"I think underripe apples are always dangerous. Any organic apple is
likely to be o.k., but usually it's safer to convert them to
applesauce (cooked), making them more digestible. Storage or stress
can affect the apple's allergenicity, and incomplete ripening leaves a
variety of toxins. Malonic acid, even at a very low concentration, can
cause severe bowel inflammation.
Most people can eat an occasional random apple without getting
symptoms such as constipation, hemorrhoids, headache, sniffles,
sneezing, gas, joint pain, restless sleep, etc., so in moderation the
toxins don't matter too much. But when there's a symptom, the fruits
with ripening problems should be high on the list of suspects.