BAFF plays a central role in acquired immunity. The disclosure identifies BAFF-responsive genes that are substantially upregulated by administration of BAFF and substantially downregulated by treatment with a BAFF antagonist. Specific genes are: NF-κB2, CD23, H2-Mβ (the beta chain of H2-DM), Fig-1, and OBF-1. The disclosure provides methods and compositions for: monitoring the activity of a BAFF antagonist in a mammal; monitoring BAFF activity in a mammal; identifying a mammal to be treated with a BAFF antagonist; and related uses. Such methods include detecting one or more molecules selected from the group consisting of Fig-1 molecule, OBF-1 molecule, and H2-Mβ molecule in a biological sample of the mammal, and optionally further detecting NF-κB2 molecule and/or CD23 molecule in the biological sample.