Animation of numerous unique antibody molecules in different colours. Antibodies, or immunoglobulins, are proteins that are shaped like a letter Y. The two arms each have an identical receptor specific for a particular antigen, such as a viral spike protein. This allows each arm to bind to the antigen when it encounters it. There are many different receptors, each with a specific protein target. The leg of the Y is the same for all antibodies. It binds to receptors on the surface of white blood cells when the arms are bound, triggering a response that kills the pathogen.