How are white blood cells formed?

Most white blood cells are formed in the bone marrow, the soft tissue inside the bone hollow. Two types of white blood cells, T lymphocytes and B lymphocytes, are produced in the lymph nodes and spleen. 

Within the bone marrow, all white blood cells originate from a common, single type of unspecialized cell known as the hematopoietic stem cell (HSC) or the pluripotent stem cell. Stem cells differentiate or specialize in different stages. 

First, pluripotent stem cells differentiate into either myeloid or lymphoid stem cells.  

The lymphoid stem cells specialize further, leading to the lymphoid cell line, which evolve into T cells and B cells. T cells mature in the thymus gland. 

Specialization of the myeloid stem cells leads to myeloblasts, which further differentiate into neutrophils, eosinophils, basophils, and monocytes.