Phagocytic cell (Function of Immune Cells)

Writer: Mustafa A. Shahrori, MD.

Keywords: Phagocytes, Neutrophils, macrophages, dendritic cells chronic granulomatous disease (CGD), Chediak-Higashi syndrome, phagocytosis, pathogen evasion

Overview

Phagocytic cells, including neutrophils, monocytes, macrophages, and dendritic cells, form a crucial first line of defense in the immune system through their ability to engulf and destroy pathogens. These cells employ a sophisticated arsenal including lysosomal enzymes and reactive oxygen species generated during the respiratory burst. Neutrophils act as rapid first responders with their distinctive multilobed nuclei and specialized granules, while macrophages take on diverse tissue-specific roles from pathogen clearance to antigen presentation. Dendritic cells bridge innate and adaptive immunity through superior antigen presentation capabilities. Clinical disorders like Chronic Granulomatous Disease, Leukocyte Adhesion Deficiency, and Chediak-Higashi Syndrome highlight the critical importance of proper phagocyte function

Phagocytosis

Phagocytes, including macrophages, neutrophils, eosinophils, and basophils, are specialized immune cells that engulf and destroy pathogens through phagocytosis (Figure 1), a key process in innate immunity. While eosinophils and basophils have phagocytic capability, their primary roles are in defense against parasites and allergic responses, respectively.

The phagocytic process occurs in several distinct steps:

1.     Engulfment: Pathogens are recognized, captured, and engulfed into a membrane-bound vesicle called a phagosome.

2.     Fusion: The phagosome fuses with a lysosome (containing digestive enzymes) to form a phagolysosome.

3.     Enzymatic action (Table 1): Lysosomal enzymes, including proteases, nucleases, and lysozymes which break down peptidoglycans, work together to destroy the engulfed pathogens.

Figure 1. Phagocytosis. The figure illustrates the key steps of phagocytosis, including recognition, engulfment, and degradation of the target within phagolysosomes by immune cells such as macrophages, neutrophils, and dendritic cells.

. Table 1: Normal Phagocyte Killing Mechanisms

Abbreviations. NADPH, nicotinamide adenine dinucleotide phosphate.

Respiratory burst and reactive molecules

During phagocytosis, phagocytes undergo a process called respiratory burst, which generates antimicrobial reactive molecules. This process was discussed in detail in the Respiratory Burst article.

Pathogen evasion mechanisms

Some pathogens have evolved strategies to evade phagocytic destruction:

• Tuberculosis (Mycobacterium tuberculosis): Modifies the phagosome to prevent lysosome fusion, allowing it to proliferate within macrophages and evade antibody-mediated defenses, leading to the formation of caseating granulomas.

• Lung abscess-causing bacteria: Can block phagosome-lysosome fusion.

• Legionella pneumophila: Creates a replicative niche inside the phagosome.

·         Listeria monocytogenes: Escapes the phagosome into the cytosol, avoiding lysosomal destruction.

Neutrophils

Neutrophils, the most abundant leukocytes in circulation, serve as the primary first responders to infection and tissue injury.

Morphology and development

Neutrophils are characterized by their distinctive multilobed nucleus, which typically contains 3-5 segments connected by thin chromatin filaments (Figure 2). This nuclear morphology facilitates rapid migration through narrow tissue spaces.

The term "polymorphonuclear leukocyte" (PMN) derives from this distinctive nuclear configuration. Neutrophils progress through several developmental stages in the bone marrow:

 Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band cell → Mature neutrophil.

Figure 2. Neutrophil. A type of granulocyte and a key component of the innate immune system. They contain granules filled with antimicrobial proteins and enzymes that help eliminate microbes.

Granule types and contents

Neutrophils contain several types of granules that sequentially fuse with phagosomes during pathogen engulfment:

Primary (Azurophilic) granules:

• Myeloperoxidase

• Defensins

• Bacterial permeability-increasing protein

• Neutrophil elastase

• Cathepsin G

• Proteinase 3

Secondary (specific) granules:

• Lactoferrin

• Lysozyme

• Collagenase

• NADPH oxidase components

• Leukocyte alkaline phosphatase (LAP)

Tertiary granules:

• Gelatinase

• Cathepsins

• Matrix metalloproteinases

  
  

ALERT: Elevated leukocyte alkaline phosphatase (LAP) scores are seen in leukemoid reactions, infections, and polycythemia vera, while decreased LAP scores are characteristic of chronic myeloid leukemia (CML). This represents a key diagnostic distinction between reactive neutrophilia and CML.

