Which of the following is NOT a component of blood?

Fibrin is indeed not considered a component of blood in the same way as red blood cells, white blood cells, or platelets are. Red blood cells, or erythrocytes, are responsible for transporting oxygen throughout the body. White blood cells, or leukocytes, play a crucial role in the immune response, helping to protect the body against infection. Platelets, or thrombocytes, are involved in blood clotting, assisting in the prevention of excessive bleeding.

Fibrin, on the other hand, is a protein that plays a critical role in blood coagulation but is not a cellular component of blood. Instead, fibrin is produced from fibrinogen during the clotting process when there is an injury to blood vessels. Therefore, while fibrin is important for the mechanism of healing and prevents blood loss, it does not constitute a component of blood itself like the other options.