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White Blood Cells
Written by Tim Sheppard MBBS BSc. Last updated 31/1/15

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What are white blood cells?

Please note that although colours are used in the images of white blood cells throughout, white blood cells are called 'white' for a reason! The colours shown are simply to make distinction easier, and to represent some aspect which may be helpful in understanding properties or function.

Blood is inevitably very important for transporting oxygen around, for which it contains red blood cells. But with this essential function, it needs to be taken care of. It contains platelets as part of its clotting mechanisms, and also white blood cells.

White blood cells, which are also known as leukocytes, are cells involved in protecting the body from infection and diseases. They are called white because they don't contain the red protein haemoglobin (the protein used to bind oxygen), and they are generally considerably larger than their red counterparts.

Unlike red cells, they contain a nucleus, because unlike red cells they have not received their full compliment of protein when they are first thrown into the blood stream. White blood cells, then, provide a useful source of genetic information for scientists, since a blood sample with red blood cells alone would lack such a contribution.

They come in several different varieties, because there's several different ways in which the body has been designed to cope with invading threats. Because they have different functions, the cells have different sizes, and their nuclei are different shapes, with many have more than one lobe.

Leukocytes can be divided broadly into one of two categories:

Granulocytes, comprising neutrophils, eosinophils and basophils, are white blood cells which contain granules to serve their purpose.

Mononuclear leukocytes, comprising lymphocytes and monocytes, contain just one nucleus (as the name suggests) which is rarely or barely lobed, and which are less likely to contain granules.

A more thorough explanation of what each cell does follows.


What is a neutrophil?

Neutrophils are the most common type of white blood cell - which is probably because they are a particularly helpful kind of white blood cells. Rather than being specific for only one particular threat (as many lymphocytes can be), neutrophils are effective combat machines.

Also known as polymorphonuclear neutrophils or PMNs, the neutrophil is a bit of a sleuth, following the chemical trail to where this invader has arrived (chemotaxis); they can 'eat' anything which they think is bad news (phagocytosis), and - rather cleverly - can be involved in something called antibody dependent cellular cytotoxicity, which involves destruction of a particular target if it is coated in antibodies.

The neutrophil has a multi-lobed nucleus; it tends to have 3-4 lobes, but if it has too many then it suggests something wrong.

A particularly helpful thing to have kicking about, the biggest disappointer when it comes to the neutrophil is that it doesn't stick around for very long. After a few hours in the blood, it jumps out and rests around for a while, and if nothing happens, it dies. Fortunately for us we have a large pool of neutrophils in reserve, so if they're needed they can be produced on demand!


What is a monocyte?

Ah, the monocyte. The big one. By no means the most common in the blood stream (though more common than the eosinophils and basophils), yet the monocyte is a hefty white blood cell with a very important job. It can vary enormously - sometimes containing vacuoles or granules, and with an uncertain lifespan - but importantly, this is the cell which will eventually become an antigen presenting cell such as a macrophage.

The monocyte circulates around in the blood stream for a while; then, when it decides to pop off into the tissue of the body, it can turn into a macrophage; the monocyte is also the precursor to the dendritic cell, which does a similar thing to the macrophage. Like the neutrophil, the macrophage can perform phagocytosis, which involves gobbling up dangerous or foreign 'bodies' such as bacteria and viruses. This is really important because it enables the macrophage to kill or inactivate the threat it has phagocytosed, and to activate lymphocytes to kill others. If this goes wrong (e.g. when a bacteria manages to escape the macrophage), then you can get very ill.

Impressively, these phagocytes can chase their prey quite dramatically. It's not exactly a high-speed car chase, but it's an impressive feat all the same. The macrophages also act like vacuum cleaners; when something gets into the body which shouldn't be there, there may be a huge reaction - a cellular battle - to try and get rid of it, so the macrophages have the job of cleaning up all the mess and phagocytosing the debris.


What is an eosinophil?

Eosinophils are so named because they stain well with the acid dye eosin. They, like neutrophils, are granulocytes which are capable of phagocytosis, and they are involved in two important functions:

- Control of parasitic infections
- Allergic reactions

Eosinophils can actually be quite a problem if you're somebody who suffers from disorders relating to allergies such as asthma. It is these cells which kick up some of the fuss - indeed, they start producing chemicals which cause all sorts of problems in your airways.

However, they are also very important, since the absence of such a response would inevitably make you more prone to particular diseases. It's a tight balance, and unfortunately sometimes it doesn't seem to go quite to plan.

Like monocytes, eosinophils have an uncertain lifespan.


What is a basophil?

These dark little things are also involved in allergies, but they have a different kind of job to do. Importantly, these babies are incredibly rare - if you looked through a millilitre of blood, you'd expect to find less than 100,000 of them - and while I'm sure that seems like a lot, it doesn't seem so much when you consider there are around 500,000,000,000 red blood cells per millilitre. That's fifty thousand times as many!

So, what do these rare cells get up to? Despite being rare in the vast scheme of things, there are still ample for them to respond in allergies by releasing particular chemicals to help the process along. They tend to be a relatively late arriver to the scene of an allergy, but they contain a chemical called histamine which is one of the reasons the cells look so dark, and it encourages inflammation.

Once again, the cell is too rare for its lifespan to be well-tracked.


Further Reading