White blood cells, or leukocytes, are cells in human blood (and also in the lymphatic system, spleen, and other tissues) that are produced in bone marrow and are critical to the proper function of the immune system. A healthy human has between 7000 and 25,000 white blood cells per drop of blood; a human with leukemia has up to 50,000 white blood cells per drop. 
There are several types of white blood cells, but the most common are neutrophil granulocytes, eosinophil granulocytes, basophil granulocytes, monocytes, macrophages, and granulocytes. All granulocytes engulf invaders of different sorts and digest them. Neutrophils attack bacteria, basophils release histamine (thus being complicit in allergies), and eosinophils attack parasites. Lymphocytes include killer T cells, and are found primarily in the lymphatic system; these and other lymphocytes destroy infected human cells. Monocytes enable T cells to recognize their viral targets; these cells are critical in developing immunities. When monocytes are found in human tissue, they are called macrophages.
A number of diseases can strike white blood cells, the most outstanding of which is leukemia. Leukopenia is the opposite, in which there is a deficit of white blood cells.
The most common type of blood cell is the red blood cell. These cells are used by all vertebrates as the primary method of delivering oxygen from either lungs or gills to the rest of the body. They are commonly called erythrocytes or RBCs to set them apart from other blood cells like leukocytes (white blood cells).
Most of a red blood cell is hemoglobin, a complex iron-based molecule. Heme groups in hemoglobin contain the iron, which links to oxygen to transport it throughout the body; they also carry some of waste carbon dioxide back to the lungs. About 2% of all oxygen and almost all carbon dioxide are dissolved in blood plasma, and myoglobin in muscle cells (related to hemoglobin) stores oxygen there as well. Hemoglobin’s iron is the reason erythrocytes are red. When depleted of oxygen, the hemoglobin takes on a bluish hue.
Red blood cells in mammals do not have a cell nucleus, and therefore do not have DNA; they also do not have other organelles like mitochondria, and produce energy through glycolysis of glucose – a fermentation process – followed by lactic acid production. Their glucose uptake is not regulated by insulin. They are shaped like disks with indentations on top and bottom, optimizing them for exchanging oxygen by maximizing surface area. They are also flexible, allowing them to fit into the tinest capillaries. Adult humans have 2-3 x 1013 red blood cells at any given time, with men and people living in high altitudes having more. Blood types are differentiated by different surface glycoproteins.