Red blood cells, or erythrocytes, are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen (O₂) to the body tissues via the blood flow through the circulatory system. They take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries.

These cells' cytoplasm is rich in haemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the blood's red color.

In humans, mature red blood cells are oval and flexible biconcave disks. They lack a cell nucleus and most organelles to accommodate maximum space for haemoglobin. 2.4 million new erythrocytes are produced per second. The cells develop in the bone marrow and circulate for about 100–120 days in the body before their components are recycled by macrophages. Each circulation takes about 20 seconds. Approximately a quarter of the cells in the human body are red blood cells.

Red blood cells are also known as RBCs, red cells,red blood corpuscles (an archaic term), haematids, erythroid cells or erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte translated as "cell" in modern usage).

A blood cell, also called a hematocyte, is a cell produced by haematopoiesis and normally found in blood. In mammals, these fall into three general categories:

Together, these three kinds of blood cells add up to a total 45% of the blood tissue by volume, with the remaining 55% of the volume composed of plasma, the liquid component of blood. This volume percentage (e.g., 45%) of cells to total volume is called hematocrit, determined by centrifuge or flow cytometry. Hemoglobin (the main component of red blood cells) is an iron-containing protein that facilitates transportation of oxygen and other respiratory gases to tissues.

Red blood cells primarily carry oxygen and some carbon dioxide through the use of hemoglobin, and have a lifetime of about 120 days. In the process of being formed they go through being a monopotent stem cell.

White blood cells, or leukocytes (also spelled "leucocytes", leuco- Ancient Greek "white"), are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five[1] different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. They live for about 3 to 4 days in the average human body. Leukocytes are found throughout the body, including the blood and lymphatic system.