Can someone explain how amoxicillin would interact with the mono antibodies?

Rash: Patients with mono who take Amoxicillin have a greater chance of developing a drug induced rash with itching. This connection is well known to the point that doctors shy away from using Amoxicillin in patients with mono.

Causing a rash?: The simple answer is that we don't know how it works. The combination of mononucleosis and Amoxicillin causes a rash in a significant minority of patients. The odds appear to go up significantly when you're already sufficiently miserable with the virus, as if adding insult to injury. Sorry if that's your situation, but you're not alone. It gets better. Keep cool, overheating makes you more itchy.