Interleukin 16 is a cytokine released by a variety of cells including lymphocytes and epithelial cells expressing the cell surface molecule CD4. It recruits and activates other cells expressing CD4, including lymphocytes, monocytes, and eosinophils.
Landi et al (2015) found it to be significantly downregulated in long term CFS patients. It is a pro-inflammatory cytokine and the reduction shown may indicate immunodeficiency. In mice studies low IL-16 is correlated with impaired development of B cells. Most studies into human disease seem to show increases in IL-16 not a decrease (asthma, rheumatoid arthritis, Lupus, MS). The other condition which shows a decrease is HIV and IL-16 may repress HIV-1 replication (Indinavir used in HIV patients increases circulating IL-16). IL-16 has a developmental role in the human immune system. It is also related to neuronal development and may account for brainfog symptoms.