What exactly is that clear liquid that you get from spider bites?

I am not a doctor or a biologist, so I may be totally full of it. But I believe that I am more or less correct.

When the spider but you, your body (skin) saw that as an intrusion (I believe the technical word is “insult”), and it rushed white blood cells to the location of the bites. These would defend the body against biological issues that resulted from the bites. They release histamines to cut down on allergic reactions, among other things.

But they don’t know when to stop, and therefore they will create lots of white blood cells – too many, in fact. (Same theory in allergic reactions and why you need anti-histamines during the pollen season)

If I am not mistaken, the clear stuff is surplus white blood cells that are not needed at the point of the bite/

I also have a spider bite (I assume) at the back of my neck; it is tender and about the size of a lentil. it itches like fury (which is how I first discovered it) and doesn’t seem to be oozing anything. Day three now and itching is subsiding; lump is still there and tender from all the scratching.

Lymphatic fluid? I know nothing about the biology; perhaps @elbanditoroso is saying the same thing in more complicated language.

I’d say it is lymph, too. The stuff that @elbanditoroso described. I used to open up mosquito bites so that they would hurt and scab instead of itch because I hated the sensation, and the same thing came out. It’s your body trying to heal itself.
For a while I thought it was bug fluid, too, and that I was detoxifying my body of the buggy fluid. Then I realized that the substitute liquid from the mosquito’s proboscis goes right into the flow of your bloodstream, as I’d assume it would with a spider bite as well.

I’m thinking fighter cells also from the immune system.

That clear liquid that leaks out is called lymph. The liquid the spider injects is called venom.

Lymph.