Phagocytosis

Spec Mapping — OCR H420 Module 4.1.1 — Communicable diseases, content statement on phagocytosis as a non-specific defence in animals, the roles of neutrophils and macrophages, and antigen presentation as the link between innate and adaptive immunity (refer to the official OCR H420 specification document for exact wording). This lesson covers the mechanism of phagocytosis from chemotaxis through phagolysosome formation to antigen presentation, and sets up the adaptive immune response of Lessons 8–9.

If a pathogen breaches the skin and mucous membranes, the second line of non-specific defence is phagocytosis — the ingestion and destruction of pathogens by specialised white blood cells called phagocytes. The phenomenon was discovered by the Russian zoologist Élie Metchnikoff (Илья Мечников) in 1882, when he observed mobile cells of starfish larvae engulfing introduced rose-thorn fragments and proposed that similar cells in vertebrates defend against pathogens — a hypothesis sufficiently iconoclastic that it took 25 years to be widely accepted, and won him the 1908 Nobel Prize jointly with Paul Ehrlich (whose "magic bullet" hypothesis of antibody specificity made him the founder of humoral immunology). Modern molecular cell biology has filled in the receptor systems (Toll-like receptors, Fc receptors, complement receptors) and the killing mechanisms (oxidative burst, lysozyme, antimicrobial peptides) but the elementary picture is still Metchnikoff's. OCR specification 4.1.1 requires you to describe phagocytosis in detail, including the contributions of neutrophils and macrophages and the crucial role of antigen presentation as the bridge to adaptive immunity.

Key Definitions:

• Phagocyte — a white blood cell that engulfs pathogens, particles or apoptotic cells. The main phagocytes are neutrophils, macrophages and dendritic cells.
• Phagosome — the membrane-bound vesicle formed when a phagocyte engulfs a pathogen.
• Lysosome — a membrane-bound organelle containing hydrolytic enzymes (lysozyme, proteases, nucleases, lipases) and ATP-driven proton pumps that maintain the internal pH at ~4.5.
• Phagolysosome — the compartment formed when a phagosome fuses with a lysosome, mixing the engulfed pathogen with hydrolytic enzymes.
• Antigen — any molecule (typically a protein or glycoprotein on a pathogen surface) recognised by the immune system.
• MHC (major histocompatibility complex) — a family of cell-surface glycoproteins that present peptide fragments to T cells. MHC-I is on all nucleated cells; MHC-II is on professional antigen-presenting cells (macrophages, dendritic cells, B cells).
• Opsonin — a molecule (antibody, complement C3b, mannose-binding lectin) that coats a pathogen and binds to phagocyte receptors, accelerating engulfment.

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Types of Phagocyte

There are two main phagocytes in the blood and tissues:

1. Neutrophils

• The most abundant white blood cell (40–75 % of circulating leucocytes; 4–6 × 10⁹/L blood).
• Multi-lobed nucleus (polymorphonuclear, hence "polymorphs" or PMNs), giving rise to the characteristic three- to five-lobed nuclear shape on a blood smear.
• Short-lived (~5 days in circulation, hours-to-days in tissue) — the first to arrive at an infection site.
• Cytoplasmic granules contain lysozyme, defensins, cathelicidins, myeloperoxidase, lactoferrin (which sequesters iron away from bacteria) and elastase.
• Die after phagocytosing a few pathogens — the dead neutrophils, partially digested pathogens, tissue debris and serum exudate together constitute the pus that accumulates at infected wounds. Pus is biologically informative: green pus reflects myeloperoxidase content; foul-smelling pus suggests anaerobic bacterial involvement; thick pus implies dense neutrophil infiltrate.

2. Macrophages

• Larger (~20 µm), longer-lived cells derived from blood monocytes that migrate into tissues and differentiate.
• Tissue-resident under distinctive names: Kupffer cells in the liver, microglia in the central nervous system, alveolar macrophages in the lung, osteoclasts in bone (bone-resorbing macrophages), dendritic cells in lymphoid organs (related but specialised for antigen presentation).
• Much longer-lived than neutrophils — months to years.
• Polarise functionally as M1 (classically activated by IFN-γ and LPS — pro-inflammatory, pathogen-killing, antigen-presenting) or M2 (alternatively activated by IL-4/IL-13 — tissue-repair, anti-inflammatory). M1/M2 are the extremes of a continuum; tissue-resident macrophages adopt phenotypes appropriate to their niche.
• Essential for antigen presentation — after engulfing a pathogen, macrophages display pathogen-derived peptides on their surface MHC-II to activate CD4⁺ helper T cells, beginning the adaptive response.

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The Steps of Phagocytosis

flowchart TD
    A[Pathogen enters tissue] --> B[Chemotaxis: phagocyte moves toward cytokines/chemokines]
    B --> C[Recognition: PAMPs bind to PRRs on phagocyte surface]
    C --> D[Engulfment: plasma membrane surrounds pathogen]
    D --> E[Phagosome formed inside the cell]
    E --> F[Lysosome fuses with phagosome]
    F --> G[Phagolysosome: enzymes digest pathogen]
    G --> H[Waste products exocytosed]
    G --> I[Peptides displayed on MHC II]
    I --> J[Antigen presentation to T helper cells]

Step 1 — Chemotaxis

When pathogens invade a tissue, damaged cells and resident macrophages release cytokines (small signalling proteins, including IL-8/CXCL8, MCP-1, TNF-α) and the inflammatory mediators we met in the previous lesson (histamine, complement fragments C3a and C5a). Neutrophils circulating in the blood detect these chemical signals through G-protein-coupled chemokine receptors on their plasma membrane. The chemotactic gradient guides them: receptor binding triggers actin-cytoskeletal rearrangement at the leading edge of the cell, and the neutrophil migrates by amoeboid movement up the concentration gradient.

