In the movie Bubble Boy, what immune deficiency did the main character suffer from?

Bubble boy suffered from a disease known as Severe combined immune deficiency (SCID). This disease can be a combination of various mutations that lead to the poor development of T and B cells.

As a consequence people who suffer from this disease cannot develop sufficient immune responses against common infections. These days, sufferers of SCIDS are treated with a bone marrow transplant to replace their dysfunctional immune cells.