There is an ability to distinguish self from non self , largely through the presence of unique, genetically determined proteins, the Class I and II MHC proteins 4. Precursor cells that derive from stem cells can replicate and differentiate at an astounding rate. The average person makes 3- 10 billion platelets, rbcs, neutrophils and lymphocytes in just one hour. The bone marrow can increase this by 10 fold in response to a need. Platelets stay in circulation for one week, rbcs for 120 days and neutrophils for just 8 hours.
Hormones can act on the bone marrow to stimulate the pluipotential stem cell.
People can be tissue typed. There are seven genes which code for MHC proteins. These genes are located on the short arm of chromosome number 6. Each of these seven genes have multiple alleles. The possible combination of genes that can be inherited makes for tremendous diversity in the expresion of MHC protiens.