LONDON (Reuters) - An experimental medicine from AstraZeneca (AZN.L) that helps the immune system fight tumours has won "orphan" drug status in the United States for treating malignant mesothelioma, a rare type of cancer.

Tremelimumab is one of a number of immuno-oncology products that AstraZeneca is banking on as it focuses heavily on promising new treatments for cancer.

The British drugmaker said on Wednesday its antibody treatment had been awarded the orphan designation, which aims to encourage drug development for rare conditions, by the U.S. Food and Drug Administration.

Orphan drugs enjoy seven years of marketing exclusivity and are defined as those fighting rare diseases that affect fewer than 200,000 people in the United States.

AstraZeneca aims to submit tremelimumab for U.S. regulatory approval in the first half of 2016 as a treatment for mesothelioma, an aggressive disease that most often affects the lining of the lungs and abdomen.

The drug is also being tested in combination with AstraZeneca's so-called PD-L1 treatment MEDI4736 for lung cancer, which is a much bigger market opportunity.

(Reporting by Ben Hirschler; editing by Susan Thomas)