Blood contains three main types of cells, each with a distinct role. Red blood cells (erythrocytes) are responsible for carrying oxygen from the lungs to the rest of the body and returning carbon dioxide back to the lungs for exhalation. They contain hemoglobin, a protein that binds oxygen. White blood cells (leukocytes) are part of the immune system and are crucial for fighting infections and defending the body against foreign invaders. There are several types of white blood cells, including neutrophils, lymphocytes, and monocytes, each with specific functions in immune defense. Lastly, platelets (thrombocytes) are involved in blood clotting; they gather at the site of a wound to form clots and prevent excessive bleeding. Together, these cells work to maintain health by transporting essential substances, protecting against disease, and repairing tissues.