'All That' Star Christy Knowings Dead at 46

12:13 PM PT -- Kenan Thompson took to Instagram to pay tribute to his his fellow "All That" star.

Former "All That" star Christy Knowings is dead after suffering an asthma attack that left her with brain damage ... TMZ has learned.

A family member tells TMZ ... Christy was hospitalized Friday after the attack and placed on life support.

We're told Christy remained on life support through Tuesday, when her family made the difficult decision to take her off it. She died Tuesday night at a Los Angeles-area hospital.

The actress and comedian joined Nickelodeon's "All That" in 1997 after appearing in "And Now This" -- a Rosie O'Donnell sketch-comedy special produced by the network.

Christy spent three seasons on "All That" playing several characters, including Penny Lane and Jessica on the recurring "Whateverrr!!!" sketch with Amanda Bynes.

She later appeared in 3 episodes of "Sesame Street" alongside her real-life twin brother, Chris Knowings ... and released the folk single "To the World" in 2020.

Christy is the second former "All That" cast member to die this year. As TMZ previously reported ... Kianna Underwood was killed in a hit-and-run collision in Brooklyn in January.

Christy was 46.