In contrast, people with severe ME/CFS had higher proportions of activated circulating lymphocytes, determined by CD69+ and CD38+ expression, and expressed more pro-inflammatory cytokines, including interferon-γ, tumour necrosis factor and interleukin-17, following stimulation in vitro, indicative of prolonged non-specific inflammation. Here, CD69 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.