I've tried to google the topic, however I can't find anything specific about what I'm wondering about.

On the unofficial elder scrolls wiki it says: "A Breton's natural Resistance to Magic can effectively halve
the Apprentice's disadvantage and add 100 Magicka to their already substantial Magicka reserves."
It also says: "100% Resist Magic is the only immunity that cannot be weakened. A Weakness to Magic
spell's magnitude is reduced to 0 in such a case...". From those statements alone it's not clear at all how
the mechanics would work.

If I have an item with 50% magic resistance what will happen?:
1. Will I get immune to magical effects because I have a total of 100% magic resistance that negates the
effect of the 100% weakness to magic? (Treating the passive weakness effect the same way as a weakness spell)
2. Or will I get immune to magic because the weakness and resistance effects are applied in succession
so that the total effect of any harmful spell cast will be reduced to 0 by the magic resistance thereby rendering the
weakness to magic pointless?
3. Is the item's enchantment affected by my current 50% weakness to magic, thereby reducing the bonus to 25%?
In that case would I get a resulting 25% weakness to magic, so that I get immune to magic if I have an
item with 100% magic resistance?
4. Or will the 50% from the item cancel out the 50% weakness I originally had, so that I would need an item (or item set)
with 150% magic resistance to be immune?

What if I'm using a spell giving 50% magic resistance instead of an enchanted item? (Apply the same questions as above)

I had a Breton Apprentice Arch-Wizard custom class.

If u get a Mundane Ring (+50% resist magic), then u r COMPLETELY IMMUNE to magic. This means no offensive spell will ever harm u.
u r immune to paralysis spells, but NOT from paralysing creatures.

Therefore, another cool Ring to get is the ring of Vitality (100% immunity to Poison, Disease and Paralysis).

With only two items u r immune to FOUR effects...


the only exception i think is poison weapons, i think they can ignore the ring's immunity...

As said above by Pdesbois nicely 100% resist can not be weakened at all, its reached immunity level.
The way I think defence, resist, weakness all works is this..

Player has Reflect spell - not reflected - P has Absorb - not absorbed - Player has resist magic % - lowered effects by % - Player has resist to element / paralysis % - effects further lowered by a % of the percentage from magic resistence.....
Player has weakness to magic - effects raised by % - player than gets hit with a weakness to spell than elemental damage - effects increased by multipule of magic weakness and element weakness.

But with 100% resist none of this apart from absorb / reflect maybe, is taken into account.

I hope this makes sense, I don't think I can personally put it in another way.

1. Will I get immune to magical effects because I have a total of 100% magic resistance that negates the
effect of the 100% weakness to magic?

If you have both a Magic Resistance 100 and a Weakness to Magic 100 effect on you, you will have no resistance or weakness to Magic.

If you get hit by a Weakness to Magic 100 spell while under the influence of a Magic Resistance 100 effect, the Weakness to Magic effect will be reduced to 0 and have no effect.


2. Or will I get immune to magic because the weakness and resistance effects are applied in succession
so that the total effect of any harmful spell cast will be reduced to 0 by the magic resistance thereby rendering the
weakness to magic pointless?

No harmful magic will affect you once you are at Magic Resistance 100.


3. Is the item's enchantment affected by my current 50% weakness to magic, thereby reducing the bonus to 25%?
In that case would I get a resulting 25% weakness to magic, so that I get immune to magic if I have an
item with 100% magic resistance?

Item enchantments ignore your magic resistance or weakness.


4. Or will the 50% from the item cancel out the 50% weakness I originally had, so that I would need an item (or item set)
with 150% magic resistance to be immune?

Yes.

http://www.uesp.net/wiki/Oblivion:Magical_Effects#Resistances_and_Weaknesses

According to this UESP wiki article resistances and weaknesses are added to produce a total that leaves a character either somewhat resistant or somewhat weak. If correct that means a Breton with a total of 100% resistance AND 100% weakness to magic would have neither resistance nor weakness.

If the game sums weaknesses and resistances separately, then applies them separately results would, of course, be very different. I haven't tested it myself so I don't really know. Maybe someone would like to do that. Maybe I'll do it. Easy enough on the PC, just hand a Breton Apprentice a mundane ring, head him over to Kvatch and see if he takes any damage from the scamp fire spells.

Mara

Since there is some conflict in the above, I'll post too:

With no other effects, a Breton Apprentice will have 50% weakness to magic.

A Breton Apprentice with the Mundane Ring will have no weakness or resistance to magic. Magical attacks on a Breton Apprentice wearing the Mundane Ring will affect the character with normal strength.

Yep. I just tested it. A lvl 1 Breton Apprentice wearing the Mundane Ring took 6 points damage from each stunted scamp fire spell.

Mara

Thanks a lot for the answers:)
I'm still interested in knowing if a spell instead of an item would work the same way.

Yes, a resist magic spell would work the same way as an enchantment. It is additive with the enchantment.

Note that Elemental Resistances are calculated separately. For example an Apprentice with 100% fire resist will be immune to Fire attacks, even if the character is weak to magic.

The Apprentice sign seems to be a complete bust. If you consider the effects you can get from Sigil stones enchantments, you need five stones to compensate for that weakness, and only two to get the benefit. I.e compared with a sign that does nothing, you're two Sigil Stones in debt before you start!

If you're just aiming for maximum magicka, then you do better as an Atronach, and if you need to compensate for the weakness, you do better as ANYTHING else.

This is a total surprise to me!

I played for a long time as a Breton Apprentice with mundane ring and NO offensive magic was affecting me at all! I entered Oblivion gates, faced mages in other places, had no other magic resistance gear / effect... ?

i did not use mods that affected this either... how come?

/shrug

Do you have an old save you could check? My guess is you took Mage rather than Apprentice as your birthsign. I'm using total Vanilla.

Just to answer, it was Apprentice sign I took, which is why i could make spells with weakness to magic effect.

I have no saves, as some 'occult forces' made my game crash constantly and i totally gave up the game.

As a side note, spells cast by the Player and reflected by the NPC cannot be resisted even if you had 200% resistance so watch what you shoot at and how much damage a single spell does. This is especially true for touch spells where you have absolutely no chance to dodge a reflected spell. At long distances they can be dodged if you're quick enough.

In the case of reflected spells, I don't know if the weakness counts, but for sure, the resistance does not.

I got that impression, bcoz liches can reflect spells cast at them and I had the 100% resist. I thought it was somre sort of bug.