The only human cell without a nucleus is the red blood cell, also known as an erythrocyte.

Here's why:

* Function: Red blood cells' primary function is to carry oxygen throughout the body. This requires a large surface area for gas exchange, which is maximized by their biconcave shape.

* Nucleus: The nucleus contains DNA, which is responsible for producing proteins. In red blood cells, the nucleus is ejected during development to make room for more hemoglobin, the protein that binds oxygen.

* Consequences: The lack of a nucleus means red blood cells cannot repair themselves or reproduce. Their lifespan is about 120 days, after which they are broken down in the spleen.