Structure of antigens recognized by cold agglutinins

At this time, three major groups of antigens recognized by cold agglutinins (CAs) are known: Iij antigens are defined by branched, linear, and branched and linear type 2 chains, respectively. They are protease- and sialidase-resistant. Whereas Ii antigens are developmentally regulated, j is expressed on newborn and adult erythrocytes in equal strength. The antigens of the Sia complex are biochemically related to the Iij antigens: they are sialylated type 2 chains. Consequently, they are protease resistant but sialidase-sensitive. Sia-l and Sia-b antigens are differentiation antigens, Sia-lb is not developmentally regulated. The structure of the Pr/Sa antigens is defined by the O-glycans of the glycophorins of the human red blood cell membrane. The immunodominant component is represented by N-acetyl-neuraminic acid. Therefore, Pr/Sa antigens are protease-sensitive. With one exception (Pra), the antigens of this complex are also sialidase-sensitive. They are not developmentally regulated.