The five Kosas are annamaya kosa (the physical body), pranayama kosa (the energetic body), manomaya kosa (the mental body), vijnanamaya kosa (the intellectual body) and anandamaya kosa( the soul body). All five layers are found beneath the skin layer. The kosas or sheaths are not clear cut layers but they intermingle, they influence each other, depend on each other.
Just as we say that yoga is the union of body, mind and soul, this union can only be created if the kosas are in harmony with each other. Neglecting one of the kosas will have a negative effect on the other kosas. If for example we neglect the annamaya kosa, we cannot penetrate into the other kosas without causing imbalance, strain and stress. It also works the other way, we cannot do our asanas without the observance from the mental and intellectual bodies. As previously stated the work we do in the asanas is a preparation for our pranayama practice and our pranayama practice feeds into the asana practice. Both also allow us to penetrate deeper within to access the mental, intellectual and soul body.
All kosas are equally as important, we should not pay attention to one more than the other.