Phagocytosis

How phagocytosis works

Phagocytosis is cellular eating at scale. A typical neutrophil arrives at an infection site, finds a bacterium, wraps it up, and disposes of it inside the cell in about a minute. Then it does the same to the next bacterium, and the next. A swarm of neutrophils can clear a billion bacteria from a wound in a day. Macrophages work similarly but live longer and eat continuously over months in tissue.

The process unfolds in five stages.

Stage 1: Chemotaxis. Phagocytes patrol blood and tissue. When infection occurs, complement cleavage at the site releases C5a, bacterial protein synthesis releases formyl peptides (fMLF), and tissue cells secrete chemokines (CXCL8/IL-8 for neutrophils, CCL2 for monocytes). Phagocytes detect these gradients through G-protein-coupled receptors and crawl up them, polarizing their cytoskeleton and migrating at ~10-20 μm/minute.

Stage 2: Recognition. Receptors on the phagocyte's surface engage the target. Pattern-recognition receptors (TLR2, TLR4, dectin-1, mannose receptor) bind microbial molecules (lipoteichoic acid, LPS, β-glucan, mannans). Opsonin receptors (FcγR for IgG-coated targets, CR1 and CR3 for complement-coated targets) bind the immunoglobulin or complement tags applied by the immune system. Scavenger receptors bind oxidized lipids and damaged self-molecules. Different receptors trigger somewhat different downstream pathways and shape what happens next.

Stage 3: Engulfment. Receptor clustering activates Src-family kinases, which phosphorylate ITAMs (in FcγR) or trigger guanine nucleotide exchange factors (for CR3). This activates Rho-family GTPases — Cdc42, Rac1 — which drive actin nucleation through WASP and Arp2/3 complex. New F-actin polymerizes beneath the contact patch, pushing the membrane outward into pseudopods that climb up around the target. The Fc-receptor route uses an extending "zipper" mechanism where each new contact pulls the membrane tighter; the CR3 route uses a "sinking" mechanism where the target settles into a depression. Either way, the membrane closes over the top within 30-90 seconds.

Stage 4: Phagosome formation. The pinched-off vesicle is initially indistinguishable from the cell surface — same membrane, same composition. Within minutes the phagosome matures: Rab5 accumulates (early endosome marker), then Rab7 (late endosome), then LAMP1 and LAMP2 (lysosome). Vacuolar H+-ATPase pumps protons into the lumen, acidifying from pH ~7 toward 4.5. NADPH oxidase components (p22phox, p47phox, p67phox, p40phox, Rac2) assemble on the cytoplasmic face and pump electrons into the lumen, generating superoxide.

Stage 5: Phagolysosome maturation and killing. Lysosomes fuse with the phagosome, dumping cathepsins B/D/L/S, antimicrobial peptides (α and β defensins, cathelicidin LL-37), lactoferrin (iron chelation), and lysozyme (peptidoglycan cleavage). In neutrophils, myeloperoxidase combines H2O2 with chloride to make hypochlorous acid — household bleach inside the phagosome. Most ingested bacteria are killed within 15-30 minutes. The phagocyte then recycles components, ejects digested debris through exocytosis, and returns to scanning for the next target.

Worked clinical example: bacterial pneumonia clearance

A patient inhales Streptococcus pneumoniae. Within minutes, complement C3b deposits on the bacterial capsule through the alternative pathway. Alveolar macrophages — the tissue-resident phagocytes lining the alveoli — recognize the C3b through CR3 (CD11b/CD18). One alveolar macrophage starts engulfing pneumococci at ~25 per minute. Cycle time per bacterium: ~2 seconds receptor engagement + ~30 seconds pseudopod extension + ~30 seconds closure = ~60 seconds per cycle, but the macrophage runs multiple cycles in parallel across different membrane regions.

