White Blood Cells (Leukocytes): Overview and Practice Questions

Leukocytes, commonly known as white blood cells, play a central role in the body’s immune defense system. They are responsible for identifying, targeting, and eliminating pathogens such as bacteria, viruses, fungi, and other foreign substances.

In the context of respiratory health, leukocytes are especially important because the lungs are constantly exposed to airborne particles and infectious agents.

For respiratory therapists, understanding leukocytes is essential for interpreting lab results, recognizing infection and inflammation, and making informed clinical decisions in both acute and chronic respiratory conditions.

What Is a Leukocyte?

A leukocyte is a type of blood cell involved in the immune response. Unlike red blood cells, leukocytes contain a nucleus and do not carry oxygen. Instead, their primary function is to protect the body from infection, inflammation, and tissue damage. Leukocytes circulate in the bloodstream and lymphatic system but can also migrate into tissues when an immune response is triggered.

Normal leukocyte counts typically range from 4,000 to 11,000 cells per microliter of blood, though this range can vary slightly depending on the laboratory and patient population. An increase in leukocytes is called leukocytosis, while a decrease is known as leukopenia. Both conditions are clinically significant and frequently encountered in respiratory care.

Types of White Blood Cells and Their Functions

Leukocytes are divided into five main types, each with distinct roles in immune defense. Understanding these differences is particularly important for respiratory therapists, as specific patterns often correlate with respiratory disease processes.

Neutrophils

Neutrophils are the most abundant type of leukocyte and are often the first responders to infection. They play a critical role in fighting bacterial infections by engulfing and destroying pathogens through a process known as phagocytosis.

In respiratory care, elevated neutrophil levels are commonly associated with:

• Bacterial pneumonia
• Acute exacerbations of COPD
• Sepsis related to pulmonary infections
• Aspiration pneumonia

Note: A left shift, which refers to an increase in immature neutrophils (bands), often signals an acute bacterial infection and may indicate worsening respiratory status.

Lymphocytes

Lymphocytes are key players in adaptive immunity and include T cells, B cells, and natural killer (NK) cells. They are especially important in viral defense and long-term immune memory.

Respiratory conditions commonly associated with lymphocyte changes include:

• Viral pneumonia
• Influenza
• COVID-19
• Tuberculosis

Note: Low lymphocyte counts, or lymphopenia, are frequently seen in severe viral infections and are associated with poor outcomes in acute respiratory failure.

Monocytes

Monocytes are large leukocytes that migrate into tissues and differentiate into macrophages or dendritic cells. In the lungs, macrophages are essential for clearing debris, pathogens, and dead cells from the alveoli.

Monocyte elevations may be seen in:

• Chronic lung infections
• Tuberculosis
• Sarcoidosis
• Prolonged inflammatory lung conditions

Note: For respiratory therapists, monocyte involvement often suggests a more chronic or evolving disease process rather than an acute infection.

Eosinophils

Eosinophils are involved in allergic responses and parasitic infections. They are especially relevant in respiratory care due to their role in asthma and other eosinophilic lung diseases.

Elevated eosinophil levels are associated with:

• Allergic asthma
• Eosinophilic pneumonia
• Drug-induced lung reactions
• Certain parasitic infections

Note: Eosinophil counts are increasingly used to guide asthma management and determine responsiveness to corticosteroids and biologic therapies.

Basophils

Basophils are the least common leukocytes and play a role in allergic and inflammatory reactions by releasing histamine and other mediators.

Although less frequently emphasized, basophils contribute to:

• Hypersensitivity reactions
• Severe allergic asthma
• Anaphylaxis

Note: Their role is often indirect but still clinically relevant in airway inflammation and allergic respiratory disease.

Leukocytes and Respiratory Infections

The respiratory system is constantly exposed to environmental pathogens, making leukocytes a frontline defense mechanism. When infection occurs, leukocytes migrate to the lungs, airways, and alveoli to combat invading organisms.

In bacterial pneumonia, neutrophils dominate the immune response, often leading to purulent sputum and consolidation seen on imaging. Viral infections, by contrast, tend to trigger lymphocyte-mediated responses and may produce more diffuse interstitial patterns on chest imaging.

