Atreca has not released the MOA of it's drug ATRC-101. What we do know is that it's a immunoglobulin G, subclass 1 (IgG1) antibody. Nothing new, there's 19 drugs of that class already approved by the FDA. Plenty of side effects and can develop resistance.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5649311/