Neutropenia and Neutrophil Dysfunction

Neutrophils are the most numerous circulating leukocytes and the front line of defence against bacterial and fungal infection. Disorders in this group fall into two broad categories: neutropenia, a reduced neutrophil count, and neutrophil dysfunction, in which adequate numbers of neutrophils fail to work normally. Both increase susceptibility to infection.

Definition

Neutropenia is a reduction in the absolute neutrophil count below the population reference range; neutrophil dysfunction is impaired neutrophil activity (adhesion, migration, phagocytosis, or microbial killing) despite adequate cell numbers.

Scope

This topic covers the definition and grading of neutropenia, its main acquired and inherited causes (including severe congenital neutropenia), and the principal disorders of neutrophil function such as chronic granulomatous disease, in which the phagocyte oxidase is defective. It frames how these disorders are recognised and categorised in the laboratory. It is a reference overview and does not provide diagnostic thresholds for, or management of, any individual.

Core questions

• How is neutropenia defined, graded, and distinguished from neutrophil dysfunction?
• What are the main acquired and inherited causes of neutropenia?
• How do inherited neutrophil function defects such as chronic granulomatous disease arise?
• Why do quantitative and qualitative neutrophil defects predispose to infection?

Key concepts

• Absolute neutrophil count
• Acquired versus congenital neutropenia
• Severe congenital neutropenia
• Agranulocytosis
• Cyclic neutropenia
• Neutrophil function (phagocytosis and oxidative killing)
• Chronic granulomatous disease
• Phagocyte NADPH oxidase

Mechanisms

Neutropenia can result from decreased production in the marrow, increased peripheral destruction or consumption, or redistribution of cells; common acquired causes include drug effects, infection, and immune mechanisms, while inherited forms such as severe congenital neutropenia arise from germline mutations that disrupt neutrophil maturation and survival (Klein 2011; Boztug 2011). Neutrophil dysfunction, by contrast, reflects a defect in how the cell performs - for example, in chronic granulomatous disease, mutations in components of the phagocyte NADPH oxidase impair the respiratory burst, so neutrophils ingest organisms but cannot kill certain catalase-positive bacteria and fungi effectively (Nauseef 2019). In both situations the shared consequence is a reduced capacity to control microbial invasion, and the laboratory work-up aims to establish whether the problem is one of cell number, cell function, or both (Dale 2017).

Clinical relevance

Neutropenia and neutrophil dysfunction are important because they explain a pattern of recurrent or severe bacterial and fungal infection, and their recognition shapes how such susceptibility is investigated at the laboratory level. This entry describes the categories and mechanisms of these disorders for reference; it does not provide diagnostic cut-offs or treatment recommendations for any individual.

Epidemiology

Severe congenital neutropenia is rare, while acquired neutropenia - particularly drug-related and infection-related - is far more common; ethnic variation in baseline neutrophil counts is also recognised and must be considered when interpreting an individual count (Dale 2017).

Evidence & guidelines

Authoritative narrative syntheses describe the evaluation of neutropenia across age groups (Dale 2017) and the genetic basis of severe congenital neutropenia (Klein 2011; Boztug 2011), while reviews of the phagocyte NADPH oxidase frame the prototypic functional defect, chronic granulomatous disease (Nauseef 2019).

Seminal works

• klein-2011
• dale-2017
• nauseef-2019

Frequently asked questions

What is the difference between neutropenia and neutrophil dysfunction?

Neutropenia means there are too few neutrophils, whereas neutrophil dysfunction means the neutrophils are present in adequate numbers but do not work properly; both can leave a person more vulnerable to infection.

Why does chronic granulomatous disease cause infections despite normal neutrophil counts?

In chronic granulomatous disease the neutrophils can engulf microbes but a defect in the phagocyte NADPH oxidase impairs the oxidative killing step, so certain bacteria and fungi survive inside the cell.