The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes. For human blood transfusions, it is the most important of the 44 different blood type classification systems currently recognized by the International Society of Blood Transfusions (ISBT) as of
December 2022. A mismatch in this, or any other serotype, can cause a potentially fatal adverse reaction after a transfusion, or an unwanted immune response to an organ transplant. The associated anti-A and anti-B antibodies are usually IgM antibodies, produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses.
Jan Janský, who invented type I, II, III, IV system
Red blood cells (RBCs), scientific name erythrocytes (from ancient Greek erythros 'red' and kytos 'hollow vessel', with -cyte translated as 'cell' in modern usage), also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen (O2) to the body tissues—via blood flow through the circulatory system. Erythrocytes take up oxygen in the lungs, or in fish the gills, and release it into tissues while squeezing through the body's capillaries.
There is an immense size variation in vertebrate red blood cells, as well as a correlation between cell and nucleus size. Mammalian red blood cells, which do not contain nuclei, are considerably smaller than those of most other vertebrates.
Mature red blood cells of birds have a nucleus, however in the blood of adult females of penguin Pygoscelis papua enucleated red blood cells (B) have been observed, but with very low frequency.
Scanning electron micrograph of blood cells. From left to right: human red blood cell, thrombocyte (platelet), leukocyte.
Two drops of blood are shown with a bright red oxygenated drop on the left and a darker red deoxygenated drop on the right.
