AC is safer than dc because you do not require as much voltage to make it travel along any line or conductor.

AC changes direction (polarity) as it pulses across a conductor.

By stepping up the frequency of the alternating current - you can pack in more power safely and make it travel a lot farther than DC. Then when you need more voltage - you can step this frequency down via transformers and create more voltage.

Very high frequency AC can be generated in in very low voltage and passed through the body to light special low wattage light bulbs, etc. It is a trick Tesla emplyed to great effect to show how safe AC power was. Edison (who clung to DC out of ignorance) countered by hiring kids and paying them a dime for stray puppies and cats to electrocute. After killing some folks stolen pets with AC he then petitioned the state prison to use AC to elecrocute people - which they did.

So AC power can be stepped up to great frequency and carry huge amounts of power without meeting the resistance which direct current does. DC of the same power output as is currently carried by AC would melt all our powerlines and set them bursting into flames.

In essence, the energy required to sustain the high frequency wave forms keeps the current from emitting its full power until it is stepped down in frequency. Yet, this same high frequency can travel across conductors with almost no resistance.

DC passes almost all of its energy along the conductor. It "fights" the conductor.
Much of any AC energy exists in a field around the conductor. The high frequency waves of AC generate powerful magnetic fields which are busy taking up much of the power.

So it's not so much that the body doesn't have time to react to the current - but that a large amount of the current will bypass the body when compared to DC.

For example touch a hot DC line and its current will immediately destroy the local tissue it comes into contact with. Touch an AC line and the current will travel through and around your body discharging into other nearby conductors.

Hope that helps!

SDAI-Tech1