Neutrophil recruitment and activation

The mobilization of neutrophils to infection sites follows a carefully orchestrated sequence (Figure 3):

1.     Margination and rolling: Mediated by selectins (P-selectin and E-selectin) on endothelial cells interacting with sialyl-Lewis X on neutrophils.

2.     Firm adhesion: Facilitated by β2 integrins (LFA-1, Mac-1) on neutrophils binding to ICAM-1 on activated endothelium.

3.     Diapedesis (Transmigration): Neutrophils squeeze through endothelial junctions-Platelet endothelial cell adhesion molecule 1 (PECAM-1)- to enter tissues.

4.     Chemotaxis: Directed migration along chemical gradients toward the infection site.

Key chemotactic factors include:

• Complement component C5a

• Interleukin-8 (IL-8)

• Leukotriene B4 (LTB4)

• N-formylmethionine peptides (from bacteria)

• Platelet-activating factor

Figure 3. Neutrophil recruitment and activation. Upon infection or tissue injury, Neutrophils migrate to infection sites, engulf pathogens through phagocytosis, and release antimicrobial substances to eliminate threats.

Neutrophil extracellular traps (NETs)

Beyond phagocytosis, neutrophils can release neutrophil extracellular traps (NETs), networks of extracellular fibers composed primarily of DNA from the neutrophil nucleus along with histones and antimicrobial proteins.

NETosis represents a form of programmed cell death distinct from apoptosis and necrosis. NETs physically trap pathogens while concentrating antimicrobial substances in their vicinity, but excessive NET formation has been implicated in autoimmune disorders and thrombosis.

Neutrophil morphological changes in disease

Several morphological changes in neutrophils serve as important diagnostic indicators:

• Left shift: Increased immature neutrophils (bands, metamyelocytes) in circulation, indicating acute infection or inflammation.

• Toxic granulation: Darker, more prominent granules in the cytoplasm during severe infections.

• Döhle bodies: Blue-gray cytoplasmic inclusions representing aggregates of rough endoplasmic reticulum.

• Hypersegmentation (Figure 4): Neutrophils with ≥5 nuclear lobes, characteristic of megaloblastic anemia due to vitamin B12 or folate deficiency.

Figure 4. Hypersegmented neutrophil.

Caponetti, G. (2014, May 20). Hypersegmented neutrophil [Image]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Hypersegmented_neutrophil_-_by_Gabriel_Caponetti,MD.jpg

Role in fungal defense

Unlike bacterial infections (where complement is crucial) or mucocutaneous fungal infections (where T-cells dominate), neutrophils provide essential protection against invasive fungal pathogens. This explains why neutropenic patients face substantially higher risk for life-threatening fungal dissemination compared to those with other immune deficiencies.

Neutropenia following chemotherapy dramatically increases risk for invasive fungal disease. Cancer patients with low neutrophil counts commonly develop candidemia, presenting with fever and positive blood cultures showing pseudohyphae and germ tube formation.

Monocytes and Macrophages

Monocytes and their tissue-resident derivatives, macrophages, represent a critical component of both innate and adaptive immunity, serving multiple roles in homeostasis and disease

Monocyte development and characteristics

Monocytes (Figure 5) develop from common myeloid progenitors in the bone marrow and circulate in the bloodstream for 1-3 days before migrating into tissues. They are characterized by:

• Large size (15-18 μm in diameter)

• Kidney-shaped nucleus

• Abundant "frosted glass" cytoplasm

• Presence of fine azurophilic granules

Monocytes constitute approximately 2-10% of circulating leukocytes. Their name derives from Greek roots meaning "single nucleus," distinguishing them from the multilobed neutrophils.

Figure 5. Monocyte to macrophage. Monocytes differentiate into macrophages upon entering tissues, playing a key role in immune defense and tissue repair.

