HTR1A and systemic lupus erythematosus: As for the interaction between 5-HT and immune cells, another study showed that hypomethylation of the 5-HT1A receptor promoter region and high expression of 5-HT1A receptors in the peripheral blood lymphocytes increase the 5-HT level in plasma and lead to the proliferation of T and B cells, which accounts for the process of SLE (21).