Ferromagnetism describes the property of some metals to exhibit interactions with magnets. Steel is ferromagnetic, aluminum isn’t. That’s why you can stick a magnet to a refrigerator but not to a beer can.
The iron in our blood isn’t ferromagnetic. It doesn’t respond to magnets. In fact, there isn’t anything in our body that’s ferromagnetic. If there were things in our bodies that responded to magnets, we couldn’t use an MRI machine for human diagnostics.
Next time you see someone with a glorified refrigerator magnet strapped to her wrist, think about how silly it is to think that wearing such a magnet could have any possible physiological effect. Even though it’s silly to think this, considering that science tells us it’s absurd, this concept has generated studies just in case we don’t understand something about magnets and our bodies. Don’t worry, science understands magnets and our bodies just fine, and wearing magnets doesn’t have any effect on the body.
If physics and anatomy were more widely understood, predatory scam artists would have much less luck swindling the gullible into purchasing, among other things, magnets for medical uses.