Brent Cornell

    
The immune system has the capacity to distinguish between body cells (‘self’) and foreign materials (‘non-self’)

• It will react to the presence of foreign materials with an immune response that eliminates the intruding material from the body

All nucleated cells of the body possess unique and distinctive surface molecules that identify it as self

• These self markers are called major histocompatibility complex molecules (MHC class I) and function as identification tags
• The immune system will not normally react to cells bearing these genetically determined markers (self-tolerance)
  
  

Any substance that is recognised as foreign and is capable of triggering an immune response is called an antigen (non self)

• Antigens are recognised by lymphocytes which bind to and detect the characteristic shape of an exposed portion (epitope)
• Lymphocytes trigger antibody production (adaptive immunity) which specifically bind to epitopes via complementary paratopes

Antigenic determinants include: 

• Surface markers present on foreign bodies in the blood and tissue – inluding bacterial, fungal, viral and parasitic markers
• The self markers of cells from a different organism (this is why transplantation often results in graft rejection)
• Even proteins from food may be rejected unless they are first broken down into component parts by the digestive system

Self vs Non-Self

    
Self markers (MHC class I) are present on the surface of all nucleated body cells and identify the cell as part of the organism

• Different organisms have distinct self markers which prevent transplantation of tissues (unless a very close genetic match)

Red blood cells are not nucleated and hence do not possess the same distinctive and unique self markers as all other body cells

• This means that red blood cells can be transferred between individuals without automatically causing immune rejection

However, red blood cells do possess basic antigenic markers which limit the capacity for transfusion (the ABO blood system)

• Red blood cells may possess surface glycoproteins (A and B antigens) either independently (A or B) or in combination (AB)
• Alternatively, red blood cells may possess neither surface glycoprotein (denoted as O)

As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups

• AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
• A blood groups cannot receive B blood or AB blood (as the B isoantigen is foreign and will stimulate antibody production)
• B blood groups cannot receive A blood or AB blood (as the A isoantigen is foreign and will stimulate antibody production)
• O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)

An additional glycoprotein (Rhesus factor) is either present or absent, resulting in positive and negative blood groups

Summary of the ABO Blood Groups

The Consequence of an Incompatible Blood Transfusion