White blood cells (leukocytes) defend the body against infection and clear away damaged tissue. They are made in the bone marrow and travel in the blood to wherever they are needed.
Neutrophils
The most common white cell and the first to arrive at a bacterial infection. They swallow and kill bacteria and are the main cell in pus.
Lymphocytes
B cells make antibodies, and T cells kill infected cells and direct the immune response. Viral infections often raise the lymphocyte count.
Monocytes, eosinophils and basophils
Monocytes leave the blood and become macrophages that clean up debris. Eosinophils rise with allergies and parasites; basophils release histamine in allergic reactions.
The white count
A normal adult white count is about 4,000 to 11,000 cells per microliter. A high count suggests infection, inflammation, steroids or a blood cancer; a low count raises the risk of serious infection.
| Cell | Share of white cells | Main job |
|---|---|---|
| Neutrophils | 50 to 70% | Kill bacteria |
| Lymphocytes | 20 to 40% | Antibodies and viral defense |
| Monocytes | 2 to 8% | Become macrophages |
| Eosinophils | 1 to 4% | Allergy and parasites |
| Basophils | Under 1% | Release histamine |
At the bedside
An absolute neutrophil count under 500 cells per microliter means a high risk of infection. A fever in that patient is an emergency that needs cultures and antibiotics fast.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).