Welcome to the forum, though I am sorry for your need to be here. Many here have used immunotherapy in several forms. As you may know, Pembrolizumab is an anti-PD-1 product. Nivolizumab (Opdivo) is as well. Both have practically identical response rates and side effect profiles. Good to know when you see reports on either. Here is some basic information on current melanoma treatments that may interest you: https://chaoticallypreciselifeloveandmelanoma.blogspot.com/2017/08/melanoma-intel-primer-for-current.html
Side effects to anti-PD-1 can vary a great deal from person to person. It can cause a wide variety of somewhat scary and crazy things – and my blog covers most of them! However, here is a report on the most common side effects (written specifically regarding nivo, but the same can be said of pembro): https://chaoticallypreciselifeloveandmelanoma.blogspot.com/2013/10/side-effects-of-nivolumab.html
These days we know a great deal more about how to manage these drugs, including their side effects, than we did when they were first approved (nivo in 2011 and pembro in 2014). Most side effects can be managed with steroids and a drug holiday if needed. Be sure to report any untoward effects you may experience to your doc sooner rather than later as that is when such side effects are easier to manage.
For myself, having taken nivo for 2 1/2 years after advancing to Stage IV melanoma with brain and lung mets, I experienced itchy rashes and vitiligo, dry mouth with oral sores, joint pain, fatigue, and wheeze. (I have asthma.) It wasn’t the most fun ever, but it wasn’t the worst – HA! I continued to work full time, 12 hour shifts, missing only 3 days in those years. The best news is that I remain NED for melanoma with my last dose having been in June of 2013 with no further treatment.
I am sorry you must, but you can do this!!! Ask more questions as you have the need. This board is comprised of many amazing, smart and caring peeps. I wish you my best. Celeste