A B-cell receptor (BCR) is a protein found on the surface of B-cells, a type of white blood cell that plays a crucial role in the immune system. The BCR is responsible for recognizing specific antigens, which are foreign substances like bacteria or viruses. When a BCR binds to an 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 and can recognize a specific antigen, allowing the immune system to target a wide variety of threats. This specificity is essential for the adaptive immune response, which provides long-lasting protection against infections.