THY1 and infection: However, when comparing the impact of secondary infection to late observations made on day 28 after the initial infection, other differences were observed: an increase in the area positive for endothelial cells (CD31+) and activated antigen-presenting cells (MHC class II+) and a decrease in the area covered by stem cells in addition to immature B cells and thymocytes (CD90+) (Supplementary Figure S11).