Ii antigen system

Ii Antigen System

The Ii antigen system is a blood group system that is characterized by the presence of specific antigens on the surface of red blood cells. These antigens are known as the I and i antigens, which are carbohydrate structures found on glycoproteins and glycolipids. The system is important in transfusion medicine and hematology due to its role in certain autoimmune hemolytic anemias.

Structure of N-acetyllactosamine, a component of the Ii antigen system.

Structure and Biochemistry

The I and i antigens are determined by the structure of carbohydrate chains on the surface of red blood cells. The I antigen is a branched structure, while the i antigen is a linear structure. These antigens are composed of repeating units of N-acetyllactosamine, which is a disaccharide consisting of N-acetylglucosamine and galactose.

The conversion from the i antigen to the I antigen occurs during early childhood as a result of enzymatic activity that adds branching to the linear i antigen structure. This conversion is usually complete by the age of 18 months.

Clinical Significance

The Ii antigen system is clinically significant in the context of cold agglutinin disease, a type of autoimmune hemolytic anemia. In this condition, autoantibodies target the I antigen on red blood cells, leading to their destruction at low temperatures. This can cause symptoms such as anemia, jaundice, and Raynaud's phenomenon.

The presence of anti-I antibodies can also complicate blood transfusions, as these antibodies can cause agglutination of donor red blood cells that express the I antigen.

Genetic Basis

The expression of the I and i antigens is controlled by the activity of specific glycosyltransferase enzymes. The gene responsible for the branching enzyme that converts i to I is located on chromosome 6. Variations in the expression of this enzyme can lead to differences in the levels of I and i antigens on red blood cells.

Related Blood Group Systems

The Ii antigen system is related to other blood group systems that involve carbohydrate antigens, such as the ABO blood group system and the Lewis blood group system. These systems also involve the modification of carbohydrate structures on the surface of red blood cells.

Related Pages

• Blood group systems
• Autoimmune hemolytic anemia
• Cold agglutinin disease
• Transfusion medicine