Having drawn red blood cells and platelets, ‘Drawing blood’ still left a vast population of cells to include, namely the white blood cells. Illustrating all the varieties of red blood cell shape and size took a lot of work but at least they’re all the same cell and a simple one at that. Adding white blood cells to the collection poses a completely different and much bigger challenge. It’s not even as if they’re all white. And the closer you look, the more subdivisions become apparent. Many names for broad classes of cell date from historical descriptions of appearance, typically with little functional insight. What’s more, this is often based on what they look like under the microscope after the additional of chemical stains. You don’t have little bags of purple granules patrolling your blood looking for germs. A ‘basophil’ is literally a ‘base lover’ where the base is a chemical dye with a positive charge, rather than any implication of loose morals. So all the name actually tells you is these cells contain molecules with lots of negative charge that bind the dye.
Modern techniques are constantly revealing the varied identity and function of white blood cells in more meaningful ways. The proteins they post on their surface influence how they detect and respond to pathogens, allergens or other immune cells. The molecules they make and secrete, where and when they do this – for better or worse – are central to inflammatory, immune and allergic responses.
Returning to the historical approaches, those coloured granules nevertheless provide a useful way to start considering specific types of white blood cells, welcome to the granulocytes…
Granulocytes
Granulocyte is a descriptive term describing cells full of granules but more specifically covers four classes of cell which can all release these granules, often in an explosive flood, as part of an immune response. Usually this is beneficial, if an invading parasite is being blitzed but less welcome when innocuous pollen grains lead to itchy eyes and a streaming nose.
Neutrophils
Under most conditions, neutrophils are the most common white blood cell in the human body. They are the shock troops, usually being mobilised fully mature to rapidly reach sites of infection and directly eliminate invading pathogens. Neutrophils are agile, able to squeeze into tight spaces wherever pathogens might have sneaked in. They have three main avenues of attack, all represented in this cartoon. Their classical role is in phagocytosis which literally means ‘eating cells’. They’re not fussy and will gobble up any foreign microorganism they can engulf and rapidly eliminate them. As indicated by the main heading, they are full of granules containing toxic enzymes and chemicals including bleach! They use these to dissolve the cells they ingest but their second line of attack is to release these like explosive grenades, indiscriminately damaging all pathogens in their vicinity. Unfortunately, these bioweapons attack any biological tissue so there is often collateral damage of host organs. This is a major contributor to the pathology of autoimmune conditions and chronic inflammation. Their third weapon are NETS, which stands for Neutrophil Extracellular Traps but are very much like actual nets. Rather than casting them, it might be better to describe neutrophils as vomiting up NETS. They are formed from a sticky, stringy mixture of the neutrophil’s own DNA, proteins and granules, spewed out to trap and kill bacteria.
Basophils, eosinophils and mast cells by contrast typically take a less head-on approach to fighting infection. Their granules, notably those of eosinophils, may contain toxic elements but tend to include signalling molecules called cytokines which modulate the wider immune and healing response. They can interact with antibodies, recruit other immune cells to fight infection, stimulate blood flow or tissue repair. Many of these effects are complex, context-dependent and still being uncovered. Their names are a useful shorthand but we now know that there are many subtypes of each with characteristics much more subtle than the colour they stain under the microscope. All three cell types are central to allergies and their involvement in conditions such as asthma is a key area of research.
Eosinophils
This cartoon refers to one of the earliest roles attributed to eosinophils, attacking parasites such as nematode worms. Their granules contain specialised enzymes and proteins which can directly break down the parasite’s external layers and recruit antibodies to enhance this attack.
Basophils
When basophils encounter a site of infection they often release histamine. This stimulates increased blood flow and promotes the recruitment of other immune cells to help out. Histamine also produces an itchy or even painful sensation in nerve endings, the increased blood flow leads to blotchy skin. These symptoms will be familiar to anyone who suffers from eczema or hay fever as this cartoon shows.
Mast cells
Mast cells have many similarities to basophils and for a long time there was confusion over whether they were distinct cell types. We now know they are in different branches of the granulocyte family tree. Whereas mature basophils circulate in the blood, immature mast cells take up residence within connective cells and remain there as mature cells. They can rapidly release their granules in response to an external trigger leading to a local flood of histamine. The increased blood flow, lump of accumulated fluid and painful itching is what we recognise as an insect bite.
You can find all the granulocytes on a range of cool merchandise at my RedBubble store
You can download these vector art images of granulocytes at scidraw.io