Monocyte subsets and functions

Monocytes vary by CD14 and CD16 expression, influencing phagocytosis, inflammation, and surveillance.

Macrophage differentiation and polarization

Once monocytes enter tissues, they differentiate into macrophages under the influence of local factors. Macrophages (Figure 6) display remarkable plasticity and can adopt different phenotypes:

• M1 macrophages (Classically activated):

  • Induced by IFN-γ and LPS.

  • Produce pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IL-12).

  • Express high levels of MHC class II for efficient antigen presentation.

  • Generate reactive oxygen and nitrogen species.

  • Primarily microbicidal and tumoricidal.

• M2 macrophages (Alternatively activated):

  • Induced by IL-4, IL-13, IL-10, and TGF-β.

  • Produce anti-inflammatory cytokines (IL-10, TGF-β).

  • Involved in tissue repair, angiogenesis, and fibrosis.

  • Express high levels of scavenger and mannose receptors.

  • Contribute to helminth clearance and allergy responses.

Figure 6. Macrophage. A type of white blood cell that engulfs pathogens, clears debris, and regulates immune responses.

Tissue-specific macrophages

Macrophages adopt specialized phenotypes depending on their tissue location (Figure 7):

• Kupffer cells (Liver): Monitor blood for pathogens and clear senescent erythrocytes.

• Alveolar macrophages (Lungs): Clear inhaled particulates and pathogens.

• Microglia (Central nervous system): Maintain neuronal health and respond to CNS injury.

• Osteoclasts (Bone): Resorb bone matrix during bone remodeling.

• Histiocytes (Connective tissue): Patrol for tissue damage and infection.

• Splenic macrophages: Remove aging erythrocytes and process blood-borne antigens.

Figure 7. Macrophage differentiation. Monocytes differentiate into distinct macrophage subsets based on tissue signals, acquiring specialized functions in immunity and tissue repair.

Macrophage functions

Macrophages perform a diverse array of functions essential for homeostasis and defense (Figure 8):

1.     Phagocytosis: Clearance of pathogens, cellular debris, and apoptotic cells.

2.     Antigen presentation: Processing and presentation of antigens to T cells via MHC class II.

3.     Cytokine production: Secretion of inflammatory mediators that orchestrate immune responses.

4.     Tissue repair: Promotion of wound healing through the release of growth factors.

5.     Iron homeostasis: Recycling of iron from senescent erythrocytes.

6.     Lipid metabolism: Clearance of modified lipoproteins and regulation of cholesterol efflux.

Figure 8. Macrophage function.

Granuloma formation

In response to certain persistent pathogens or foreign materials, macrophages can form granulomas (Figure 9) (Figure 10), organized structures composed of epithelioid macrophages often surrounded by lymphocytes.

NOTE: Tuberculosis represents the prototypical granulomatous disease, characterized by "caseating granulomas" with central necrosis. M. tuberculosis survives within macrophages by preventing phagolysosomal fusion, emphasizing how pathogen evasion strategies directly influence disease pathology. Proper identification of the morphological features of granulomas is crucial for differential diagnosis among infectious, autoimmune, and foreign body reactions.

Figure 9. Granuloma formation. A structured aggregation of immune cells, primarily macrophages, formed in response to persistent infections or foreign substances, serving to contain and isolate pathogens.

Figure 10.Granuloma formation.

Mukhopadhyay, S. (2009, April 23). Non-necrotizing granuloma in a lymph node in the neck [Scientific figure]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Granuloma_mac.jpg

Dendritic Cells

Dendritic cells (Figure 11) (DCs) represent a specialized lineage of antigen-presenting cells that excel at capturing, processing, and presenting antigens to T lymphocytes, thereby initiating adaptive immune responses.

Figure 11. Dendritic cell.

