Editing Blood type (section)

===ABO blood group system===
[[Image:ABO blood group diagram.svg|right|thumb|350px|''ABO blood group system'': diagram showing the carbohydrate chains that determine the ABO blood group]]
[[Image:Punnett_square_blood_types.svg|right|thumb|350px|Simplified [[Punnett square]] of the possible genotypes and phenotypes of children given genotypes and phenotypes of their mother (rows) and father (columns) shaded by phenotype]]
{{Main|ABO blood group system}}

The ABO blood group system involves two antigens and two antibodies found in human blood. The two antigens are antigen A and B. The two antibodies are A and B. The antigens are present on the red blood cells and the antibodies in the [[serum (blood)|serum]]. Regarding the antigen property of the blood all human beings can be classified into four groups: those with antigen A (group A), those with antigen B (group B), those with both antigen A and B (group AB) and those with neither antigen (group O). The antibodies present together with the antigens are found as follows:{{citation needed|date=November 2021}}
# Antigen A with antibody B
# Antigen B with antibody A
# Antigen AB with neither antibody A nor B
# Antigen null (group O) with both antibodies A and B

There is an [[Agglutination (biology)|agglutination]] reaction between similar antigen and antibody (for example, antigen A agglutinates the antibody A and antigen B agglutinates the antibody B). Thus, transfusion can be considered safe as long as the serum of the recipient does not contain antibodies for the blood cell antigens of the donor.{{citation needed|date=November 2021}}

The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A and anti-B antibodies are usually [[immunoglobulin M]], abbreviated [[IgM]], antibodies. It has been hypothesized that ABO IgM [[antibodies]] are produced in the first years of life by sensitization to environmental substances such as food, [[bacteria]], and [[virus]]es.<ref>{{cite web|title=Position statement: Red blood cell transfusion in newborn infants |url=https://www.cps.ca/en/documents/position/red-blood-cell-transfusion-newborn-infants |publisher=Canadian Pediatric Society |archive-url=https://web.archive.org/web/20180519120449/https://www.cps.ca/en/documents/position/red-blood-cell-transfusion-newborn-infants |archive-date=19 May 2018 |date=April 14, 2014}}</ref> The original terminology used by [[Karl Landsteiner]] in 1901 for the classification was A/B/C; in later publications "C" became "O".<ref name=Oor0>{{cite journal |last1=Schmidt |first1=P. |last2=Okroi |first2=M. |title=Also sprach Landsteiner – Blood Group 'O' or Blood Group 'NULL' |journal=Transfusion Medicine and Hemotherapy |date=2001 |volume=28 |issue=4 |pages=206–208 |doi=10.1159/000050239 }}</ref> Type O is often called ''0'' (''zero'', or ''null'') in other languages.<ref name="Oor0"/><ref>{{cite web |url=https://www.bloddonor.dk/fileadmin/Fil_Arkiv/PDF_filer/Andre/Your_Blood__June_2006.pdf |title=Your blood&nbsp;– a textbook about blood and blood donation |access-date=2008-07-15 |page=63 |archive-url = https://web.archive.org/web/20080626184746/https://www.bloddonor.dk/fileadmin/Fil_Arkiv/PDF_filer/Andre/Your_Blood__June_2006.pdf |archive-date = June 26, 2008}}</ref>
{| class="wikitable plainrowheaders" style="text-align:center;"
|+Phenotype and genotype of blood types
|-
! scope="col"|[[Phenotype]]
! scope="col"|[[Allele|Alleles]]
|-
!scope="row"| A
| ABO*A1.01
|-
!scope="row"| B
| ABO*B1.01
|-
!scope="row"| AB
| ABO*A1.01, ABO*B1.01
|-
!scope="row"| O
| Two nonfunctional ABO genes
|}