Antony was actually a Plebeian. Starting in the early republic, already the 1-1 correlation between poor=plebeian and rich=patrician was starting to get blurred. There were relatively poor patricians, and fabulously wealthy plebeians. Pompey Magnus, for example, was also a Plebeian, and was stupidly wealthy, even compared to other Patricians. He was also the leading man of the state for a time, and this was because he was of the Senatorial class, which didn’t necessarily mean Patrician exclusively by that time.

While the Patricians did have a sort of “noble air” to them, Plebeians by the late republic had their own strength, which was arguably stronger than any perceived “nobility”: the tribunate. ONLY plebeians could be elected to the position of tribune of the plebs, and the Romans stuck to this rigidly because it was a religious observance as well as a political one. There were oaths to the gods made by the plebeian body of around 500-400 BCE in which the plebs swore to uphold the legitimacy and the sacrosanct nature of their tribunes in order to curb patrician hegemony, and it worked. By the late republic the tribune of the plebs was a powerful figure in Roman politics, as evidenced by the Gracchi brothers, who would have succeeded in their land reforms had their enemies not illegally killed them and sent the republic spiraling into empire.

So, by the late republic, a “noble” was someone of the senatorial class, and this didn’t necessary mean plebeian or patrician; both were nobles in equal spades so long as they had the money, and some, like Cicero, weren’t even technically either; they were other Italians who had come up from humble beginnings via their skills. But the tribune of the plebs remained an important plebeian only office because only it could veto motions put forth in the senate in the name of the people of Rome. Hence, Antony served under Caesar as tribune because he answered the Caesar, and Caesar needed to be able to control that veto power to enact the laws he wanted to see enacted.