For over a century blood transfusion has saved millions of lives.
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Humans have evolved an immune system over eons to recognise self from non-self and then deal with threats in two complex but defined pathways.
Firstly, the Innate Immune System dates back hundreds of millions of years and acts as frontline defence against all types of foreign pathogens. Parts of this are shared by early invertebrates.
Secondly, the Adaptive Immune System uses macrophage and lymphocyte cells to provide a targeted response. Both systems are interdependent and protect us against foreign threats and even cancer.
Once alarm signals (foreign antigens) are received, the Adaptive System invokes two mechanisms of defence - a Cell Mediated Response or an Antibody Response. They are triggered differently.
All our nucleated cells have HLA antigens on them. This antigen group is the gateway that leads down the pathway of a Cell Mediated Immunity (CMI, T cells) response and hence tissue rejection (e.g. liver, kidney, transplant failure) rather than an antibody only one (B cells).
Red cells are non-nucleated and normally do not express HLA antigens on their surface. Thus CMI issues are not invoked with pure red cell transfusions.
The transfusion of incompatible red cells into patients generates an antibody response directed at non-self-antigens on the donor cell's surface, destroying them. It is this pathological process that 'blood grouping' and 'cross-matching' techniques have been developed to prevent.
Antigens on red cells are aggregated into 'Groups' based on their DNA profile and consist of proteins or glycoproteins (protein + sugars). These blood groups vary in their ability to generate antibodies.
There are at least 33 different Blood Groups recorded on red cells and all of us have variations of these.
Experience and scientific research for over a century has clearly demonstrated that only a relatively small number of red cell antigen groups evoke Clinically Significant Antibodies when transfused into an antigen negative patient.
Important examples include the ABO, Rh (Rhesus), Kell (Cellano), Kidd and Duffy groups. Other very rare antigens may cause transfusion reactions. Specialty units always investigate these events. Extremely rare red cell groups are sometimes found.
To ensure safety, all donor blood is screened by the Transfusion Service for infective agents and the red cells are grouped for ABO and Rh phenotypes. The patient's transfusion history is always obtained.
The recipient red cells are grouped by the transfusion laboratory scientists to match the donor's ABO and Rh type.
The donor red cells are then cross-matched (mixed) in the laboratory against the recipient's serum to exclude any possible reaction. Simultaneously the patient's serum is cross-matched against a panel of specially selected red cells that have on their surface all the known clinically significant antigens.
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