The body's first line of defence against harmful pathogens is the skin and stomach acid, the second is white blood cells. Vaccination involves exposing the body's immune system to a weakened or harmless version of the pathogen in order to stimulate white blood cells to produce antibodies.
Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens.
Bacteria come in many shapes and sizes, but even the largest are only ten micrometres long (ten-millionths of a metre). Bacteria are living cells and can multiply rapidly into favourable conditions. Once inside the body, they release poisons or toxins that can make us feel ill.
Viruses are many times smaller than bacteria. They are among the smallest organisms known, consisting of a fragment of genetic material inside a protective protein coat. Viruses can only reproduce inside host cells, damaging them when they do so. Once inside, they take over the cell and make hundreds of thousands of copies of themselves. Eventually, the virus copies fill the whole host cell and it bursts open. The viruses then pass out through the bloodstream, the airways, or by other routes.
Population Growth of Microogranisms
In the correct conditions (with warmth, moisture and nutrients) bacteria can multiply rapidly. The human body can provide these conditions for bacteria to multiply, for example in a cut. Each bacterium splits into two up to every 20 minutes. So, after one 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…