ARPC1B deficiency syndrome is a rare condition characterized by combined immune deficiency, recurrent infections, allergies, asthma, and autoinflammation. It is also known as platelet abnormalities with eosinophilia and immune-mediated inflammatory disease (PTLEID) or immunodeficiency-71 with inflammatory disease and congenital thrombocytopenia (IMD71). ARPC1B deficiency syndrome is attributed to mutations in the gene encoding actin-related protein 2/3 complex subunit 1B (ARPC1B), a member of the Arp2/3 complex and essential for driving the dynamics of the actin cytoskeleton. These mutations disrupt protein structure and cause impaired T-cell and neutrophil migration and proliferation, altered immunologic synapses formation, defective regulatory T cell (Treg) function, increased levels of immunoglobulin E (IgE) and immunoglobulin A (IgA), and thrombocytopenia. ARPC1B is the only ARPC1 isoform found in hematopoietic cells, whereas other tissue types express both ARPC1B and the other isoform ARPC1A, which may explain the predominant immunologic and hematologic manifestations. The inheritance pattern is autosomal homozygous recessive with variable penetrance.

Only a few individuals have been diagnosed with the ARPC1B deficiency syndrome worldwide, and the exact mechanisms and full spectrum of disease severity remains unclear. Clinical onset of disease is very early with a mean of 2 months of age. Both sexes are affected equally.