Racism and prejudice is one of the central themes of To Kill a Mockingbird. We see this in many ways. Let me give a few examples.
First and most obviously, we see isolation and racism in the trial of Tom Robinson. Very few white people will believe in his words, no matter how convincing. Therefore, he is made to stand alone in a world where he is unjustly accused of a crime that he did not commit. We also see this very clearly in the mob scene. White men come to harm or kill Tom; the only one who defends him is Atticus. The text does not say, but it is clear that Tom feels isolation.
Second, we see this in the very geography of Maycomb. The blacks live away from the main areas of the Maycomb and certainly not where the white live. On several occasions it is mentioned that the blacks live past the Ewells beyond the town dump. In other words, they are out of sight. From this perspective, racism has isolated them. Here is one example of this point:
A dirt road ran from the highway past the dump, down to a small Negro settlement some five hundred yards beyond the Ewells‘.