 When an APC, such as a dendritic cell, binds a foreign antigen it becomes activated and moves into the lymph nodes (sites for antigen sampling by T cells) via afferent lymphatic vessels.Naïve T cells in the circulation regularly move through the lymph nodes via HEV in order to scan the APC for foreign antigens.When they encounter such an antigen, the cell becomes activated, resulting in the immune system mounting a response against the causative agent of the infection.