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Stem cells are cells that have not yet undergone final differentiation. These cells can differentiate and become specific cells or continue to divide into more stem cells. Different types of stem cells are used for different diseases.

For autism, the two types of stem cells with the most evidence of benefit are cord blood stem cells and mesenchymal stromal cells (MSCs).

Many studies have shown that MSC suppresses inflammation. It is believed that this contributes to their benefit for autism. When MSCs are given by IV infusion, most of the MSC are filtered out of the blood by the patient's lungs and do not go to other parts of the body. Nonetheless, during this process, the patient's immune system receives signals released from the MSC. Afterward, the patient's entire metabolism is likely to have less inflammation and less auto-immune activity. 

Mesenchymal stromal cells can be found in many tissues of the body. They can be harvested from bone marrow, fat (adipose tissue), umbilical cord tissue, and even from dental pulp or menstrual blood. In the case of autism, MSCs are usually obtained from bone marrow (BM-MSC) or umbilical cord tissue (UC-MSC).
It is safe for a patient to receive MSC from their own body or from a donor. If a child with autism is large enough, and willing to undergo the harvesting procedure, they can receive BM-MSC from their own hip bone.