Alloderm is a dermal matrix from cadaver skin. The skin is processed, removing all living cells, antibodies, and identifiers from the donor. What this process leaves is a lattice of collagen. Essentially, Alloderm is a framework of protein. There is nothing "living" in Alloderm. Imagine removing everything from a house except the 2x4 framework. If your skin is the house, then the 2x4 framework is the Alloderm.
After it is placed in the body, new blood vessels will grow into this collagen matrix and it will become "incorporated," meaning that the Alloderm is eventually replaced by your body with your own tissue. It's preferred use in breast reconstruction is due to many factors:
1. It is important to isolate the expander or implant from the overlying mastectomy skin flaps. The upper two thirds of the expander is covered by the pectoralis muscle and Alloderm creates a "sling" to cover the lower third of the expander/implant. This isolates the device from the overlying skin.
2. Alloderm allows for very precise definition of the inframammary fold. This is the skin crease where the bottom portion of the breast meets the chest wall. When reconstructing a breast, this is one of the most important landmarks.
3. Alloderm allows for expansion. When creating a breast, you want a tear-drop shape. Alloderm allows for greater expansion than the pectoralis muscle. This affords a tear-drop shape when used with an expander.
I hope this description helps you. Any new advance in surgery takes awhile to catch on. Alloderm is no exception. Also, please note that Alloderm is not the only dermal matrix out there, although it does have the longest track record.