Serum differs from blood plasma in the following ways:

• How it's obtained

  Serum is obtained from blood that has clotted, while plasma is obtained from blood that has been prevented from clotting with an anticoagulant. 

• What it contains

  Serum lacks clotting factors, such as fibrinogen, that are present in plasma. Both serum and plasma contain similar components like antibodies, hormones, and electrolytes