Which feature distinguishes white blood cells from red blood cells?

White blood cells, or leukocytes, are distinguished from red blood cells, or erythrocytes, primarily by the presence of a nucleus. Unlike red blood cells, which lose their nucleus during maturation and are primarily responsible for transporting oxygen, white blood cells retain their nuclei throughout their life cycle. This nucleus is crucial as it contains the cell’s genetic material and is involved in the regulation of the cell's functions, including immune response and the production of antibodies.

This characteristic of having a nucleus allows white blood cells to perform a variety of complex functions necessary for the immune system, such as identifying and neutralizing pathogens. The other features mentioned pertain specifically to red blood cells or are unrelated to the distinguishing features of blood cells. For instance, hemoglobin is a protein found in red blood cells that carries oxygen, while nutrient transport is a function associated with plasma and not a defining feature of blood cell types. Additionally, blood cells are produced in the bone marrow, not the lungs. Thus, the presence of a nucleus is a definitive characteristic that sets white blood cells apart from red blood cells.