Respiratory therapists often correlate leukocyte patterns with:

• Sputum characteristics
• Chest X-ray or CT findings
• Arterial blood gas abnormalities
• Oxygen requirements and ventilatory support

Note: Understanding leukocyte behavior helps guide expectations for disease progression and recovery.

Leukocytes in Chronic Respiratory Disease

Chronic respiratory conditions involve ongoing inflammation, and leukocytes are central to this process. In COPD, neutrophils and macrophages contribute to airway remodeling, mucus hypersecretion, and progressive airflow limitation. Elevated leukocyte counts during exacerbations often signal infection and the need for antibiotics or escalated therapy.

In asthma, eosinophils play a dominant role in airway inflammation, bronchoconstriction, and hyperresponsiveness. Monitoring eosinophil levels can help predict exacerbation risk and tailor long-term treatment strategies.

Interstitial lung diseases often involve monocytes and lymphocytes, reflecting immune-mediated injury and fibrosis rather than acute infection.

Leukocytes and Mechanical Ventilation

For patients receiving mechanical ventilation, leukocyte counts are closely monitored due to their association with infection, inflammation, and ventilator-associated complications.

Ventilator-associated pneumonia typically presents with:

• Rising leukocyte counts
• Neutrophil predominance
• Worsening oxygenation
• Increased secretions

Systemic inflammation reflected by leukocytosis can also indicate sepsis or acute respiratory distress syndrome (ARDS), both of which significantly impact ventilator management and patient outcomes.

Note: Respiratory therapists use leukocyte trends to anticipate changes in lung compliance, oxygenation requirements, and weaning readiness.

Leukopenia and Respiratory Risk

Low leukocyte counts pose a serious risk in respiratory patients. Leukopenia may result from chemotherapy, immunosuppressive therapy, bone marrow disorders, or severe viral infections.

Patients with leukopenia are more susceptible to:

• Opportunistic respiratory infections
• Rapid progression of pneumonia
• Poor wound healing following airway procedures

Note: Respiratory therapists must exercise heightened infection control measures and closely monitor these patients for subtle signs of respiratory deterioration.

Clinical Interpretation for Respiratory Therapists

Leukocyte values should never be interpreted in isolation. Respiratory therapists integrate these lab findings with clinical assessment, imaging, and respiratory mechanics.

Key considerations include:

• Trends rather than single values
• Differential counts instead of total leukocyte count alone
• Correlation with vital signs and oxygenation
• Response to antibiotics, steroids, or bronchodilators

Note: This holistic approach supports accurate clinical judgment and improves patient outcomes.

White Blood Cell Practice Questions

1. What does an elevated overall white blood cell (WBC) count most commonly indicate?
An active immune response, often due to infection, inflammation, stress, or trauma.

2. What type of infection is suggested by elevated neutrophils and bands?
An acute bacterial infection.

3. What is the normal adult range for total white blood cell count?
Approximately 4,500 to 11,500 cells/mm³.

4. What term describes an abnormally elevated white blood cell count?
Leukocytosis

5. What does leukocytosis most often reflect clinically?
A vigorous immune response to infection, inflammation, or trauma.

6. At what WBC level should clinicians be concerned about a serious infection?
When the WBC count exceeds 20 × 10³/µL.

7. What term describes a white blood cell count below the normal range?
Leukopenia (leukocytopenia)

8. What does leukopenia often indicate about immune system function?
The immune system may be overwhelmed or suppressed.

9. Which conditions commonly cause leukopenia?
Severe infection, AIDS, chemotherapy, bone marrow suppression, or immunosuppressive therapy.

10. Why is a WBC count with differential ordered in postoperative patients with low-grade fever?
To help rule out bacterial infection, such as pneumonia.

11. Why are blood cultures ordered when sepsis is suspected?
To identify the causative organism and guide antibiotic therapy.

12. What is the complete blood count (CBC)?
A laboratory test that measures white blood cells, red blood cells, and platelets.