For mild bacterial loads (10⁴-10⁵ CFU), resident alveolar macrophages clear the inoculum and the patient remains asymptomatic. For larger inocula or virulent serotypes, the macrophages secrete IL-8 (CXCL8) which recruits neutrophils from blood. The first neutrophils arrive in 4-6 hours; peak influx at 24-48 hours. A single inflamed alveolus may contain 10⁵-10⁶ neutrophils — and at 10-20 phagocytic events per neutrophil per minute, total clearance capacity reaches 10⁹ bacteria/hour. Hence the speed of pneumonia resolution: with adequate phagocyte recruitment and adequate opsonization (anti-capsule IgG from prior exposure or vaccination), bacterial burden falls 100-1000× per day.

If phagocytosis is impaired — chronic granulomatous disease, neutropenia from chemotherapy, or HIV-induced neutrophil dysfunction — the same inoculum can progress to severe pneumonia with bacterial counts exceeding 10⁸ CFU/mL in lung. Mortality from invasive pneumococcal disease ranges from 5% in healthy adults to 25-50% in immunocompromised hosts.

Stages of phagocytosis

Stage	Duration	Key molecules	Output
Chemotaxis	Minutes to hours	C5a, fMLF, CXCL8 ↔ GPCRs	Phagocyte arrives at infection site
Recognition	Seconds	PRRs (TLR, dectin), FcγR, CR1/3, scavenger	Target identified, signaling initiated
Engulfment	~30-90 seconds	Cdc42, Rac1 → WASP → Arp2/3 → F-actin	Pseudopods wrap target
Phagosome formation	Seconds to minutes	Membrane fission, Rab5 acquisition	Sealed intracellular vesicle
Phagolysosome maturation	5-30 minutes	V-ATPase, NADPH oxidase, lysosomal fusion	Acidified, oxidative, hydrolytic environment
Killing & digestion	15-60 minutes	ROS, HOCl, defensins, cathepsins	Microbe dead, contents degraded
Resolution	Hours to days	Efferocytosis, neutrophil apoptosis	Inflammation subsides, tissue repair

The architecture is conserved across species — even single-celled amoebae use the same actin-based engulfment, suggesting phagocytosis predates immunity by a billion years.

Variants and molecular details

• Efferocytosis. Phagocytosis of apoptotic cells, signaled by surface "eat me" molecules (phosphatidylserine flipped to the outer leaflet) that bind phagocyte receptors (TIM-1, TIM-4, BAI1, stabilin-2). The engulfed dying cell is silently digested — no inflammation. Defective efferocytosis contributes to lupus and atherosclerosis.
• LC3-associated phagocytosis (LAP). A subset of phagosomes recruits autophagy machinery (LC3, Beclin-1) to enhance phagolysosomal killing and antigen presentation. Important for antifungal immunity and clearance of dying cells.
• NETosis. A neutrophil suicide program where chromatin decondensed by PAD4 is extruded as DNA-protein traps (NETs) studded with antimicrobial granules. Catches large extracellular pathogens (some bacteria, fungal hyphae) that resist phagocytosis. Excess NETosis drives autoimmunity and thrombosis.
• Trained immunity. Prior exposure to certain stimuli (BCG vaccine, β-glucan, oxidized LDL) epigenetically reprograms macrophages to respond more vigorously to subsequent challenges. Heterologous protection lasts months.
• M1 vs M2 polarization. Macrophages adopt distinct functional states: M1 (IFN-γ + LPS-driven) is pro-inflammatory, microbicidal, antigen-presenting; M2 (IL-4/IL-13-driven) is tissue-repair, anti-inflammatory, profibrotic. Tumor-associated macrophages skew M2 and support cancer growth.
• Phagosome maturation arrest. Mycobacterium tuberculosis blocks Rab7 acquisition, preventing phagolysosome fusion — replicates inside an arrested phagosome for years. This is the cellular basis of latent TB.

