Question

when using magnetic beads to separate labeled cells, what is the difference between positive and negative selection?

Answer 1

Magnetic-activated cell sorting (MACS) is a method for separation of various cell populations depending on their surface antigens(CD molecules) invented by Miltenyl Biotec .The name MACS is a registered trademark of the company.

With this method, the cells can be separated positively or negatively with respect to the particular antigen(s).

Positive selection:

In positive selection the cells expressing the antigen(s) of interest, which attached to the magnetic column, are washed out to a separate vessel, after removing the column from the magnetic field. This method is useful for isolation of a particular cell type, for instance CD4 lymphocytes.

Negative selection:

In negative selection the antibody used is against surface antigen(s) which are known to be present on cells that are not of interest. After administration of the cells/magnetic nanoparticles solution onto the column the cells expressing these antigens bind to the column and the fraction that goes through is collected, as it contains almost no cells with these undesired antigens.

(General procedure of MACS:

The MACS method allows cells to be separated by incubating with magnetic nanoparticles coated with antibodies against a particular surface antigen. This causes the cells expressing this antigen to attach to the magnetic nanoparticles. Afterwards the cell solution is transferred on a column placed in a strong magnetic field. In this step, the cells attached to the nanoparticles (expressing the antigen) stay on the column, while other cells (not expressing the antigen) flow through. With this method, the cells can be separated positively or negatively with respect to the particular antigen(s) )