Review The roles of cells and cytokines in the pathogenesis of psoriasis Susana Coimbra 1,2,3 , PhD, Ame ´rico Figueiredo 4 , PhD, Elisabeth Castro 1,2 , PhD, Petronila Rocha-Pereira 2,5 , PhD, and Alice Santos-Silva 1,2 , PhD 1 Biochemistry Laboratory, Department of Biological Science, Faculty of Pharmacy and 2 Institute of Molecular and Cellular Biology (Instituto de Biologia Molecular e Celular [IBMC]), University of Porto (Universidade do Porto), Porto, Portugal, 3 Centre for Investigation into Health Technologies (Centro de Investigac ¸a ˜o das Tecnologias da Sau ´de [CITS]), Northern Polytechnic Health Institute (Instituto Polite ´cnico da Sau ´de Norte), Cooperative Higher Education, Polytechnic and University (Cooperativa de Ensino Superior, Polite ´cnico e Universita ´rio [CESPU]), Gandra- Paredes, Portugal, 4 Dermatology Service, Coimbra University Hospital (Hospitais da Universidade de Coimbra), Coimbra, Portugal, and 5 Centre for Investigation in Health Sciences (Centro de Investigac ¸a ˜o em Cie ˆncias da Sau ´ de [CICS]), University of Beira Interior (Universidade da Beira Interior), Covilha ˜ , Portugal Correspondence Susana Coimbra/Alice Santos Silva Servic ¸o de Bioquı ´mica Faculdade de Farma ´cia da Universidade do Porto Rua Anı ´bal Cunha 164 Porto 4050-047 Portugal E-mails: ssn.coimbra@gmail.com, assilva@ff.up.pt Conflicts of interest: The authors declare no conflict of interest. Abstract Psoriasis is considered an immune chronic disease in which T cells are accepted as important. Nowadays, it is believed that psoriasis is most likely a T helper (Th)1/Th17 induced inflammatory disease. However, some other cells, such as endothelial cells, dendritic cells, monocytic cells, neutrophils, keratinocytes, and several cytokines, appear to have, at different stages of the disease, an important role in its pathogenesis. For instance, the response to psoriasis therapy is dependent not only on the inactivation of Th1 and Th17 immune responses but also on the inactivation of dendritic cell products. Moreover, interleukin (IL)-23 deregulation appears to be an independent factor in the pathogenesis of psoriasis. Indeed, currently, the IL-23/Th17 axis is believed to be crucial in psoriasis patho- genesis, and its inhibition appears to be important for therapeutic achievement. This review presents the roles and interactions of cells and cytokines that are related to psoriasis pathogenesis. Introduction Psoriasis was initially considered to represent a disease of abnormal epidermal keratinocyte proliferation, mani- festing in the characteristic thickening and scaling of erythematous psoriatic plaques caused by epidermal keratinocyte hyperplasia, parakeratosis, leukocyte infil- tration, and neoangiogenesis. The cutaneous inflamma- tory infiltrate was considered as a secondary event. Later, T cells were identified as important players in the initiation and maintenance of psoriasis. 1 The leukocyte infiltrate in psoriasis consists predominantly of T cells, and their infiltration seems to precede epidermal hyper- plasia. 389 ª 2012 The International Society of Dermatology International Journal of Dermatology 2012, 51, 389–398