13. What are white blood cells also called?
Leukocytes

14. What are red blood cells called?
Erythrocytes

15. What are platelets called?
Thrombocytes

16. What information does the WBC differential provide?
The number and proportion of each type of white blood cell.

17. Which two white blood cell types make up the majority of circulating WBCs?
Neutrophils and lymphocytes

18. Which white blood cell types normally make up only a small percentage of the total count?
Basophils, eosinophils, and monocytes.

19. Why is the absolute WBC differential more clinically useful than percentages alone?
Because percentage changes can be misleading when one cell type increases significantly.

20. How is the absolute differential count calculated?
By multiplying the total WBC count by the percentage of each cell type.

21. What is neutrophilia?
An absolute increase in circulating neutrophils.

22. What are immature neutrophils called?
Bands

23. Why are immature neutrophils referred to as “bands”?
Because their nucleus has a band-shaped appearance.

24. What are mature neutrophils called?
Segmented neutrophils (segs)

25. What does the presence of elevated bands and segs indicate?
A severe or acute bacterial infection.

26. What causes the bone marrow to release bands into circulation?
Severe infection requiring increased neutrophil production.

27. What is neutropenia?
A decrease in the number of circulating neutrophils.

28. Why does neutropenia increase infection risk?
Because neutrophils are essential for fighting bacterial infections.

29. Which conditions are commonly associated with neutropenia?
Bone marrow disease, leukemia, lymphoma, HIV infection, and chemotherapy.

30. What does an elevated WBC count most often result from?
An increase in neutrophils or lymphocytes responding to infection.

31. Why is lymphocyte analysis important in HIV infection?
HIV targets and destroys CD4 T lymphocytes.

32. What CD4 count is commonly associated with opportunistic infections?
Less than 200 × 10⁶/L

33. What opportunistic infection is commonly seen with severe lymphopenia in HIV?
Pneumocystis jirovecii pneumonia

34. What does a falling lymphocyte count suggest in an immunocompromised patient?
Progressive immune system failure.

35. Why are antibiotics not recommended after foreign-body removal unless infection is evident?
Because inflammation and bronchial hyperresponsiveness can occur without infection.

36. What WBC-related findings support starting antibiotics?
Fever, purulent secretions, and elevated WBC count.

37. What does bronchorrhea indicate after airway irritation?
Increased airway secretions requiring airway clearance therapy.

38. Why might inhaled corticosteroids and bronchodilators be used after foreign-body removal?
To treat transient bronchial hyperresponsiveness causing cough and wheeze.

39. How does trauma affect the white blood cell count?
It often causes leukocytosis due to stress and inflammation.

40. What is the key clinical takeaway when interpreting WBC values?
They must be evaluated alongside the differential, clinical signs, and patient context.

41. What is another name for white blood cells?
Leukocytes

42. Which formed elements of the blood are complete cells with nuclei?
White blood cells (WBCs)

43. What is leukopoiesis?
The production of white blood cells.

44. How do leukocytes differ from red blood cells in structure?
Leukocytes are nucleated and do not contain hemoglobin.

45. What are the two principal categories of white blood cells?
Granular and agranular leukocytes

46. Which white blood cells are classified as granular leukocytes?
Neutrophils, eosinophils, and basophils.

47. Which white blood cells are considered agranular?
Lymphocytes and monocytes

48. What is diapedesis?
The process by which white blood cells migrate between capillary endothelial cells.

49. Approximately how many white blood cells are present per drop of blood?
About 5,000 to 10,000 cells.

50. What term describes an abnormally high white blood cell count?
Leukocytosis

51. List three common causes of leukocytosis.
Infection, surgery, and anesthesia.

52. What term describes an abnormally low white blood cell count?
Leukopenia

53. List three causes of leukopenia.
Radiation exposure, shock, and chemotherapy.

54. What percentage of the total white blood cell population is found circulating in the blood?
Approximately 2%

55. Where are most white blood cells normally located?
In lymphatic fluid, skin, lungs, lymph nodes, and spleen.

56. What is a differential white blood cell count?
A test that measures the number of each type of circulating white blood cell.

57. What does a differential white blood cell count help identify clinically?
The type and severity of infection or immune response.

