# Serotype

A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their shared reactivity between their surface antigens and a particular antiserum, allowing the classification of organisms to a level below the species. A group of serovars with common antigens is called a serogroup or sometimes serocomplex.
Serotyping often plays an essential role in determining species and subspecies. The Salmonella genus of bacteria, for example, has been determined to have over 2600 serotypes. 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: O1 and O139.
Serotypes were discovered in hemolytic streptococci by the American microbiologist Rebecca Lancefield in 1933.

## Procedure

Serotyping is the process of determining the serotype of an organism, using prepared antisera that bind to a set of known antigens. Some antisera detect multiple known antigens and are known as polyvalent or broad; others are monovalent. For example, what was once described as HLA-A9 is now subdivided into two more specific serotypes ("split antigens"), HLA-A23 and HLA-A24. As a result, A9 is now known as a "broad" serotype. For organisms with many possible serotypes, first obtaining a polyvalent match can reduce the number of tests required.
The binding between a surface antigen and the antiserum can be experimentally observed in many forms. A number of bacteria species, including Streptococcus pneumoniae, display the Quellung reaction visible under a microscope. Others such as Shigella (and E. coli) and Salmonella are traditionally detected using a slide agglutination test. HLA types are originally determined with the complement fixation test. Newer procedures include the latex fixation test and various other immunoassays.
"Molecular serotyping" refers to methods that replace the antibody-based test with a test based on the nucleic acid sequence – therefore actually a kind of genotyping. By analyzing which surface antigen-defining allele(s) are present, these methods can produce faster results. However, their results may not always agree with traditional serotyping, as they can fail to account for factors that affect the expression of antigen-determining genes.

## 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 often 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 to HLA (MHC) locus :
| HLA Locus | # of Serotypes | Broad Antigens | Split Antigens |
| A | 25 | 4 | 15 |
| B | 50 | 9 | |
| C* | 12 | 1 | |
| DR | 21 | 4 | |
| DQ | 8 | 2 | |
| DP* | | | |
| *DP and many Cw require SSP-PCR for typing. ||||

## Bacteria

Most bacteria produce antigenic substances on the outer surface that can be distinguished by serotyping.
* Almost all species of Gram-negative bacteria produce a layer of lipopolysaccharide on the outer membrane. The outermost portion of the LPS accessible to antibodies is the O antigen. Variation in the O antigen can be caused by genetic differences in the biosynthetic pathway or the tranporter used to move the building-blocks to the outside of the cell.
* The flagella on motile bacteria is called the H antigen in serotyping. Minute genetic differences in the components of the flagella lead to variations detectable by antibodies.
* Some bacteria produce a polysaccharide capsule, called the K antigen in serotyping.
The LPS (O) and capsule (K) antigens are themselves important pathogenicity factors.
Some antigens are invariant among a taxonomic group. Presence of these antigens would not be useful for classification lower than the species level, but may inform identification. One example is the enterobacterial common antigen (ECA), universal to all Enterobacterales.

### E. coli

E. coli have 187 possible O antigens (6 later removed from list, 3 actually producing no LPS), 53 H antigens, and at least 72 K antigens. Among these three, the O antigen has the best correlation with lineages; as a result, the O antigen is used to define the "serogroup" and is also used to define strains in taxonomy and epidemiology.

### Shigella

Shigella are only classified by their O antigen, as they are non-motile and produce no flagella. Across the four "species", there are 15 + 11 + 20 + 2 = 48 serotypes. Some of these O antigens have equivalents in E. coli, which also cladistically include Shigella.

### Salmonella

The Kauffman–White classification scheme is the basis for naming the manifold serovars of Salmonella. To date, more than 2600 different serotypes have been identified. A Salmonella serotype is determined by the unique combination of reactions of cell surface antigens. For Salmonella, the O and H antigens are used. There are two species of Salmonella: Salmonella bongori and Salmonella enterica. Salmonella enterica can be subdivided into six subspecies. The process to identify the serovar of the bacterium consists of finding the formula of surface antigens which represent the variations of the bacteria. The traditional method for determining the antigen formula is agglutination reactions on slides. The agglutination between the antigen and the antibody is made with a specific antisera, which reacts with the antigen to produce a mass. The antigen O is tested with a bacterial suspension from an agar plate, whereas the antigen H is tested with a bacterial suspension from a broth culture. The scheme classifies the serovar depending on its antigen formula obtained via the agglutination reactions. Additional serotyping methods and alternative subtyping methodologies have been reviewed by Wattiau et al.

### Streptococcus

Streptococcus pneumoniae has 93 capsular serotypes. 91 of these serotypes use the Wzy enzyme pathway. The Wzy pathway is used by almost all gram-positive bacteria, by lactococci and streptococci (exopolysacchide), and is also responsible for group 1 and 4 Gram-negative capsules.

## Other organisms

Many other organisms can be classified using recognition by antibodies.
* The malaria pathogen Plasmodium falciparum is notorious for its many surface antigen variants. A certain vaccine candidate is designed to cover all of these serotypes.
* Toxoplasma gondii can be classified into serotypes.
* Trypanosoma cruzi, which causes Chagas disease, can be serotyped using whole parasites.