FRANKFURT, Sept 21 (Reuters) - European regulators on Thursday granted market approval for Merck KGaA and Pfizer's immuno-oncology drug Bavencio to treat a rare and aggressive type of skin cancer called Merkel cell carcinoma.

In May, Bavencio was approved by the U.S. Food and Drug Administration to treat bladder cancer and in March to treat Merkel cell carcinoma.

Bavencio, also known as avelumab, belongs to a class of drugs called PD-L1 or PD-1 inhibitors that help the immune system attack cancer by blocking a mechanism tumours use to evade detection. (Reporting by Ludwig Burger; Editing by Maria Sheahan)