1a and 1b with antigens
2a and 2b on surface.
Serotype or serovar refers to distinct variations within a species of bacteria or viruses or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their cell surface antigens, allowing the epidemiologic classification of organisms to the sub-species level. A group of serovars with common antigens is called a serogroup.
Serotyping often plays an essential role in determining species and subspecies. The Salmonella genus of bacteria, for example, has been determined to have over 4400 serotypes, including Salmonella enterica serovar Typhimurium, S. enterica serovar Typhi, and S. enterica serovar Dublin. Vibrio cholerae, the species of bacteria that causes cholera, has over 200 serotypes, based on cell antigens. Only two of them have been observed to produce the potent enterotoxin that results in cholera: 0:1 and 0:139.
Serotypes were discovered by the American microbiologist Rebecca Lancefield in 1933.
Role in organ transplantation 
The immune system is capable of discerning a cell as being 'self' or 'non-self' according to that cell's serotype. In humans, that serotype is largely determined by human leukocyte antigen (HLA), the human version of the major histocompatibility complex. Cells determined to be non-self are usually recognized by the immune system as foreign, causing an immune response, such as hemagglutination. Serotypes differ widely between individuals; therefore, if cells from one human (or animal) are introduced into another random human, those cells are oftentimes determined to be non-self because they do not match the self-serotype. For this reason, transplants between genetically non-identical humans often induce a problematic immune response in the recipient, leading to transplant rejection. In some situations this effect can be reduced by serotyping both recipient and potential donors to determine the closest HLA match.
Human leukocyte antigens 
Serotypes according the HLA (MHC) locus
||# of Serotypes
|*DP and many Cw require SSP-PCR for typing.
See also 
External links