I wanted to talk about depression but first I have to talk about inflammation

Monday’s post was inspired by this paper on depression and inflammation.  Let’s dig in.

First question:  what is inflammation?  Allow me to translate the wikipedia article.

First, local cells detect that something is there that should not be.  Not every cell can do this.  Wiki’s list is:  macrophages, dendritic cells, histiocytes, Kupffer cells and mastocytes.  Every cell on that list is part of the immune system except for Kupffer cells, which are part of the liver.*  These cells all have pattern recognition receptors whose job is to identify molecules that represent a threat.  In a perfect world, they would recognize every threat and ignore every host molecule and harmless foreign matter.   In the real world, my body spent years convinced that tree pollen was a mortal threat and the only cure was blinding sinus pain.

When a particle activates the pattern recognition receptors they release mediators (the names, types, and specialities of these are easily a blog post in and of themselves) which cause blood vessels to increase in diameter (=vasodilation).  The most immediate obvious of this is increased blood flow, but the dilation also increases the permeability of the blood vessel walls, which allows fluid and protein to leak from the blood vessels into organ tissue.  With the help of a mediator, this also allows white blood cells, especially neutrophils ( the source of the best immune system chase scenes) into the organ tissue.  The additional fluid also increases flow to the lymph nodes, which is the in-body equivalent of getting a pathogen sample to the CDC: the lymph nodes study the fluid for pathogens in their tiny little lymph labs** and, having identified it, trigger the development of a counter-attack.    The mediators also increase the sensitivity of nerves to pain, to incentivize you not to re-injure the area.

And that’s how a physical insult becomes an acute inflammatory reaction.  If the insult is chronic, or if the inflammatory reaction becomes self reinforcing (e.g. inflammation makes an ingrown toenail more ingrown), it becomes chronic inflammation, which we will talk about on Friday, and hopefully get to the actual article on depression on Monday.

*If you’re me, you may have to remind yourself that dendritic cells != dendrites.

**Some day I’m going to learn how that process actually works and not pretend it’s a tiny reenactment of The Andromeda Strain, but the image will do for our purposes.