Pull-down assay is a technique used to study protein interactions. By binding a target protein or protein domain to an immobilized affinity column (such as agarose beads or affinity resin), and then adding the test sample to the mixture, the proteins that interact with the target protein are "pulled down" from it. By subsequently analyzing the pulled down proteins, the interacting partners of the target protein can be identified and studied. This technology helps reveal direct or indirect interactions between proteins and provides important information for understanding cell signaling, protein complexes and biological processes.