Blood cells are born in your bones, in the bone marrow that is!  Most bones in the body are hollow and are filled with a fatty, cellular substance called bone marrow.  It is here that blood cells reproduce and mature - more than 100 billion cells per day! 

What are stem cells?

All blood cells start out as stem cells.  Stem cells are cells that haven't yet decided what to kind of cell to be.  Some stem cells are pluripotent, meaning they can become any cell type.  Other stem cells are limited to certain categories of cells, such as hematopoeitic stem cells which can only become blood cells. 

Hematopoeitic stem cells get chemical messages from the body that tell them to divide and develop into a particular kind of blood cell, depending on what is needed.  Once the cells have fully matured, like soldiers preparing for battle, they are given a "mission."  Some cells are ordered to seek and destroy germs, and others are given jobs as support.  Once they are released into the blood stream, they can travel all over the body to carry out their duties.  T-cells are unique in that they must travel to the thymus to finish their education before becoming a fine-tuned fighting machine. 

What are "blasts?"

Once a stem cell is told what type of cell to become, it turns into a blast.  The term "blast" comes from the Latin root for "building."  Hematopoeitic (blood-producing) blasts are baby blood cells.  There are several types of blood cell blasts.  For example a baby red blood cell is called an normoblast and a baby lymphocyte is called a lymphoblast.  Immature blood cells are rarely found in the bloodstream.  The number of blasts in the blood may increase when production of that blood cell type is revved up.  For example, after an acute episode of hemorrhage (bleeding), there may be high numbers of immature red blood cells in the blood.  Two types of these baby red blood cells include normoblasts and reticulocytes.  When a large number of lymphoblasts, myeloblasts, or normoblasts are found in the blood, a severe infection, bleeding, hemolysis (blood cell destruction), or cancer should be considered as a possible cause. 

Why is the size of blood cells important?

In general, the larger a blood cell is, the younger it is.  For example, a complete blood count (CBC) blood test often reports the "mean corpuscular volume" or MCV.  This is measure of the average size of the red blood cells.  Younger red blood cells are typically larger, so a high MCV may indicate that red blood cells are being produced more quickly. 

The MCV may also be affected by deficiencies of enzymes or proteins.  For example, iron deficiency leads to small, light-colored red blood cells (i.e., a low MCV).  A deficiency of folic acid or vitamin B12 will lead to a high MCV.

The mean platelet volume (MPV) can be used to determine how quickly platelets are being produced.  Younger platelets and rapidly-produced platelets are generally larger in size.

What are "bands?"

Bands are an immature form of neutrophils.  A higher number of bands in the blood stream may indicate that the body is producing neutrophils quickly.  This may indicate an infection is underway!