Robert Downey Jr. marked his 55th birthday by heaping praise on the frontline workers amidst the coronavirus pandemic.

The 'Avengers: Endgame' star used his big day to pledge support and his gratitude to those working hard to keep the world safe amidst the virus, which has claimed the lives of thousands of people worldwide.

He wrote on Instagram: '''If u don't like something, change it. If you can't change it, change your attitude...' #mayaangelou #happybirthday #Poet #Activist 1928-2014 It's an honour to share a birthday with this great #woman, and a necessity to express my #gratitude to the following - #Healthcare workers, Personal care and home health aides, First responders, Anyone in an active service job, As well as law enforcement, the National Guard, all Armed Services, etc ... Anyone braving exposure in the #service of others - Proudly, RDJ (sic)''

Robert's friends and co-stars rushed to social media to send their birthday wishes, with Chris Evans writing on Twitter: ''Happy birthday to one of my absolute favorites! Love you 3000, @RobertDowneyJr (sic)''

Gwyneth Paltrow shared: ''Before we put this day to bed, I must wish my dear, dear @robertdowneyjr a happy birthday. Here he is, making a toast at our wedding, during which his humour elicited my usual look of pure joy/shock whenever he is near. He is my brother, friend and supporter. A mega genius, and the weirdest comedy writer of all time. I know all of our @marvel family join me in wishing him a very happy day. Do you love Robert? Let him hear you if you do. We love you RDJ (sic)''

Mark Ruffalo wrote: ''Sending you all the virtual hugs for your birthday, @RobertDowneyJr ... I love you to life, 3000. It's an honour knowing such a generous and complex man. (sic)''