CD4 and rheumatoid arthritis: In the most frequently used model of RA–collagen-induced arthritis, the presence of collagen type II-specific CD4+ T cells leads to the activation of collagen-specific B lymphocytes together with the secretion of various pro-inflammatory cytokines (TNF-α and IL-6), although the anti-inflammatory IL-10 and IL-1 receptor antagonist can also be detected in the affected tissues [197,200].