I'm not sure how it works in other version of D&D, but in 4e you get absolutely no bonus to defense with your weapon, and that doesn't quite make sense. When you're in combat with someone, you don't just stand there and take the hit, you either try and dodge the attack or you use your weapon to block or redirect the attack.
In D&D 4e weapons do not inherently help your defense. Why?
This is true in the majority of RPGs I've seen, including all prior versions of D&D. The problem is one of level of abstraction; how specific do you really want to get with differences in defensive capabilities between different weapons? Some of the older editions (and certain versions of Traveller, for example) had varying to-hit based on your weapon and the target's armor. 4e explicitly takes a much more abstract view of combat, and parrying as something other than a reactive defense power would be inconsistent with its general view of combat.
Probably an easy fix is to assume that parrying is part of what makes up the base 10+half level to AC, and to give a bonus to-hit or combat advantage when attacking an unarmed target (because parrying weapons with your bare hands is... tricky. Doable in some cases (gauntlets, monk, etc), but hard).
I've seen a couple systems where dedicated parrying weapons (such as the main gauche and duelling cloaks) were represented as shields, providing a bonus to AC but little to no offensive capabilities. Swashbuckling Adventures did this well for 3.5 (including parrying feats), and since it's based on 7th Sea I imagine they did as well. For a 3.x variant that breaks out weapon bonuses to defense, take a look at Codex Martialis. The 40k RPG family (Dark Heresy, Deathwatch, and Rogue Trader) also feature parrying explicitly, with some weapons being well-balanced for it, and others being more difficult or impossible to parry with (chain-axes, if I recall correctly). If you're looking to create a parrying system, you could mine these for ideas.
There are weapons that give a bonus to AC. Off the top of my head, the parrying dagger and most of the double weapons have the defensive property, which gives you +1 AC for wielding them with another weapon in your other hand (or wielding them in both hands, in the case of the double weapons).
There are also feats that give you an AC bonus for certain weapon styles, such as polearms/staffs (Hafted Defense), or dual-wielding (Two Weapon Defense).
4e made a conscious decision to prefer speed and balance over "realistic" simulation. This is, after all, the game that decided a spell called "fireball" is actually a fire square. Adding the complexity of per weapon defenses would complicate and slow down the game. It also adds more variables, creating more opportunities for unbalanced things to sneak in.
You could add such a system with simple bonuses (A longsword grants +2 AC, a dagger grants +1 AC), but it gets wonky quickly. Wouldn't a wide dwarven waraxe be more effective against arrows than a longsword? If the dagger fighter can get inside longsword fighter's reach, doesn't the advantage move from the longsword to the dagger? Should a dagger get a bonus against an unarmed opponent (no defense like putting a sharp object in the way!) but suffer a penalty against a two-handed sword which easily swats the dagger away?
In-game, the defense bonus is assumed; it's the bonus you're getting from Dex/Int. You're assumed to be making the best use of whatever you're using. If you have a dagger, you'll strive to get inside the long sword user's reach, while the long sword user is trying to keep you at range. And of course you're making trade offs, trying to also get your own licks in. (If you don't want to get your own licks in, that's what the full defense is for.)
You could simply alter the parry system.
Back in the day, there was a time when I decided to add an active parry to 1st ed AD&D combat. It was a "to hit" roll, with whatever bonuses you would normally get. You were essentially attempting to hit your opponents weapon while they were trying to hit you... which is what parrying with a weapon seems to be all about.
The simplest way to handle this was to let the higher roll win. If your roll plus bonuses exceeded your opponents, then you blocked their attack. If their attack roll and bonuses exceeded your parry attempt, then that roll was applied to your AC to see if they actually scored a hit and caused damage.
Yes, this made combat a drawn out experience, much like the swashbuckling sword fights we see in the movies, were 95% of all the sword swings would hit the opponent's blade. Yes, it was an active thing, where if the PC did not say "I'll parry that" at the time of an attack, they did not get the option to parry it. It was never an automatic thing.
Sure.... for some PCs, with their incredible strengths and amazing magical weapons, in melee combat they were virtually untouchable. There were always spells and catapults and falling flaming trees to kill THOSE guys... but that's another story.
