Gellar, who played Kathryn Merteuil in the film, left a comment on Blair's Instagram post about her condition. She offered her love and offered to help in any way she could.
"I love you, I'm proud of you, and there is nothing I wouldn't do for you," Gellar wrote.
Blair, who played Cecile Caldwell in Cruel Intentions, was glad to see Gellar's response and responded with great appreciation.
"I know that and am beyond grateful," Blair wrote, adding an emoji of two pink hearts.
Numerous other celebrities commented on the post, as well, including Gellar's husband, Freddie Prinze Jr.
"Love you Mama," Prinze Jr. wrote.
Blair opened up about her MS in a lengthy post on Saturday. She revealed that she was diagnosed in August and that the crew of her new Netflix show have been beyond accommodating for her.
"I was in this wardrobe fitting two days ago. And I am in the deepest gratitude. So profound, it is, I have decided to share," Blair wrote. "The brilliant costumer Allisa Swanson not only designs the pieces Harper Glass will wear on this new Netflix show, but she carefully gets my legs in my pants, pulls my tops over my head, buttons my coats and offers her shoulder to steady myself. I have multiple sclerosis."
"I am in an exacerbation. By the grace of the lord, and will power and the understanding producers at Netflix , I have a job. A wonderful job. I am disabled. I fall sometimes. I drop things. My memory is foggy. And my left side is asking for directions from a broken gps. But we are doing it. And I laugh and I don't know exactly what I will do precisely but I will do my best," she continued.
She then thanked numerous friends who have helped her with her condition, including Gellar and Prinze.
Photo Credit: Getty Images for GILT / Michael Kovac