White blood cells, or leukocytes, are your body’s immunity cells and are there to protect you from illness and disease. They circulate through your bloodstream and tissues and fight viruses, bacteria, cancer cells, and any other invaders that could potentially threaten your health. They produce antibodies that then attach to the invader and destroy it. 

White blood cells are produced in the bone marrow, and they take up around 1% of your total blood. The major difference between red blood cells and platelets is that the white blood cells have nuclei. There are three major classes of white blood cells – lymphocytes, granulocytes, and monocytes. Lymphocytes are divided into B cells, T cells, and natural killer cells. Granulocytes are white blood cells with granules in their cytoplasm and are divided into neutrophils, eosinophils, and basophils. 

A high white blood cell count or leukocytosis means that your body produces more white blood cells than it should, and there can be many reasons that cause this. A viral or bacterial infection, inflammation, burns, rheumatoid arthritis, corticosteroids, pregnancy, allergies, smoking, or stress can all cause the level of white blood cells to elevate. However, elevated levels can also be connected to more serious conditions such as leukemia, polycythemia vera, myelofibrosis, or lymphoma.

A low white blood cell count or leukopenia means that your body is doesn't produce enough white blood cells and is more prone to any infection. As with a high white blood cell count, there are also many conditions associated with a low white blood cell count. Chemotherapy, radiation therapy, myelodysplastic syndrome, autoimmune disorders such as lupus or rheumatoid arthritis, anaplastic anemia, leukemia, HIV, or malnutrition are some of the conditions that may cause low levels of white blood cells.