Answer very briefly, the following:- Red Blood cells have no nucleus, then why do we call them cells?

RBC are formed in the liver and spleen when the organisjn is foetus. After birth  these are formed in the red bone marrow. At the time of formation of RBC’s they have  nucleus but later on they loose nucleus. Those enucleated cells perform all the  functions of a cell so these are called cells.

RBC’s are surrounded by semipermeable plasma membrane. It contains homogenous  cytoplasm and it is without the nucleus, endoplasmic reticulum, mitochondria,  ribosomes and centrioles. RBC’s get advantage due to this condition. Corpuscles have  more space to hold haemoglobin. It can as well give more oxygen by the haemoglobin  to the tissue cells. So the enucleated condition is more useful to the tissue cells. The  necessity of work has made them without nucleus and perform all the functions of the  cells so they are called cells.