58. What is the term for white blood cells leaving the bloodstream?
Emigration

59. What immune function involves engulfing and digesting pathogens?
Phagocytosis

60. Which white blood cells are most active in phagocytosis?
Neutrophils and macrophages

61. What is chemotaxis?
The chemical attraction of white blood cells to a site of injury or infection.

62. Which granulocyte has a nucleus with 2 to 5 lobes and fine, pale granules?
Neutrophil

63. What percentage of circulating white blood cells are neutrophils?
Approximately 55% to 70%.

64. What is the primary role of neutrophils in immune defense?
Rapid response to bacterial infection using enzymes and oxidants.

65. Which granulocytes have a bilobed nucleus and account for 1% to 3% of WBCs?
Eosinophils

66. What substances are released by eosinophils?
Histaminase and antiparasitic enzymes.

67. Which white blood cells are involved in parasitic infections and allergic reactions?
Eosinophils

68. Which granulocytes contain large dark granules and make up less than 1% of WBCs?
Basophils

69. What is the primary role of basophils?
Mediating inflammatory and allergic reactions.

70. Which substances are released by basophils?
Heparin, histamine, and serotonin.

71. Which agranulocyte has a large, dark nucleus and increases during viral infections?
Lymphocyte

72. What percentage of circulating white blood cells are lymphocytes?
Approximately 20% to 35%.

73. Which lymphocytes produce antibodies?
B lymphocytes

74. Which lymphocytes attack viruses, fungi, cancer cells, and transplanted tissue?
T lymphocytes

75. Which lymphocytes provide nonspecific immune defense against tumors and microbes?
Natural killer (NK) cells

76. Which white blood cell is the largest in circulation and has a kidney-shaped nucleus?
Monocyte

77. What percentage of circulating white blood cells are monocytes?
Approximately 3% to 8%.

78. What do monocytes become once they enter tissues?
Macrophages

79. What is the primary function of macrophages?
Phagocytosis of pathogens and removal of dead tissue.

80. What is a “band” cell?
An immature neutrophil released during severe infection.

81. What structural feature distinguishes white blood cells from red blood cells?
The presence of nuclei in mature white blood cells.

82. What is the general function of white blood cells?
To protect the body from pathogens and provide immunity.

83. Which white blood cells are the most abundant phagocytes?
Neutrophils

84. Which white blood cells help recognize foreign antigens?
T lymphocytes

85. What laboratory finding suggests bone marrow response to severe infection?
An increased number of band neutrophils.

86. A high white blood cell count is called __________ and often indicates __________.
Leukocytosis; infection.

87. A low white blood cell count is called __________. Name one cause.
Leukopenia; radiation exposure.

88. What does HLA stand for, and what do these antigens represent?
Human leukocyte antigens; markers representing the antigens of all cells in an individual.

89. Which white blood cells are most involved in early, nonspecific immune defense?
Neutrophils

90. Which white blood cells play a major role in long-term, adaptive immunity?
Lymphocytes

91. What type of white blood cell increases during parasitic infections?
Eosinophils

92. Which white blood cell type is primarily responsible for antigen presentation?
Monocytes (macrophages)

93. Why are neutrophils often the first cells to arrive at an infection site?
They respond rapidly to chemotactic signals.

94. What change in the WBC differential is commonly seen in viral infections?
An increase in lymphocytes (lymphocytosis).

95. What WBC abnormality increases the risk of opportunistic infections?
Neutropenia

96. Which white blood cells release cytokines to regulate immune responses?
Lymphocytes and monocytes

97. What is the primary function of memory B cells?
To provide rapid antibody production upon re-exposure to an antigen.

98. Which WBCs are most associated with immediate hypersensitivity reactions?
Basophils

99. What happens to white blood cells after they migrate into tissues?
They perform immune functions and may differentiate into specialized cells.

100. Why is the absolute neutrophil count clinically important?
It helps assess infection risk and immune competence.

Final Thoughts

Leukocytes play a critical role in immune defense and are highly relevant to respiratory care. From acute conditions such as pneumonia to chronic diseases like asthma and COPD, leukocyte patterns offer important insight into disease activity, severity, and response to treatment.

For respiratory therapists, understanding how leukocytes function and how to interpret their levels supports better clinical judgment, more effective patient monitoring, and stronger collaboration with the healthcare team.