In my lab, human DNA is extracted from whole-blood samples.
I don't actually do the extractions and I am not familiar with the specific protocol but I understand that platelets and red blood cells do not have nuclei and therefore do not have DNA so I presume that isolating the sample's white blood cells is part of the procedure.
I also understand that different types of white blood cells have multi-lobed nuclei but I do not know how that affects the DNA which can be extracted from them.
I've seen a few papers, such as this one: http://www.ncbi.nlm.nih.gov/pubmed/15113441, state that they extracted DNA from specific types of white blood cells; lymphocytes in this case. These papers to not focus on, or even address, immune function.
Why extract DNA from only one type of white blood cell?
Do lymphocytes contain DNA that is more representative of the patient's genome than other types of white blood cells?
Are certain types of white blood cells easier to isolate than others?
Is this just the standard DNA-from-blood extraction procedure or are there multiple options to choose between depending on your research goals?