Your cat's aggression is an inherent aspect of its predatory nature and behaviors like:

• Stalking
• Chasing
• Leaping
• Pouncing
• Swatting
• Biting

Are all common displays and they are always a major component of any play session.
Normally this isn't an issue and it's just how cats play and helping to satisfy your cat's predatory whims can be pretty fun! But when your cats play becomes play aggressive and by that I mean when it actually scratches and bites you, then things can get very uncomfortable and painful.
A cat that doesn't understand that such aggression is painful and even dangerous for its owner means playtimes can turn from a pleasure into an ordeal.
The reason for play aggression happening is that a bored under exercised and lonely cat becomes over stimulated during play and typical play-time mock aggression becomes the real thing.
Because all cat play is based around the predatory feline nature, an over stimulated cat vents this excess through an escalation of its normal predatory play.
So, instead of swatting at you with claws sheathed, the cat extends them and instead of mouthing your hand, the cat gives it a sharp bite.
Unfortunately, the problem can get worse as the cat is play aggressive because it's not getting enough stimulating, interactive play time; but because the cat demonstrates this behavior whenever it gets played with, it's owner plays with it less...
which results in more play aggression and so on and so on in a spiral.
What to look for if your cat is about to change mood and become aggressive.
The first thing is to pay attention to your cat's body language and expression.
Normal, non-aggressive feline play behavior includes the 'play face', with a half-open mouth and heavily lidded eyes; the sideways hop (often with arched back); and a lightly switching tail (it's going from side to side, but slowly and gently.
) With play aggression, when your cat's getting too excited, its body language will alter dramatically.
Its ears will go back, its tail will start lashing violently from side to side, and its pupils will enlarge.
Its movements will also become significantly more vigorous and energetic: there'll be increased speed and force to its playing.
If you think your cat's getting overexcited and is displaying the signs and symptoms I have outlined above then the best thing to do is to stand up and walk away - before it actually starts to demonstrate the play aggression! You can resume play as soon as the cat has calmed down a bit; the idea is to stop before the cat has the chance to vent its energy on you.
If you have not picked up the signs in time and you have already been bitten or scratched then do not reward the cat with any attention especially negative attention, but simply stand up and walk away.
If the cat persists or is becoming difficult to deal with the isolate the cat in the room by itself until it calms down, which can take anything from five to twenty minutes.
Since play aggression is almost always due to boredom and an excess of energy, the best thing you can do is to provide lots of opportunities for stimulating interactive play with your cat.
For example: get it running around; chasing things; climbing; providing stimulating toys and so on.
The key points for dealing with play aggression are:

• If play aggression happens then walk away and be consistent with this behavior
• Don't use physical punishment
• Provide lots of opportunities and toys for stimulating interactive play
• Make sure any cat play is on your terms and don't allow your cat to initiate play as this is habit-forming, and teaches it that you can be manipulated

In summary then, cat aggression is part of the cat's nature and is normal but play aggression is not normal but happens when the cat gets over excited.
If play aggression happens then take the steps outlined to deal with it.
Remember though that the cat is not being personal and just needs to be 'trained' out of it.