Besides, another study also explored the immune reactions in Chlamydia psittaci pneumonia and they found psittacosis cases with pneumonia had higher levels of serum cytokines (G-CSF, IL-2, IL-6, IL-10, IL-18, IP-10, MCP-3, and TNF-α) than bronchitis cases (17). Here, TNF is linked to susceptibility to pneumonia measurement.