How does the avidin-biotin complex (ABC) method work?

The ABC methodology involves the following steps.

1. The application of an unlabeled primary antibody onto the tissue section, which binds specifically to the target antigen. 
2. The application of a biotinylated secondary antibody, which attaches to the primary antibody and introduces biotin molecules at the site of antigen binding. 
3. Application of a complex of avidin and biotin horseradish peroxidase conjugate (ABC). Avidin binds strongly to biotin, forming a stable complex. 
4. Development of the peroxidase by using DAB or other chromogenic substrates. This reaction produces a colored label at the site of the antigen, allowing for its visualization under a microscope.