Why erythrocytes are more unusual than the rest of the cells?

Erythrocytes is another name used for red blood cells present in blood. These absorb oxygen present in the environment and supply it to all parts of the body. They also absorb from all parts of the body carbon dioxide  generated as a waste product for its removal from the body. Red blood cells areshaped like flat, round disks that are thicker on the edges as compared to the center.

One very unusual feature of red blood cells is absences of nucleus. While most types of cells have a nucleus that control many of the cell functions, a mature red blood cell has no nucleus. Also the cell membrane of red blood cells very flexible. This allows them to squeeze through tiniest of the blood vessels.