Physiology chapters
White blood cells and immunity
The five types of white blood cell, their normal counts and jobs, how neutrophils and macrophages fight infection, and how lymphocytes provide specific immunity through antibodies and T cells.
- Normal total white count is about 4,000–11,000/µL; neutrophils are the commonest (about 60–70%), then lymphocytes (about 20–30%).
- Neutrophils and macrophages are the main phagocytes of innate immunity; eosinophils fight parasites; basophils release histamine.
- B cells make antibodies (humoral immunity); T cells kill infected cells and coordinate the response (cell-mediated immunity).
On this page
Overview
White blood cells (leucocytes) defend the body against infection. Most of them work outside the blood: the blood carries them to where they are needed.
There are five main types. Three have granules in their cytoplasm (granulocytes): neutrophils, eosinophils and basophils. Two do not (agranulocytes): monocytes and lymphocytes.
Normal counts
Total white cell count: about 4,000–11,000 per µL of blood.
| Cell | Share of white cells (approx.) |
|---|---|
| Neutrophils | 60–70% |
| Lymphocytes | 20–30% |
| Monocytes | 3–8% |
| Eosinophils | 1–4% |
| Basophils | Under 1% |
A widely used memory aid for the order from most to least common: "Never Let Monkeys Eat Bananas": neutrophils, lymphocytes, monocytes, eosinophils, basophils.
Neutrophils
- Nucleus with 3–5 lobes; fine granules.
- Made in the marrow; live only hours in blood and a day or two in tissues.
- The first cells to arrive in acute bacterial infection.
How neutrophils kill bacteria
- Margination and rolling along the vessel wall.
- Diapedesis: squeezing between endothelial cells into the tissue.
- Chemotaxis: moving towards chemicals from bacteria and damaged tissue (such as C5a and leukotriene B4).
- Phagocytosis: engulfing the bacterium, helped by opsonins (antibody and complement C3b) that coat it.
- Killing: by enzymes and by reactive oxygen species made in the respiratory burst (NADPH oxidase).
Neutrophilia (raised neutrophils) is typical of acute bacterial infection. Pus is mainly dead neutrophils.
Monocytes and macrophages
Monocytes circulate for a day or so, then enter tissues and become macrophages. Tissue macrophages have special names: Kupffer cells (liver), alveolar macrophages (lung), microglia (brain), osteoclasts (bone).
Macrophages:
- Are powerful phagocytes, able to engulf larger particles than neutrophils.
- Present antigens to T cells.
- Release cytokines (such as IL-1 and TNF) that cause fever.
- Dominate chronic inflammation and granulomas.
Eosinophils
- Bilobed nucleus; large red granules.
- Attack parasites (especially worms) by releasing toxic proteins.
- Help limit allergic reactions.
Eosinophilia points to parasitic infection (worms) or allergy (asthma, hay fever).
Basophils
- Dark blue granules containing histamine and heparin.
- Like tissue mast cells, they carry IgE on their surface and release their granules in allergic (type I hypersensitivity) reactions.
Lymphocytes
Lymphocytes provide specific (acquired) immunity. They live in lymph nodes, spleen and other lymphoid tissue, and many recirculate.
| Type | Matures in | Main job |
|---|---|---|
| B lymphocytes | Bone marrow | Become plasma cells that make antibodies (humoral immunity) |
| T lymphocytes | Thymus | Cell-mediated immunity |
| Natural killer (NK) cells | Bone marrow | Kill virus-infected and tumour cells without prior sensitisation (innate) |
T cells
- Helper T cells (CD4): release cytokines that activate B cells, cytotoxic T cells and macrophages. They are the main target of HIV.
- Cytotoxic T cells (CD8): kill cells infected with viruses and some tumour cells.
- Regulatory T cells: damp down the response and prevent autoimmunity.
T cells recognise antigens only when presented on MHC molecules: MHC class II (on antigen-presenting cells) to CD4 cells, and MHC class I (on all nucleated cells) to CD8 cells.
Innate and acquired immunity
| Feature | Innate | Acquired |
|---|---|---|
| Present from birth | Yes | Develops after exposure |
| Specific to one microbe | No | Yes |
| Memory | No | Yes (faster, stronger second response) |
| Components | Skin, mucus, neutrophils, macrophages, NK cells, complement | B cells, antibodies, T cells |
Vaccines work by creating memory B and T cells without causing the disease.
Antibodies (immunoglobulins)
| Class | Key facts |
|---|---|
| IgG | Most abundant in blood; main antibody of the secondary response; the only one that crosses the placenta |
| IgM | First antibody made in a primary response; a large pentamer; good at activating complement |
| IgA | In secretions (saliva, tears, breast milk, gut mucus); protects mucous membranes |
| IgE | Binds mast cells and basophils; allergy and parasites |
| IgD | On the surface of naive B cells |
IgM appears first, but IgG is the one that crosses the placenta and gives newborns protection.
Clinical relevance
Leucocytosis
How it happens: infection, inflammation, stress, steroids, or leukaemia.
What you see: neutrophilia in bacterial infection; lymphocytosis in some viral infections; eosinophilia in worms and allergy.
Neutropenia
How it happens: chemotherapy, some drugs, marrow failure.
What you see: high risk of serious bacterial and fungal infection; fever in a neutropenic patient is an emergency.
HIV infection
How it happens: the virus destroys CD4 helper T cells.
What you see: opportunistic infections and some cancers as CD4 counts fall.
Expect: normal counts and the commonest white cell, the first cell in acute infection, the steps of phagocytosis, causes of eosinophilia, the contents of basophil granules, B vs T cells and where they mature, CD4 vs CD8, and the features of each immunoglobulin class, especially IgG and IgM.
Quick revision
- Total WBC 4,000–11,000/µL; neutrophils 60–70%, lymphocytes 20–30%.
- Neutrophils: first in acute bacterial infection; kill by phagocytosis and respiratory burst.
- Monocytes become tissue macrophages.
- Eosinophils: parasites and allergy. Basophils: histamine and heparin.
- B cells (marrow) make antibodies; T cells (thymus) give cell-mediated immunity.
- IgG crosses the placenta; IgM first; IgA secretions; IgE allergy.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What is the most common white blood cell?
The neutrophil, making up about 60–70% of white cells.
Which white cell increases in parasitic infection?
The eosinophil.
Which antibody crosses the placenta?
IgG.
What is the difference between B and T lymphocytes?
B cells mature in the bone marrow and make antibodies; T cells mature in the thymus and provide cell-mediated immunity.