What is the primary function of red blood cells?

Red blood cells (RBCs), also known as erythrocytes, primarily function to transport oxygen from the lungs to the tissues throughout the body. This is achieved through the hemoglobin molecule contained within the red blood cells, which binds oxygen when the RBCs pass through the pulmonary capillaries in the lungs where oxygen concentration is high. As the RBCs circulate through the body, they release this oxygen to the cells that require it for various metabolic processes.

While red blood cells are involved in other processes, their main role is indeed the transport of oxygen. They also play a secondary role in transporting a small portion of carbon dioxide back to the lungs, but this is primarily accomplished by a different mechanism involving bicarbonate in the blood plasma. The other choices, such as fighting infections and transporting nutrients, are the responsibilities of other types of cells and components in the blood, such as white blood cells and plasma. Thus, the correct response highlights the fundamental and primary role of red blood cells in oxygen transport, making it clear why this is the most appropriate answer.