What is the primary function of red blood cells?

Red blood cells, also known as erythrocytes, serve the essential function of transporting oxygen from the lungs to the body's tissues and returning carbon dioxide from the tissues back to the lungs for exhalation. This process is crucial for maintaining cellular respiration and overall metabolic activity in the body's cells.

Hemoglobin, a protein within red blood cells, binds to oxygen in the lungs and releases it in the tissues where it is needed for energy production. The efficiency of this system is a defining characteristic of red blood cells, making them vital to sustaining life.

While the other options describe important roles in the immune response, blood clotting, and pathogen destruction, they do not pertain to the primary function of red blood cells. Therefore, the correct focus on the oxygen transport capability accurately reflects the core purpose of these cells in the circulatory system.