Meanwhile, rinsing your bloodstained knickers or sheets in cold water won't affect the proteins - it'll just rinse the blood right out!
This works best if the stain is reasonably fresh. Once it dries out, the blood will clot and this, too, helps it to bind to the fabric like applying heat.
Blood - and most secretions from the human body - contains a lot of protein. The body in general has loads of protein in it, and blood is no exception.
When exposed to heat, such as by rinsing in hot water, blood "cooks". Think about how an egg white is clear-ish, then once it hits the heat it almost immediately goes white and changes texture. The same happens with blood.
Did your period come unexpectedly and now you've got blood stains on your clothes? Always use *cold* water to rinse, and often that'll be sufficient on its own to remove fresh stains. It has to be cold. Here's why...