The Meaning of Irregular Antibody Screening

Irregular antibody screening refers to the examination of all antibodies other than those of the human ABO blood group system. Screening of irregular antibodies is critical for ensuring safe blood transfusion and reducing transfusion reactions in blood recipients. Due to its importance, irregular-antibody screening is recommended to be performed as part of the standard protocol, particularly in pregnant women. However, in some regions of the world, especially in developing countries, irregular-antibody screening is not part of the standard screening protocol. This may increase the risk of blood transfusion reactions, which sometimes can be serious.

To understand what is irregular antibody screening, it's necessary to understand what is the ABO blood group system first.

ABO blood group system

Do you know your blood type? Currently, it's very easy to have your blood type checked in the hospital. But it was once thought that all humans have similar blood, and many people were dying from blood transfusions, and doctors did not know why. It was not until 1901 that blood groups were found by an Austrian scientist named Karl Landsteiner. To date, it has been well established that there are four major blood types among humans, namely A, B, O, and AB, which are collectively referred to as the ABO blood group system. One blood type may be incompatible with another, and this incompatibility is the reason for the often tragic consequences of blood transfusions before.

An adult body typically contains 5-6 liters of blood. Blood is a highly specialized tissue composed of several thousand kinds of components, including various cells such as red cells, white cells and platelets, proteins, salts, etc. Normally, 55% of the blood's volume is plasma, a fluid mainly made of proteins and salts. The ABO blood groups are classified based on the properties of red blood cells. According to the presence or absence of the antigens A and B on the surface of red blood cells, a person may thus have type A, type B, type O, or type AB blood.

* Type A blood has antigen A on red blood cells and anti-B antibody in the plasma

* Type B blood has antigen B on red blood cells and anti-A antibody in the plasma

* Type O blood has neither antigen A nor antigen B on red blood cells, and has both anti-A antibody and anti-B antibody in the plasma

* Type AB blood has both antigen A and antigen B on red blood cells, and has neither anti-A antibody nor anti-B antibody in the plasma

Fig. 1 below shows the properties of the red blood cells and the plasma of each blood type more clearly.

Fig. 1 Four Major Blood Types

Blood type

Antigen(s) on red blood cells

Antibodies in the plasma


A antigen



B antigen




Anti-A and Anti-B


A antigen and B antigen


Anti-A and Anti-B can bind to red blood cells with corresponding antigens and activate complement cascade, resulting in hemolysis. For instance, a recipient with type A blood should not be transfused with red cells from a donor with type B or AB blood, or harmful transfusion reactions will occur. So, ABO blood test is an essential part of the standard protocol before blood transfusion, as it helps avoid hemolysis from ABO blood group incompatibility.

Regular antibodies VS. Irregular antibodies

The discovery of the ABO blood group system has greatly reduced transfusion reactions and improved the safety of blood transfusion. But other factors, especially irregular antibodies, can also be the cause of clinical transfusion reactions.

Regular antibodies and irregular antibodies are the two main groups of antibodies classified roughly on the timing and triggering event of antibody production.

Regular antibodies are those directed against the ABO blood group antigens A and B. Their production occurs very early in life. Regular antibodies are often alluded to as natural antibodies by blood transfusionists.

Irregular antibodies refer to all antibodies other than those of the ABO blood group system. These antibodies do not exist under normal conditions but may be produced as a result of certain conditions such as blood transfusions, injections, maternal and child blood group incompatibility in pregnancy, and immune-stimulating of blood products or some non-perceived stimuli.

Why is irregular antibody screening important?

As an important therapeutic measure, blood transfusion is often used to rescue the patients with acute hemorrhage. Besides, blood transfusion has important therapeutic effects for the patients with chronic anemia, chronic consumption diseases, severe infection and hemorrhagic diseases.

With the development of blood transfusion technology, the incidence of rapid hemolytic reactions caused by ABO blood group incompatibility has decreased significantly. Available data show that currently, irregular antibodies are the main cause of clinical transfusion reactions. In addition, irregular antibodies can trigger hemolytic disease in newborns, pose difficulty in blood type identification, and lead to questionable clinical crossmatching cases. Further, irregular antibodies are also associated with abortion and stillbirth. So, screening of irregular antibodies is very important.

