Kingdoms are a classification system used for living organisms. Red blood cells are not living cells on their own. They are formed within the bone marrow from stem cells and lack a nucleus and other organelles.

Here's why red blood cells don't belong to a kingdom:

* They lack the characteristics of life: Red blood cells don't reproduce independently, don't have their own metabolism, and don't respond to stimuli.

* They are components of a larger organism: Red blood cells exist within the context of an animal's circulatory system and are dependent on the body for their function.

Therefore, the correct answer is that red blood cells are not classified into a kingdom. They are components of a multicellular organism.