Overview
White blood cells (leukocytes) are complete cells that defend the body against harmful agents and abnormal cells. They differ from red blood cells in structure, lifespan, abundance, and ability to exit blood vessels.
White Blood Cells vs Red Blood Cells
- White blood cells are complete cells with nucleus and organelles.
- Red blood cells lack nucleus and many organelles for oxygen transport specialization.
- White blood cells have shorter lifespans: minutes to days depending on type.
- Red blood cells average lifespan is about 120 days.
- White blood cells are less abundant than red blood cells in blood.
- White blood cells can leave blood vessels and enter tissues via diapedesis.
Abundance and Lifespan Details
| Cell Type | Typical Count per Microliter | Relative Abundance | Typical Lifespan |
|---|
| Red blood cells | ~5,000,000 | Much more common | ~120 days |
| White blood cells | ~5,000–10,000 | Much less common | Minutes to days, type dependent |
Diapedesis and Tissue Migration
- White blood cells use blood vessels for transport to target sites.
- They squeeze between vessel cells to enter tissues; process is diapedesis.
- Diapedesis allows rapid response at infection or injury sites.
Leukocyte Classification
- Two main groups based on granules after staining.
- Granulocytes: neutrophils, eosinophils, basophils.
- Agranulocytes: lymphocytes, monocytes.
- Granulocytes have lobed nuclei and short lifespans.
Granulocytes: Neutrophils
- Most abundant leukocyte: about 50–70% of leukocytes.
- Granules visible with neutral stains; cells appear light lilac.
- Nucleus has 2–5 lobes; more lobes with cell age.
- Primary defenders against bacteria via phagocytosis.
- Attracted to infection sites by chemicals like cytokines.
Neutrophil Granule Contents and Functions
| Substance | Location | Function |
|---|
| Lysozyme | Neutrophil granules | Breaks down bacterial cell walls |
| Hydrogen peroxide | Neutrophil granules | Contributes to microbial killing |
| Defensins | Neutrophil granules | Disrupts microbial membranes |
Granulocytes: Eosinophils
- Comprise about 2–4% of leukocytes.
- Granules stain with acidic eosin; nuclei typically 2–3 lobes.
- Involved in allergic reactions and parasitic worm infections.
- Participate in chronic inflammation and some autoimmune diseases.
- Elevated eosinophils (eosinophilia) indicate related conditions.
Granulocytes: Basophils
- Rarest granulocyte: less than 1% of leukocytes.
- Granules stain dark blue with basic stains and are larger.
- Intensify local inflammatory responses.
- Contain histamines and other inflammatory mediators.
Agranulocytes: Lymphocytes
- Include natural killer (NK) cells, B cells, and T cells.
- NK cells kill virus-infected, cancerous, or abnormal cells.
- B cells mediate humoral immunity by producing antibodies.
- T cells mediate cellular immunity by attacking diseased or foreign cells.
- B and T cells form memory cells after exposure for faster responses.
Specific vs Nonspecific Immunity
| Type | Key Cells | Targeting | Mechanism |
|---|
| Nonspecific immunity | Neutrophils, other leukocytes | General harmful agents | Broad attack and inflammation |
| Specific immunity | B cells, T cells | Specific antigens | Antibodies (humoral) and cell-mediated attack |
Agranulocytes: Monocytes and Macrophages
- Monocytes constitute about 2–8% of leukocytes.
- Larger cells that circulate then enter tissues.
- Differentiate into macrophages within tissues.
- Perform phagocytosis of bacteria and antigenic material.
- Eliminate threats directly or recruit other immune cells.
Key Terms & Definitions
- Leukocytes: White blood cells involved in immune defense.
- Diapedesis: Movement of white blood cells out of vessels into tissues.
- Granulocytes: Leukocytes with visible cytoplasmic granules and lobed nuclei.
- Agranulocytes: Leukocytes with small or less visible granules.
- Phagocytosis: Engulfing and digesting pathogens or debris.
- Cytokines: Chemicals that attract and activate immune cells.
- Humoral immunity: Antibody-mediated specific immunity by B cells.
- Cellular immunity: T cell-mediated destruction of diseased or foreign cells.
- Eosinophilia: Higher than normal eosinophil levels in blood.
Action Items / Next Steps
- Review granulocyte features: staining, nucleus lobes, and functions.
- Memorize leukocyte percentages and roles in immunity.
- Understand diapedesis and phagocytosis mechanisms.
- Distinguish specific vs nonspecific immunity and key cell types.