for example ina cut each bacterium will multiply into two every twenty minutes. so after an hour a single bacterium could have reproduced to give eight bacteria.
lines of defence
When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the disease. The body’s first line of defence is its natural barriers which include:
- chemicals in tears
- chemicals in sweat
- stomach acid.
The body’s first line of defence is called passive immunity, which means preventing the pathogen from entering in the first place. If a pathogen manages to get into the body, the second line of defence takes over, which is called active immunity. The white blood cells have key functions in this.
lines of defence (continued)
white blood cells can:
- ingest pathogens and destroy them
- produce antibodies to destroy pathogens
- produce antitoxins that neutralise the toxins released by pathogens.
On average an adult human will catch two to four colds every year, and a child will catch even more (because their immune system is still developing. This happens for two main reasons:
- there are hundreds of different cold viruses
- the viruses have a high mutation rate
A high mutation rate means that the DNA of the virus frequently changes, so the human body has to destroy the virus using a different antibody each time, which makes it harder to develop immunity.
types of white blood cells
- There are several types of white blood cell, each with a different function. But there are two main groups. The first of these groups surrounds and digests bacteria. They can pass easily through blood vessel walls into the surrounding tissue, and move towards pathogens or toxins.
- The second produces antibodies to label microorganisms. Pathogens contain certain chemicals that are foreign to the body. These are called antigens. Each of this group of white blood cell carries a specific type of antibody - a protein with a chemical ‘fit’ to a certain antigen. When a white blood cell with the appropriate antibody meets the antigen, the white blood cell reproduces quickly to make many copies of the antibody that neutralises the pathogen.