How was insulin obtained before the advent of $rDNA$ technology? What were the problems encountered?

(N/A) Before the advent of $rDNA$ technology,insulin used for the treatment of diabetes was extracted from the pancreas of slaughtered cattle and pigs.
The major problem encountered was that insulin obtained from animal sources was not identical to human insulin.
Because it was a foreign protein,it caused some patients to develop allergies or other types of immune reactions.