Affinity chromatography is a biochemical technique for separating one type of molecule from a mixture by exploiting its natural, specific binding to another molecule.

Essentially, it works like this: You have an adsorbent, which is basically a solid matrix for the binding to occur on. The adsorbent is usually made up of very small, insoluble particles or beads. You covalently link one molecule to this matrix. You then add your mixture (such as blood serum or liquefied tissues/cells) to the matrix, either by mixing them in a container (batch preparation) or by putting the matrix in a chromatography column and pumping the liquid through.

Because one of the molecules is stuck to the immobile matrix, the other will bind to it and be retained. You can then wash away the rest of the mixture, and "elute" the second molecule off with a solution that "discourages" binding. This method can be used to purify a biomolecule such as a particular type of protein or a specific DNA fragment from a mixture of others.

One of the most common matrix-bound molecules used is an antibody. If you have an antibody to a particular protein (produced by injecting a sample of the protein into a rabbit or goat and purifying the antibody from some of their blood), you can purify this protein from any other sample using this method. Another common method binds a known DNA sequence to the matrix and uses it to isolate proteins which bind to that sequence. This can detect transcription factors and other proteins that work to regulate gene expression by binding to the gene itself.

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