The MACS technique, also known as annexin V columns, is a sperm selection technique that separates apoptotic spermatozoa (sperm that are going to die) from healthy spermatozoa.
The MACS process is carried out thanks to magnetic particles attached to annexin V, a protein that is able to detect spermatozoa that have initiated apoptosis. The membrane of apoptotic spermatozoa undergoes a modification compared to healthy spermatozoa in that it externalises a molecule called phosphatidylserine, to which annexin V can bind.
Thus, if the seminal sample is mixed with annexin V (bound to magnetic particles), annexin V will bind to those spermatozoa that have externalised phosphatidylserine. If the semen sample is subsequently passed through a magnetic field, the apoptotic sperm bound to annexin V, and thus to the magnetic particles, will be retained.