MAVO Year 4 · Transport and immune defence · Section 13.7

Transplants and blood transfusions

Transplants and rejection

A transplant replaces or supplies tissue or an organ from a donor. The recipient's immune system may recognise donor antigens as foreign and cause rejection, involving immune cells and sometimes antibodies. Matching relevant tissue characteristics reduces risk. Many recipients also need medicines suppressing immune responses.

In autoimmune diseases, immune responses damage the body's own tissues. For example, rheumatoid arthritis involves immune-mediated inflammation in joints. Autoimmunity can involve antibodies, immune cells or both.

Blood groups and antibodies

Blood groups depend on antigens on red blood cells. Two important systems are ABO and RhD. In the usual ABO pattern:

• A: A antigens on red cells; anti-B in plasma.
• B: B antigens on red cells; anti-A in plasma.
• AB: both A and B antigens; neither anti-A nor anti-B normally present.
• O: neither A nor B antigens; both anti-A and anti-B in plasma.

Group O cells still have other surface antigens. Likewise, AB plasma is not free of every type of antibody.

Blood transfusions

Incompatible red cells can react with a recipient's antibodies, causing clumping and destruction of cells. Blood services therefore test compatibility, including ABO, RhD and other relevant factors.

For red-cell transfusions, O-negative is commonly called the universal donor type and AB-positive the universal recipient type in the simplified ABO/RhD model. This is not a rule for every blood component or patient. Plasma compatibility follows different principles, and clinical crossmatching remains essential.

RhD and pregnancy

A person with the RhD antigen is RhD-positive; without it, RhD-negative. If an RhD-negative pregnant person encounters RhD-positive red cells from a foetus, sensitisation may occur and anti-D antibodies can form. In a current or later RhD-positive pregnancy, these antibodies can cross the placenta and damage foetal red cells.

Preventive anti-D immunoglobulin is offered in relevant situations to people not already sensitised. It helps prevent their own lasting anti-D response; it is not a cure for sensitisation that has already occurred.

Blood-group testing

Red cells are mixed with solutions containing anti-A, anti-B or anti-D. Clumping shows that the corresponding antigen is present. Anti-A reacts with A antigen, and anti-B reacts with B antigen.