Many animals, including all vertebrates, have essentially the same blood cells as humans. Other types of circulatory systems, such as those in arthropods and mollusks, are not properly blood, but do share some similarities. Humans and other vertebrates have three types of blood cells: red blood cells or erythrocytes, white blood cells or leukocytes, and platelets or thrombocytes.
Animals with an open circulatory system have hemolymph, a fluid combining blood and interstitial fluid, rather than blood, and it contains only one type of cell, hemocytes. Hemocytes, like the leukocytes of humans and other animals, are part of the immune system. They are phagocytic cells that ingest foreign particles and pathogens and serve in signaling within the body. Animals with hemolymph use hemocyanin instead of hemoglobin to transport oxygen.
Human blood cells are very similar to those of other animals, though there are a few interesting differences. Both human and animal blood cells can have A or B antigens, or both or neither, resulting in blood typing of A, B, O, or AB. They can also have Rhesus (Rh) antigens. All of these antigens are attached to the surface of the red blood cells.
Red blood cells in humans differ from those of many animals in that they do not have nuclei. Most animals, besides mammals, have nuclei in their mature red blood cells. Mature mammalian red blood cells lose their nucleus and organelles in order to carry more hemoglobin, and they do not need to use any oxygen themselves. Erythrocytes are round in all mammals except camels, who have oval erythrocytes.
Crocodile icefishes are the only known vertebrate species that do not have red blood cells or hemoglobin. They live in very oxygen-rich cold water and oxygen is freely dissolved in their blood rather than attached to hemoglobin in red blood cells. Among other animals, the size of erythrocytes can vary widely.
Platelets and leukocytes do not differ significantly between humans and other vertebrates. There are five types of leukocytes: neutrophils, eosinophils, basophil, lymphocytes, and monocytes. Humans have all five in their blood, but some animals, such as fish, have less. Leukocytes are an important part of the immune system and help to fight off disease. Platelets, which also have no nuclei, help the blood clot to prevent excessive bleeding. The platelets of different animals can be more or less adhesive; those of horses are among the most adhesive.