Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services for trouble breathing, swelling of the tongue or throat, fainting, severe confusion, or blue or grey lips.
These symptoms may indicate anaphylaxis or another medical emergency rather than routine serum sickness.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Arrange prompt medical advice if you develop a new rash, fever, and joint pain within days or weeks of starting a medicine or receiving an injection, infusion, antivenom, or immune treatment.
Bring the product name and the date it was given.
Seek urgent same-day assessment if you have:
Much less urine than usual, dark urine, or blood in the urine
New swelling around the eyes, hands, legs, or feet
A rapidly worsening or painful rash
Blisters, peeling skin, or sores in the mouth, eyes, or genitals
Severe weakness, persistent high fever, or difficulty walking
Symptoms that continue to worsen after the suspected trigger is stopped
And for anyone, someone who:
is vomiting and cannot keep fluids down
Every sign above is quoted from the sofidex article on Serum sickness. If you are not sure, err upward — it is always reasonable to be checked.