The two mechanisms used to label cells in MACS are Direct and Indirect labeling.

Direct Magnetic Cell Labeling

In the direct labeling method, the microbead containing the antibody correlates directly to an antigen on the surface of a cell. Direct magnetic cell labeling is a faster and more specific protocol. 

Indirect Magnetic Cell Labeling 

In the indirect labeling method, a combination of monoclonal and polyclonal antibodies are used. The protocol involves initially attaching an antibody to the cell. Then a secondary antibody with a magnetic bead binds to the primary antibody. Indirect magnetic cell labeling is less specific, but is also less expensive. It is useful for sorting cells that have weak antigen expression and cannot be sorted using the direct labeling method.