Which protein is primarily responsible for transporting iron in the blood?

Transferrin is the primary protein responsible for transporting iron in the blood. It is a glycoprotein produced mainly in the liver, and its main function is to bind to free iron ions and facilitate their transport throughout the body to various tissues where iron is needed, such as the bone marrow for hemoglobin synthesis and the liver for storage as ferritin.

Transferrin plays a critical role in iron metabolism and helps regulate the levels of iron in the bloodstream, preventing toxicity due to excess free iron. The binding of iron to transferrin protects the body from oxidative damage, as free iron can participate in harmful reactions.

The other proteins listed do not serve this specific function. Albumin, while it is the most abundant protein in human plasma and transports a variety of substances, including some metals, is not the primary carrier for iron. Immunoglobulin is involved in the immune response and does not play a role in iron transport, while fibrinogen is crucial for blood clotting and has no relevance to the transportation of iron.