Which enzyme is too low in severe combined immunodeficiency?

Not always an enzyme: Severe combined immunodeficiency (scid) is diagnosed when t cells (a type of white blood cell) are absent or not working. There are at least 15 causes of scid. The very first one identified, though currently not the most common form is caused by deficiency of the enzyme Adenosine deaminase, which remove toxic products that kill t cells.

Depends: There are different types of severe combined immunodeficiency (scid). The one which most commonly has a "low enzyme" is Adenosine deaminase deficiency (ada). Other forms of scid effect other genes, which have other important effects but are not "enzymes" as such.