T follicular helper cells (Tfh) are a specialized subset of CD4+ T cells that were first identified in the human tonsil. They play a critical role in protective immunity helping B cells produce antibody against foreign pathogens.
Circulating Tfh numbers have been shown to increase in number in the blood of patients with autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis. Patients with common variable immunodeficiency caused by ICOS deficiency have been shown to have severely reduced circulating Tfh and major defects in antibody generation.
(Source: British Society for Immunology)