If a blood cell is placed in a hypotonic solution, water will flow

Into the cell, causing it to swell tremendously.

This is because osmosis is transporting water across the semi-permeable membrane from an area of low solute concentration to high in order to balance it out. 

The opposite effect will be the cell will shrink due to water leaving the cell, which is when it is placed in a hypertonic solution.

Hope this helps!