APC a APC can express MHC- and co-stimulatory molecules and present exogenous antigens to CD4+ T cells, besides presenting endogenous antigens to CD8+ T cells. Three cell types are classified as professional antigen-presenting cells; dendritic cells, macrophages, and B lymphocytes 特异性信号和协同刺激信号 APC MHCⅡ类分子 B7 抗原多肽 TCR-CD3复合物 CD28 T细胞 CD4T细胞活化双信号示意图◼ APC ◼ APC can express MHC-II and co-stimulatory molecules and present exogenous antigens to CD4+ T cells, besides presenting endogenous antigens to CD8+ T cells.Three cell types are classified as professional antigen-presenting cells: dendritic cells, macrophages, and B lymphocytes