Magnetic Separation this is used in protein purification, immunoassays, pre-processing in polymerase chain reactions.  There are a number of commercial systems on the market, the principal of which is illustrated in figure 1.  Magnetic particles are coated with a reagent which selectively attaches to the target cells when added to the cell suspension.  After an incubation period the target cells are isolated by use of a magnetic field.  This method offers several advantages over non-magnetic cell separation methods:
1. it permits the target material to be isolated directly from crude solutions such as blood, bone marrow and tissue homogenates.
2. it simplifies the reaction steps.
3. since the reaction kinetics of attachment are very fast and the time required for the separation of the particle/cell complexes by a magnetic field only takes a few minutes, the procedure is much faster than other methods.

Figure 1: Principal of magnetic separation.

Immunoassays this is a group of methods that employ antibodies to measure the concentration of an analyte.  The US National Research Laboratories Fabs (Force Activated BioSensor) concept is a cantilever based immunosensor which works by measuring interactions in much the same way as an AFM and consists of three steps, illustrated in figure 2:
1. antibodies are covalently bonded to the cantilever surface.
2. the sample solution is flowed over the surface of the cantilever, the antibodies bind to any analyte molecules present in the solution.
3. a solution containing 2mm NdFeBLa magnetic beads with covalently bounded antibodies that attach to the analyte is introduced.

Figure 2: Fabs concept.