Immunopathogenesis of psoriatic arthritis. A proposed model for psoriatic arthritis immunopathogenesis is illustrated in which a genetically primed individual is exposed to a bacterial, stress, or entheseal-related peptide. This in turn activates the innate immune response, resulting in CD8 infiltration and chemokine/cytokine release. The process is amplified with angiogenesis and cellular infiltration of involved tissues. HLA and other genes expressed may determine the exact pattern of tissue involvement. HLA, human leukocyte antigen; MHC, major histocompatibility complex; NKR, natural killer cell receptor; TCR, T-cell receptor; TLR, Toll-like receptor.