B7 (protein)

The T-cell receptor must first interact with the Major histocompatibility complex (MHC) surface protein.

This is also called "Signal 1" and its main purpose is to guarantee antigen specificity of the T cell activation.

The balance between the opposing signals generated by B7-CD28 and B7-CTLA-4 binding regulates the intensity of the T cell response.

In the TNF family of molecules, the protein 4-1BB (CD137) on the T cell may bind to 4-1BB ligand (4-1BBL) on the APC.

This interaction produces a series of downstream signals which promote the target T cell's survival and activation.

CTLA-4-knockout mice are unable to stop immune responses, and develop a fatal massive lymphocyte proliferation.