What to know about non-Hodgkin lymphoma, the cancer Jane Fonda was diagnosed with

• Jane Fonda is undergoing treatment for non-Hodgkin lymphoma.
• The actress said she feels lucky because the cancer is treatable with chemotherapy.

Actress Jane Fonda recently announced that she is undergoing treatment for non-Hodgkin lymphoma, a type of cancer that affects the immune system.

The actress, 84, shared her diagnosis on Instagram Friday. In the post, she said she has started chemotherapy treatments and is handling them "quite well" so far.

"This is a very treatable cancer," Fonda wrote. "80% of people survive, so I feel very lucky."

Around 80 out of every 100 people diagnosed with non-Hodgkin lymphoma (also known as non-Hodgkin's lymphoma or NHL) survive their illness for a year or more, according to Cancer Research UK. A slightly smaller percentage (73%) survive for five years or more, according to the American Cancer Society.

The Oscar winner and climate advocate said she plans to undergo six months of chemo, and that she wouldn't let her illness get in the way of her activism.

Lymphoma is a cancer of the immune system

Non-Hodgkin lymphoma usually originates in the lymph nodes or spots with similar tissue.

Lymph nodes are located all throughout the body, including along the neck, armpit, and groin. The lymphatic system is part of the immune system, but along with helping to fight off infections, it also helps move fluid throughout the body.

NHL affects white blood cells, or lymphocytes, which play an important role in immune system function. If not caught early, it can spread to other organs outside of the lymph system, making it much harder to treat.

The cancer most commonly affects adults, but children can get it too. Men have a slightly higher risk compared to women, according to the American Cancer Society.

NHL is more common than Hodgkin lymphoma, a similar cancer that is characterized by a specific cell signature. Patients with NHL tend to be a bit older — most are over 55 when they're diagnosed, while the median age for diagnosis of Hodgkin lymphoma is 39, according to the Moffitt Cancer Center.