We live in a world buzzing with life that takes a variety of shape and form. Many of these life forms, the microbes, are not visible to the naked eye including bacteria, virus, fungi and some parasites. We coexist in harmony with many of these micro organisms and many bacteria and fungi belonging to our natural flora are even beneficial to us. But organisms that invade and grow within us can cause us harm, illness and disease. We call these organisms' pathogens.
All multi-cellular organisms from plants, insects and mammals, have developed natural defence systems to fend off pathogens. The first line of defence is our physio-chemical barriers including the skin and mucosa, that cover internal surfaces with external contact for example, lungs, gut and mouth. These physio-chemical barriers work by reducing the foothold of microbes by constantly moving, for example shedding of dead skin cells and beating cilia of lung epithelial cells, as well as producing destructive enzymes or making the environments unpleasantly acidic. Roll over the graphic to view some of our natural defence systems that fend off pathogens.
Despite these barriers some pathogens will succeed in colonising our cells. In order to survive an active defence system, the immune system, has therefore evolved in multi-cellular organisms. Most multi-cellular organisms have an immune system that strikes against pathogens carrying certain recognised structures. This immune system is called the innate immune system. However, vertebrates have, in addition, evolved to have an adaptive immune system that can tailor an optimal "attack strategy" against a particular microbe and remember this to stop reinfections quickly. The innate and adaptive immune system, as well as the cells that link them, the antigen presenting cells, will be presented in the following sections and finally we will also look at how they all interact during different immune responses.