HHLA2 and cancer: The HHLA2 protein is expressed on antigen-presenting cells, and elevated HHLA2 is thought to be associated with a more severe pathology and poor prognosis in cancer patients [54–57], whereas several studies showed that patients with higher HHLA2 expression had better survival rate [58, 59]; this paradox can be explained, in part, by the dual role of HHLA2 in immunity and by the fact that the HHLA2 acts as a protective factor in this article.