Since starting Drawing Blood I’ve been very aware of neglecting the white half of our circulating cells. Unlike red blood cells, it’s not as simple as different shapes and expressions, white blood cells (leucocytes) are incredibly complex with new features still being discovered. Macrophages are a good example. They are commonly defined as ‘large cells that eat other cells’ which is true. To an extent. We now know that this functional definition applies to a wide range of cells that have different roles in different tissues and they’re not even all derived from the same progenitors.
In broad terms, macrophages are activated by external signals and can be called into action in a variety of roles. One is a pro-inflammatory persona, called ‘M1’ in which they launch all out war on infection, devouring pathogens and working with other immune cells to defend the body. I like to think of this as macrophage rage. At the other end of the spectrum, they can be anti-inflammatory ‘M2’ macrophages. These play a key role in repairing tissue and restoring health, laying down collagen fibres to stitch up damage. In the interets of a catchy rhyme, I call this a sage macrophage.
Before a horde of immunologists go all M1 on me, saying this is a gross oversimplification, it’s true, it is. But it’s one that has totally permeated the literature and more importantly, is a helpful starting point for explaining what they do. In reality, the balance between these behaviours, when they are adopted and for how long is critical for effective protection against infection and subsequent wound repair. A better understanding of macrophage identities will have many medical benefits.
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