Although each cell contains a full complement of DNA, cells use genes selectively. Other genes, however, are inactive most of the time.
Some genes play a role in early development of the embryo and are then shut down forever.
Many genes encode proteins that are unique to a particular kind of cell and that give the cell its character - making a brain cell, say, different from a bone cell. A normal cell activates just the genes it needs at the moment and actively suppresses the rest. Genes, through the proteins they encode, determine all body processes, including how the body responds to challenges from the environment.