Oncoproteins & Receptor Signaling
Ligand
The binding partner–often carried by a different cell, which is why proximity is the measurement.
Definition
A ligand is the molecule a receptor binds. Affinity sets how readily the pair associates, and competing ligands for one receptor is a control mechanism in its own right–CTLA-4 outcompeting CD28 for CD80 is the canonical example. In checkpointLoading... biology the ligand is typically presented by a different cell than the receptor, so engagement happens across the immune synapseLoading... and can only be confirmed by measuring proximity between two cells.