FCER2 encodes CD23, a low-affinity IgE receptor with critical roles in adaptive immunity and allergic disease. On B cells, FCER2/CD23 initiates IgE-dependent antigen uptake and presentation to T cells 1, and serves as a marker of type 2-polarized memory B cells that function as precursors of pathogenic IgE-producing plasma cells 23. CD23+ IgG1+ memory B cells expressing IL4R and germline IGHE transcript are enriched in atopic individuals and correlate with circulating IgE levels 23. On macrophages, FCER2 signaling activates the L-arginine-nitric oxide pathway for intracellular parasite killing 4. Beyond allergic responses, FCER2 participates in spermatogonial stem cell proliferation via the NOTCH2/HES1 pathway, with mutations associated with azoospermia 5. Functionally, FCER2 variants modulate inhaled corticosteroid responsiveness in asthma and chr19 rhinosinusitis; the rs2228137 R62W polymorphism increases IgE binding affinity and Egr-1 expression in B cells 6789. Proteomic studies confirm FCER2 downregulation by corticosteroids in asthma 10. These findings establish FCER2 as a multifunctional immune regulator controlling IgE responses and a pharmacogenetic determinant of corticosteroid sensitivity.