A B-cell receptor (BCR) is a protein complex found on the surface of B cells, a type of white blood cell. It plays a crucial role in the immune system by recognizing specific antigens, which are foreign substances like bacteria or viruses. When a BCR binds to its corresponding antigen, it triggers the B cell to activate and proliferate, leading to the production of antibodies. Antibodies are proteins that help neutralize or eliminate pathogens. Each BCR is unique, allowing the immune system to target a wide variety of antigens. This diversity is essential for effective immune responses and the body's ability to remember past infections, providing long-term immunity.