ARPC1B (Actin Related Protein 2/3 Complex Subunit 1B) is a gene that encodes a part of the ARP2/3 complex, a crucial regulator of actin cytoskeleton organization and cell movement. It is vital for proper immune cell function and the body's defense against infections. Mutations in ARPC1B can lead to a rare immunodeficiency disorder called ARPC1B deficiency, which compromises the immune response.