Which of the following is NOT a component of human blood?

Fibroblasts are not a component of human blood, making this the correct choice. Erythrocytes, commonly known as red blood cells, play a crucial role in transporting oxygen throughout the body. Leukocytes, or white blood cells, are essential for the immune response, identifying and combating pathogens. Plasma is the liquid portion of blood that carries cells, nutrients, hormones, and waste products. Fibroblasts, on the other hand, are specialized cells found primarily in connective tissues and are responsible for the production of collagen and other fibers. They are not part of the blood composition, which consists specifically of erythrocytes, leukocytes, and plasma.