Blood is a mixture of various cells suspended in a fluid called plasma. Although fundamentally different in both appearance and function, all blood cells originate from a single type of stem cell in the bone marrow.
Red blood cells, or erythrocytes, are made up of little sacs of haemoglobin which are designed to carry oxygen. White blood cells, or leukocytes, are part of our immune system and move in and out of blood vessels searching for any signs of infection or cellular abnormalities. Platelets, or thrombocytes, help repair the blood vessel in the event of an injury by helping the blood to clot.
Source: Al Jazeera