Disease relevance

• Chronic granulomatous disease (CGD). NADPH oxidase deficiency. Recurrent suppurative infections with catalase-positive organisms; granulomas in skin, lung, liver. Diagnosis: DHR or NBT test. Treatment: TMP-SMX prophylaxis, itraconazole, IFN-γ, hematopoietic stem cell transplant or gene therapy.
• Leukocyte adhesion deficiency (LAD). Defective β2 integrin (CD18) blocks neutrophil adhesion and emigration from blood vessels. Patients have high white counts but absent neutrophils at infection sites. Delayed umbilical cord separation, severe periodontitis, recurrent infections without pus. Curative HSCT.
• Chediak-Higashi syndrome. LYST mutation, giant lysosomes that fail to fuse with phagosomes properly. Recurrent pyogenic infections, partial albinism, neuropathy, accelerated lymphoma phase.
• Asplenia and hyposplenism. Loss of splenic macrophages eliminates the major clearance site for blood-borne encapsulated bacteria. Overwhelming post-splenectomy infection risk.
• HIV/AIDS. HIV infects and damages macrophages (CCR5-tropic strains particularly), impairing phagocytic clearance of opportunistic pathogens. Risk of disseminated MAC, PCP, Cryptococcus.
• Atherosclerosis. Macrophages engulf oxidized LDL in arterial walls and become foam cells. Defective efferocytosis of dying foam cells creates necrotic plaque cores that drive plaque instability and rupture.
• Autoimmune disease. Defective efferocytosis of apoptotic cells in lupus exposes nuclear autoantigens, driving autoantibody production. SLE patients have impaired phagocytosis assays in vitro.
• Sepsis. Severe inflammation overwhelms phagocyte capacity, NETs accumulate, and immunothrombosis (clots driven by NET-platelet interactions) damages microvasculature, contributing to organ failure.

Common pitfalls and misconceptions

• "All cells can phagocytose." No. Phagocytosis at scale is restricted to professional phagocytes — neutrophils, macrophages, dendritic cells. Non-professional cells can engulf small things but not bacteria efficiently.
• "Phagocytosis is always inflammatory." Efferocytosis of apoptotic cells is silent and anti-inflammatory. The phagocyte distinguishes targets by surface signals — phosphatidylserine on dying cells inhibits inflammation; pathogen-associated patterns and opsonins promote it.
• "Phagocytes only kill what they swallow." Neutrophils also produce NETs (extracellular killing), macrophages secrete antimicrobial peptides, and ADCC cell types kill targets without engulfment. Phagocytosis is one mode among several.
• "All phagocytosed material is destroyed." Some intracellular pathogens (TB, Listeria, Salmonella, Legionella, Brucella, Cryptococcus) survive or escape — their evolution centered on subverting phagocyte mechanisms. Dendritic cells deliberately preserve engulfed material to present it to T cells.
• "Macrophages and neutrophils are interchangeable." Neutrophils are short-lived (hours to days), fast responders with abundant granules — they swarm in acute infection then die. Macrophages are long-lived (months), tissue-resident, also do antigen presentation, repair, and homeostasis.
• "Bigger is harder to phagocytose." Mostly true — particles >5 μm tax phagocyte capacity. But Frustrated phagocytosis (where the target is too large) still triggers degranulation, NETosis, or syncytial macrophage giant cells (in granulomas), which can damage host tissue.

Therapeutic applications

• Granulocyte colony-stimulating factor (G-CSF). Filgrastim and pegfilgrastim mobilize neutrophils from marrow and shorten chemotherapy-induced neutropenia. Reduces febrile neutropenia and infection-related mortality.
• IFN-γ for CGD. Recombinant IFN-γ partially restores oxidative burst in some CGD patients and reduces serious infection frequency. Used in addition to antibiotic and antifungal prophylaxis.
• Macrophage-targeted cancer therapy. Anti-CD47 (magrolimab) blocks the "don't eat me" signal on tumor cells, restoring macrophage phagocytosis of cancer. CSF1R inhibitors reprogram tumor-associated macrophages from M2 to M1.
• Monoclonal antibodies. Therapeutic mAbs (rituximab, trastuzumab, daratumumab) opsonize target cells for phagocytosis by host macrophages — ADCP is increasingly recognized as a major effector mechanism.
• Gene therapy and HSCT for phagocyte defects. Hematopoietic stem cell transplant cures CGD, LAD, Chediak-Higashi, and severe congenital neutropenia. Gene therapy with lentiviral CYBB correction now in trials for X-linked CGD.
• Hyperbaric oxygen therapy. Increases tissue O2 tension to support NADPH oxidase activity in chronic wound infection and necrotizing fasciitis, complementing antibiotics.