Antibodies, are just the B cell receptor in a secreted form, so they can circulate in serum, which is the non-cellular part of blood - attaching to pathogens and tagging them for destruction.

抗体，就是分泌型的B细胞受体，所以它们可以在血清（血液除去细胞的部分）中循环，附着在病原体上并标记以进行破坏。