The antibody fits the spike protein like a lock and key. They key might fit other locks in humans with varying degrees of fit, on other cells in the body. The

The antibody fits the spike protein like a lock and key. They key might fit other locks in humans with varying degrees of fit, on other cells in the body. There are two such lock-and-keys on the antibody, the other key fits the immune system cells, that start a cascade that amplifies the immune response. Even if it is a weak fit for an unintended target, the response can be an issue.