Immune thrombocytopenia (ITP) is a rare autoimmune disease due to an abnormal T cell response, notably supported by splenic T follicular helper cells, that stimulates the proliferation and differentiation of autoreactive B cells. The antiplatelet autoantibodies they produce facilitate platelet phagocytosis by macrophages, essentially in the spleen. Macrophages contribute to the perpetuation of the auto-immune response as the main antigen-presenting cell during ITP. CD8+ T cells also participate to thrombocytopenia by increasing platelet apoptosis. Besides this peripheral platelet destruction, inappropriate bone marrow production also exacerbates thrombocytopenia, due to an immune response against megakaryocytes. Moreover, the level of circulating thrombopoietin, the main growth factor of megakaryocytes, is low during ITP. In this review, the major mechanisms leading to thrombocytopenia, the role of the different immune cells and the different targets of treatments are described.