Hello ,

Serum is blood without Corpuscles and Fibrinogen.

Serum is the clear liquid that can be separated from clotted blood. Serum differs from plasma, the liquid portion of normal unclotted blood containing the red and white cells and platelets. It is the clot that makes the difference between serum and plasma.  Serum = Blood - Corpuscles + Fibrinogen (Plasma protein) or Blood plasma- Fibrinogen

Hope this was helpful.