How do T-cells work to recognize invading cells or molecules?

Antigen-presenting cells show digested antigens to T cells through the MHC on their surface, which allows the T cell to “see” the antigen and recognize it as foreign. The connection between the MHC and the receptor on the T cell is the first signal necessary to activate the T cell to respond to a tumor and destroy it.

Explanation:

halogen, any of the six nonmetallic elements that constitute group 17 (group viia) of the periodic table. the halogen elements are fluorine (f), chlorine (cl), bromine (br), iodine (i), astatine (at), and tennessine (ts).