Irregular antibody screening before blood transfusion, especially for recipients in high-risk departments, can effectively reduce the incidence of hemolytic reactions after blood transfusion, ensuring the safety of clinical blood transfusion. In the United States, Japan, Australia and many other countries, irregular antibody screening has become part of the standard protocol before blood transfusion. However, in some developing countries, irregular antibody screening is often not performed or only performed for some patients. This increases the risk of harmful transfusion reactions. If irregular antibodies are present in the blood of a recipient or donor, acute or late-onset transfusion reactions can occur and even cause life-threatening events.

How is irregular antibody screening performed?

Screening of antibodies is often through performing a Coombs test, which is also known as an antiglobulin test or red blood cell antibody screening. There are two types of Coombs test: direct and indirect, as shown in Fig. 2.

Fig. 2 Direct and Indirect Coombs test
Direct and Indirect Coombs test

(By No machine-readable author provided. A. Rad~commonswiki assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims)., CC BY-SA 3.0,

Both Coombs tests are based on the binding of anti-human antibodies to human antibodies (commonly IgG or IgM). The anti-human antibodies are generated in animals and will react with human antibodies directed against red blood cells. The Coombs test involves the use of a small sample of blood from the recipient or donor and can be performed in a small glass tube, on a glass slide, or on a test strip. If agglutination of red blood cells occurs, it's a positive result. If agglutination does not occur, it's a negative result.

The Coombs test checks your blood for antibodies that attack red blood cells. If you have irregular antibodies in your plasma, you'll test positive.

In a 2011 study (Xianjun Ma et al.), scientists screened irregular antibodies in blood transfusion recipients from China by performing Coombs test. Among the 29,236 patients screened, 138 (0.47%) tested positive for irregular antibodies. The specificity of these irregular antibodies was mainly anti-M, anti-D, anti-C, and anti-E. The positive rate of irregular antibodies in females was higher than that in males, and the positive rate in pregnant women was higher than that in non-pregnant women. These differences might be due to that one can develop irregular antibodies through pregnancy. Furthermore, scientists observed a substantial reduction in the number of transfusion reactions after introducing irregular-antibody screening in hospital. These findings emphasize the need to perform irregular antibody screening as part of standard protocol, particularly in pregnant women. Further, individuals who test positive for irregular antibodies must be screened further to identify antibody subtypes to reduce blood transfusion reactions and improve the safety of blood transfusion.

In summary, irregular antibody screening is the examination of all non-ABO antibodies in the blood. This screening is very important because it helps reduce blood transfusion reactions. Simply testing ABO blood types is not enough to meet the safety requirements for blood transfusion. Irregular antibody screening is a supplement to ABO blood identification to ensure safe blood transfusion.


[1] Carel Jan van Oss, Letter to the Editor: "Natural" versus Regular Antibodies, The Protein Journal (2004).

[2] Xianjun Ma et al, Irregular-antibody Screening in Shandong Region and Clinical Impact on Blood Transfusion-a review of case from 2008 to 2010, Journal of Convergence Information Technology (2011).

[3] RONG Shiqin, Clinical value of irregular antibody screening in safe blood transfusion, Youjian Medical Journal (2013).

[4] Chen Yong et al, Significance of irregular antibody screening before blood transfusion in clinical blood transfusion, Int J Lab Med (2013).

[5] LUO Jian-hua, Clinical analysis of irregular antibody screening and clinical safety of blood transfusion in blood recipients, Journal of Clinical and Experimental Medicine (2013).

[6] MA Xianjun et al, Significance of irregular antibody screening of recipients in clinical transfusion safe, Journal of Clinical Hematology (2009).

[7] Xia Hefeng, Clinical significance of screening irregular antibody before blood transfusion, Jiangsu Med J (2013).

[8] Xu Jixun, Irregular antibody screening before blood transfusion and safety of clinical blood transfusion, Journal of Clinical Hematology (2010).

Cite this article

CUSABIO team. The Meaning of Irregular Antibody Screening.


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