repertoire = spectrum = scope?

Repertoire is a word from French, which we acquired rather recently (and it was a neologism in French rather recently)--by which i mean the last couple of hundred years. The French constructed it from monk's Latin, in which it was used to mean list. It was originally applied to theater or opera companies and meant the list of plays or operas they were able to reliably perform. It has since come to mean, by extension, a list of skills or capacities of anything.

So, in this context:

In general, deficiencies and gene variants involving T cells are better understood than B cell deficiencies in terms of defined associations with autoimmune disease. A deficient immunologic repertoire may result in diminished immunocompetence.

. . . repertoire means the various things which can the cell can be expected to accomplish immunologically. Specifically, it means that if the immunological repertoire is deficient, i.e. lacking in things which can be accomplished, the cell will not be as competent immunologically. Essentially, the sentence is a tautology.

Yes it is, and it's horrible jargon, unnecessarily verbose and unhelpful.
A sentence using "performance" or "effects" or "capability" would be simpler.