The two main types of cells in human blood are red blood cells and white blood cells. Red blood cells, or erythrocytes, are responsible for transporting oxygen from the lungs to the body’s tissues and returning carbon dioxide to the lungs for exhalation. They contain hemoglobin, a protein that binds oxygen. White blood cells, or leukocytes, play a vital role in the immune system, defending the body against infections and foreign substances. There are several types of white blood cells, each with specific functions in immune response. Together, these cells are crucial for maintaining health and supporting bodily functions.