Casein is one of the many proteins that a milk-allergic person can react to. \

Allergic reactions to dairy proteins can span the spectrum from
hay-fever-like allergic rhinitis (runny nose, sneezing) to swelling to
hives to asthma to anaphylactic shock, etc. Some people (me included) can
also have rheumatoid arthritis triggered by milk allergy. It's also been
associated with autism.
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Mark Feblowitz