The immune system is the collective term for the many mechanisms the body uses to protect us from infection. It isn’t one thing, but rather a complex system of many molecules, cells and organs which interact in very specific ways to prevent and combat infection.
Given the prevalence of herbs, exercise programs and even bottles of water that are marketed as being able to ‘boost the immune system’, and the fact that little immunology is taught in schools outside of biology subjects, it is understandable that many people picture the immune system as a kind of vague, ethereal shield.
In reality the immune system has many different components throughout the body, from specialised cells and molecules capable of combating infection, to whole organs whose main function is to develop specific immune responses. Apart from these highly specialised systems there are organs and cells that primarily perform other functions – and so are not strictly considered part of the immune system – but which still play a part in preventing infection, such as skin cells that produce proteins that kill bacteria.
One of the key features of the immune system is memory, which manifests as a resistance to repeated infections with the same microbe. This lets us consider the immune system in two parts – as the adaptive and innate immune systems. The innate immune system responds the same way to the same infection each time it is encountered. The adaptive immune system takes longer to respond, but is much more effective at combating infections, and also produces memory cells. Memory cells remember an infection and if a person is infected with the same microbe then these cells allow the adaptive immune system to respond much quicker than the first encounter – in most cases you won’t even know you’ve been infected the second time.
The footsoldiers in this army are white blood cells, which are responsible for most of the activities of the innate and adaptive arms of our body’s defences.
White blood cells are produced from bone marrow stem cells. Some mature in the marrow, while others head to other tissues to mature before being able to properly function. Some white blood cells live in specific tissues, whilst others circulate in the bloodstream, and can be recruited to the site of an infection. While there can be a lot of variability between the cell types produced, there are two basic mechanisms by which they kill invading organisms.
One of these mechanisms is the gobbling up of microbes to destroy them. Some white blood cells specialise at this. The process of phagocytosis (literally ‘cell eating’) involves the white blood cell binding the microbe by specialised receptors, followed by the extension of the cell’s own membrane around the microbe. This leads to the cell engulfing the microbe which is now held in an internal compartment. This compartment is fused with others containing harmful enzymes. Other enzymes assemble at the edge of the compartment and produce highly reactive chemicals to help destroy the microbe.
Once the microbe has been killed, the left over parts can be used to activate the adaptive immune system, so that the body can fight the infection more efficiently, and once it is cleared, remember it in case of future infection.
After some more processing, molecules from the microbe can be used to stimulate an adaptive immune response, or re-activate memory cells from a previous infection with the same microbe.
This animation illustrates each of the steps involved in phagocytosis. In this example the way the bacterium is recognised as foreign is by expression of the bacterial sugar mannose.
Here we can see a white blood cell chasing and then phagocytosing bacteria. Nom nom nom.
The other major mechanism of destroying microbes is the process of releasing a granule of harmful molecules, a process known as degranulation. This is especially useful for attacking organisms too big to be gobbled up, such as parasitic worms. Some cells release granules that specialise in killing host (own) cells – which can be important in protecting the body from intracellular parasites like viruses, and from cancers, which develop from our own cells.
Phagocytosis, degranulation and memory are a few of the basic mechanisms by which cells of the immune system combat invading microorganisms.
The immune system is a complex arrangement of molecules, cells and tissues which exists throughout the body, and which helps to prevent and combat infection. The major cellular processes for destroying invading microorganisms are performed by white blood cells, and include phagocytosis and degranulation.
Next time I will discuss how the innate immune system recognises things as foreign, and the role of inflammation in an immune response.