I got a pretty bad gash on my arm and when i was washing away the blood i noticed a small almost allergic rash following the trail of blood?

No Known skin diseas: Dear jdV, In 41 years of dermatology practice, I have never heard this complaint. There should be no allergic reactions to your own blood in any way shape or form. It would be nice to see a picture of this rash, And it would be hard to say what might've cause the coloration, But some people actually scrubbed the area around the wound and it will cause some temporary redness which looks like a rash