Henna is like a protein treatment. It bonds to the hair, fills in any gaps in the hair cuticle, and adds a second layer of strength. The lawsone (dye) of the henna penetrates the hair shaft and bonds to the keratin in the hair. Remember, hair is made up of keratin protein. As the dye bonds to the keratin, this makes the overall hair shaft much stronger and thicker. In this regard, henna acts much like a protein treatment.