Behnsen, J., Narang, P., Hasenberg, M., Gunzer, F., Bilitewski, U., Klippel, N., Rohde, M., Brock, M., Brakhage, A. A., & Gunzer, M. (2007, February 2). A well-resolved dendritic cell drags a conidium through a distance of up to 9 μm. The conidium, however, is not phagocytosed by the cell [Screen clip]. PLoS Pathogens. Retrieved from https://commons.wikimedia.org/wiki/File:Dendritic_cell.JPG

Development and classification

Dendritic cells originate from both myeloid and lymphoid progenitors in the bone marrow and can be classified into several subtypes:

1.     Conventional DCs (cDCs): Specialized in cross-presentation to CD8+ T cells.

2.     Plasmacytoid DCs (pDCs): Morphologically resemble plasma cells

3.     Langerhans cells: Reside in stratified epithelia, particularly the epidermis

Dendritic cell maturation

Dendritic cells exist in two functional states (Figure 12):

Immature DCs:

• Low surface expression of MHC and co-stimulatory molecules

• Efficient at antigen capture

• Reside primarily in peripheral tissues

Mature DCs:

• High expression of MHC class I and II

• Upregulated co-stimulatory molecules (CD80, CD86, CD40)

• Enhanced expression of chemokine receptor CCR7

• Migration to lymphoid organs

Figure 12. Dendritic cell maturation. 

Once internalized, antigens are processed through two main pathways:

• MHC class II pathway: For exogenous antigens; involves endosomal degradation and loading onto MHC class II molecules for presentation to CD4+ T cells.

• Cross-presentation: Allows exogenous antigens to enter the MHC class I pathway, enabling the activation of CD8+ T cells against viruses and tumors that do not directly infect DCs.

Migration and T cell activation

Following antigen capture and maturation, dendritic cells migrate to lymphoid tissues through several steps:

1.     Release from tissue anchoring via E-cadherin downregulation

2.     Expression of matrix metalloproteinases to facilitate movement through extracellular matrix

3.     Upregulation of CCR7, which responds to chemokines CCL19 and CCL21 expressed in lymphatics

4.     Entry into lymphatic vessels and trafficking to T cell zones of lymph nodes

Within lymphoid tissues, DCs provide three essential signals for T cell activation (Figure 13):

1.     Signal 1: Antigen recognition via MHC-peptide complex binding to the T cell receptor

2.     Signal 2: Co-stimulation through CD80/CD86 binding to CD28 on T cells

3.     Signal 3: Cytokine secretion that dictates T cell differentiation (e.g., IL-12 for TH1, IL-4 for TH2)

The "three-signal" model of T cell activation explains why antigen recognition in the absence of proper co-stimulation leads to anergy rather than activation. This concept underpins the mechanism of action for co-stimulation blockade therapies used in autoimmune diseases and transplantation.

Figure 13. Dendritic cells capture and process the cancer antigens and present these as peptides in association with MHC on the surface of the cells

ResearchGate. (2025, March 13). PD-L1 immunostaining: What pathologists need to know [Scientific figure]. ResearchGate. https://www.researchgate.net/figure/Dendritic-cells-capture-and-process-the-cancer-antigens-and-present-these-as-peptides-in_fig2_355545146

Clinical relevance

Langerhans cell histiocytosis (LCH):

• Clonal proliferation of CD1a+ Langerhans-like cells

• Presentation ranges from isolated bone lesions to multisystem disease

• Classified as an inflammatory myeloid neoplasm

• Treatment depends on extent: observation for localized disease, chemotherapy for multisystem involvement

Comparison Overview of Innate Immune Cells

Table 2: Innate immune cells.

Abbreviations. LAP; Leukocyte alkaline phosphatase, MPO; Myeloperoxidase  MHC II; Major histocompatibility complex class II,  IFN-γ; Interferon-gamma TNF-α; Tumor necrosis factor-alpha,  IL; Interleukin, LPS; Lipopolysaccharide, CD; Cluster of differentiation, CML; Chronic myelogenous leukemia, FcεRI; High-affinity IgE receptor c-KIT tyrosine kinase receptor (Mastocytosis), ADCC; Antibody-dependent cell-mediated cytotoxicity.

References 

Abbas, A. K., Lichtman, A. H., & Pillai, S. (2023). Cellular and molecular immunology (10th ed.). Elsevier.

Murphy, K., & Weaver, C. (2022). Janeway's immunobiology (10th ed.). W.W. Norton & Company.