The two main types of blood cells are red blood cells (RBcs) and white blood cells (WBCs). Red blood cells, or erythrocytes, are responsible for carrying oxygen from the lungs to the body's tissues and returning carbon dioxide back to the lungs for exhalation. They contain hemoglobin, a protein that binds oxygen. White blood cells, or leukocytes, are essential for the immune system, helping to protect the body against infections and foreign invaders. They come in various types, including lymphocytes, neutrophils, and monocytes, each with specific functions in the immune response.