White blood cells (WBCs) are a type of cells found in blood. They form just 1% of your blood but are entrusted with the function of immunity. So, white blood cells circulate in the blood and help the immune system fight off microbes and infections caused by them.

When the body identifies an intruder or an infection, it releases white blood cells to fight them. This often causes an inflammatory reaction.

White blood cells are produced by stem cells in the bone marrow. In fact, 80 to 90% of white blood cells are stored in the bone marrow.

Types and functions of white blood cells
White blood cells can be broadly classified into granulocytes and agranulocytes, depending on the presence or absence of small protein granules in them.

Granulocytes
There are three types of granulocytes, namely:

• Basophils: These form less than 1% of the white blood cells in the body. The basophil number usually increases during an allergic reaction. Basophils produce chemicals like histamine to control the body’s immune response.
• Eosinophils: These white blood cells are primarily responsible for responding to parasitic infections but also play a role in the body’s general immune and inflammatory responses. They have also been found to attack and kill cancer cells.
• Neutrophils: These cells form the majority of white blood cells in the body. They are essentially scavengers that identify an infectious agent like bacteria or fungi and destroy them. Neutrophils form the first line of the body’s defence mechanism.

Agranulocytes
There are two types of agranulocytes. They are:

• Lymphocytes:These white blood cells produce substances called antibodies to fight against microbes and other harmful invaders. Lymphocytes are of three types: B cells: These cells produce antibodies to help the immune system fight infections in the body. They are also called B-lymphocytes.
• T cells: These white blood cells identify and attack infection-causing cells and remove them from the body.
• Natural killer cells: As the name suggests, natural killer cells are a type of white blood
• cell that attack and kill viral and cancer cells.
• Monocytes:These cells form 2 to 8% of your total white blood cell count. These cells usually have a longer lifespan than other WBCs and are responsible for fighting off chronic infections.

What do abnormal WBC counts indicate?
Certain health conditions and diseases can give rise to abnormal white blood cell count. Depending on the disease, the WBC count can be higher or lower than normal.

High white blood cell count
When the body produces excessive amounts of WBCs, it is termed leucocytosis. If you have a high white blood cell count, it may be indicative of one of the following conditions:

• Allergic response (commonly seen in people with asthma and atopic dermatitis)
• Burns
• Trauma
• Heart attack
• Inflammatory conditions like rheumatoid arthritis, IBD, eczema
• Infections caused by bacteria, fungi, parasites or viruses
• Leukaemia

Sometimes, undergoing a surgical procedure can also raise your WBC count temporarily.

Low white blood cell count
When the body produces fewer than normal white blood cells, the condition is called leukopenia. A few conditions that may cause leukopenia include:

• Autoimmune diseases like lupus
• HIV
• Bone marrow disorders
• Lymphoma
• Bone marrow damage due to chemotherapy, radiation therapy or toxin exposure
• Sepsis
• Vitamin B12 deficiency