Tricky question; but I think the answer lies in how "from" & "against" are used in English. For ex -- against — in resistance to or defense from: " Medicines give immunity against diseases." from — (used to indicate agent or the cause) : "Medicines make you immune from diseases." While the difference between two is wafer thin & they can be used interchangeably, some patterns do exist. For ex - a. Food protects one from starvation. b. Food protects against starvation. While tangibility of agent is important factor; I feel the structure of the sentence is also quite important factor in deciding from/against. The difference is too minute.

I think to protect "from" is mostly used to indicate that you just want to "separate" something from something else, for instance preventing something from being covered by something else (to protect a car from dust or snow by covering it with a piece of material, etc). To protect "against" gives more the idea of protection against something "aggressive" (to protect against a desease, to protect an iron object against rust, by covering it with a special paint, etc).

The meanings are exactly the same. The only very small difference, is that sometimes it is more common to use a pronoun or a noun with 'from'. For example: 'Sunscreen protects YOUR SKIN from the sun's harmful rays' - Here the noun 'your skin' means that we are more likely to use 'from', but it doesn't really matter, you can still use against. 'Sunscreen protects against the sun's harmful rays' - Here the fact that there is no noun means that we are more likely to use 'against', but it doesn't really matter, you can still use from. More examples: 'I will protect YOU from him' 'These gloves protect MY HANDS from the cold.' 'This new product protects against baldness' 'This cream protects against bacteria.' Again, in all of those examples you can still use the other preposition, it is just that with a noun or pronoun from is sometimes more common. Like I said, 'from' and 'against' really are interchangeable, it has no effect on the meaning.

You have the right idea. To protect "against" suggests an attack or invasion (something actively threatening), while "protect from" suggests something passive.

The best way I can describe the two are when you are protecting from something. That is is tangible. Like your example protect from dust. Dust is an actually thing you can see, it is tangible. Protecting against disease is something you don't necessarily see or touch. You can also use protect from non tangible items as well. There is very little difference and most of the time they can be used interchangeably.