Guselkumab is a fully human monoclonal IgG1λ antibody approved for the treatment of moderate-to-severe plaque psoriasis, psoriatic arthritis (PsA), and moderate-to-severe hidradenitis suppurativa (HS). It functions as a selective interleukin-23 (IL-23) inhibitor. By binding with high affinity to the p19 subunit of IL-23, guselkumab prevents the cytokine from interacting with its receptor complex on the surface of immune cells. This blockade inhibits the IL-23 signaling pathway, which is central to the differentiation, expansion, and survival of pathogenic T helper 17 (Th17) cells. By targeting this upstream cytokine, guselkumab reduces the downstream production of pro-inflammatory mediators (like IL-17, IL-22) that drive inflammation, keratinocyte hyperproliferation, and tissue damage in these conditions.