ARPC1B and immunodeficiency disease: ARPC1B is a key factor in the assembly and maintenance of the ARP2/3 complex, which is involved in actin branching from an existing filament.[18] Stefano et al[19] analyzed 14 patients with biallelic ARPC1B mutations, and reported combined immunodeficiency, allergy, and auto-inflammation caused by the mutations in these patients.