As a wildlife biologist, I have been bitten and stung by quite a number of things, from hundreds of bats of many species, to insects of all kinds, and even an ocelot. Sometimes it hurts a lot, and sometimes it just itches a lot, but such is what I do and I usually don't complain about such.
And I am not complaining now, just giving an introduction to a yellow sac spider bite.
The yellow sac spider (Cheiracanthium sp. probably mildei) is a spider found in many houses, that minds it's own business eating bugs that might make it into your house. Having spiders in your house is generally a good thing. But this spider has a toxic bite if you scrunch it while it is in the sheets. I have found and killed a couple at my current abode, but one got me.
Here is a photo of my little finger, between the knuckle and first joint.
So what is going on here? First, the area puffed up like most bites/stings do from any insect. This is a reaction from my body's immune system and is normal, even though this is a bite from an arachnid not an insect. But there is more.
Arachnids, and in this specific case a yellow sac spider, can deliver different toxins and do some specific cell damage. Here, you are seeing the result of a cytotoxin, which liquifies cells. Yes, that is what is going on in the photo, the destruction of cells in my finger. Depending on how much venom was introduced, the damage will be such.
I have photographed this species of spider before, but no hope that I can find such this evening. So just the bite for now.