• Every organism has cells with?unique molecules?on the cell surface membrane which act as?markers?to?identify?it
• These unique markers are?macromolecules?and they allow?cell-to-cell recognition
• The?immune system?has the ability to distinguish between 'self' and 'non-self' based on these molecules
  • Microorganisms?(both pathogenic and non-pathogenic), such as?bacteria?and?viruses,?trigger an?immune response?as the immune system recognises their markers as being?non-self?
  • Molecules that trigger an immune response in this way are named?antigens
  • Antigens?are found on cell surface membranes of cancer cells, bacterial cell walls, the envelopes of viruses and even pollen grains
  • Some?glycolipids?and?glycoproteins?on the outer surface of cell surface membranes act as antigens
• Allergies?are the result of an immune response triggered by antigens on the surface of an?allergen, such as pollen

Antigens on red blood cells

• Red blood cells have?specific markers?on their surface known as?antigens?which?determine the blood group?of an individual
• If a?transfusion?is given to an individual with mismatched blood group, the antigens on the red blood cell surface will trigger an immune response
• There are two?antigen markers?that must be considered:
  • The?ABO marker?- this determines whether the individual is?blood group A, B, AB or O
  • The?Rhesus (Rh) marker?- this determines whether the individual is rhesus?positive?or rhesus?negative

Determining ABO blood types

• Blood type A?has a?type A antigen?consisting of an initial 'H' marker which is modified with another molecule called N-acetylgalactosamine
• Blood type B?has a?type B antigen?consisting of an initial 'H' marker which is modified with another molecule called galactose
• Blood type AB?has?type A and B antigens?consisting of two 'H' markers one of which is modified with N-acetylgalactosamine and the other with galactose'
• In?blood type O,?the 'H' marker is not modified and so there are no A or B antigens

Blood type is determined by the presence or absence of specific antigen markers on the surface of the red blood cells

• If a transfusion is given to someone of an?incompatible blood type,?an immune response will occur due to the presence of antibodies in the recipient's blood that bind to blood cells with non-self antigens
• An immune response may result in?agglutination?of the blood in the blood vessels and?could be fatal
  • Agglutination is when red blood cells clump together due to the binding of antigens and antibodies
• Compatible?blood types means?not?using blood that has a?different type of antigen?to the patients blood

Blood type must be compatible when carrying out a transfusion to prevent coagulation of blood in blood vessels