To leave the bloodstream, neutrophils must cross the endothelium of post-capillary venules. The process — termed diapedesis or transendothelial migration — proceeds in four steps:

1. Rolling along the inflamed endothelium, mediated by selectin–sialyl-Lewis-X interactions.
2. Activation by endothelium-displayed chemokines.
3. Firm adhesion via integrins (LFA-1, Mac-1) binding endothelial ICAM-1.
4. Transmigration between or through endothelial cells.

Once in the tissue, the neutrophil continues to move up the chemotactic gradient toward the highest cytokine concentration at the infection site. The whole journey from circulating neutrophil to phagocytosing tissue cell typically takes 15–30 minutes for an acute infection.

Step 2 — Recognition

At the infection site, the phagocyte recognises the pathogen in one of two ways:

• Direct recognition of conserved pathogen molecules (PAMPs, e.g., lipopolysaccharide, flagellin, peptidoglycan) by pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs).
• Opsonisation — the pathogen is first coated in proteins called opsonins (e.g., complement C3b, antibodies), which bind to receptors on the phagocyte and promote engulfment. Opsonised pathogens are engulfed far more efficiently than "naked" ones.

Step 3 — Engulfment

The phagocyte's plasma membrane forms pseudopodia — actin-driven extensions of the cell — that progressively wrap around the pathogen. The driving force is actin polymerisation: G-actin monomers polymerise into F-actin filaments at the leading edge, pushing the membrane outwards. The process is regulated by Rho-family GTPases (Rac1, Cdc42, RhoA), which activate the Arp2/3 complex that nucleates new actin branches. As the pseudopodia extend around the pathogen, they meet on the far side and fuse, trapping the pathogen inside a membrane-bound vesicle called a phagosome.

The whole engulfment takes ~5–15 minutes depending on pathogen size. Particles larger than ~10 µm overwhelm the phagocyte and cannot be fully engulfed — large parasites such as helminth larvae are killed instead by extracellular degranulation of eosinophils. Fungal hyphae of Candida and Aspergillus can grow longer than the phagocyte itself and rupture the phagocyte, a phenomenon called phagocyte escape that is one reason fungal infections are so dangerous in immunocompromised patients.

Step 4 — Formation of the Phagolysosome

Lysosomes (small organelles containing digestive enzymes) move through the cytoplasm and fuse with the phagosome to form a phagolysosome. The lysosomes release their contents into the new compartment.

Step 5 — Digestion of the Pathogen

Inside the phagolysosome, the pathogen is destroyed by a combination of mechanisms acting in parallel:

• Lysozyme — hydrolyses the β-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in bacterial peptidoglycan, lysing the cell wall.
• Proteases — cathepsins, elastase, neutrophil serine proteases digest pathogen proteins into peptides.
• Nucleases — digest pathogen DNA and RNA, both inactivating the pathogen and generating peptides for later antigen presentation.
• Lipases and phospholipases — digest pathogen membranes.
• Reactive oxygen species (ROS) — the NADPH oxidase complex (assembled in the phagolysosomal membrane on activation) generates superoxide (O₂⁻); superoxide dismutase converts this to hydrogen peroxide (H₂O₂); myeloperoxidase (only in neutrophils) converts H₂O₂ and chloride ion to hypochlorite (HOCl) — the active ingredient of bleach. This concerted production is called the oxidative burst or respiratory burst; it can be measured experimentally as a sudden increase in oxygen consumption by phagocytes.
• Reactive nitrogen species — inducible nitric oxide synthase (iNOS) in macrophages produces nitric oxide (NO), which combines with superoxide to form peroxynitrite, another potent antimicrobial.
• Low pH — vacuolar H⁺-ATPase proton pumps acidify the phagolysosome to pH ~4.5, optimal for cathepsin activity and lethal to most engulfed pathogens.
• Iron sequestration — lactoferrin in neutrophil granules binds iron, depriving engulfed bacteria of an essential nutrient.

The redundancy is important: chronic granulomatous disease, a genetic deficiency of NADPH oxidase, leaves patients with intact phagocytosis but severely impaired pathogen killing, demonstrating that the oxidative burst is essential rather than auxiliary.

Step 6 — Exocytosis and Antigen Presentation

Indigestible material is expelled from the cell by exocytosis. Crucially, some pathogen-derived peptides (8–25 amino acids long) generated by protease digestion in the phagolysosome are loaded onto MHC class II molecules in a specialised vesicle (the MIIC compartment) and displayed on the macrophage's plasma membrane. The phagocyte has now become a professional antigen-presenting cell (APC) ready to communicate with helper T cells.

MHC-I vs MHC-II — a critical distinction:

Feature	MHC-I	MHC-II
Cells expressing it	All nucleated cells	Professional APCs only (macrophages, dendritic cells, B cells)
Peptide source	Cytosolic proteins (endogenous)	Phagolysosome proteins (exogenous)
Peptide loading	ER, via TAP transporter	MIIC compartment after invariant-chain release
Detected by	Cytotoxic T cells (CD8⁺)	T helper cells (CD4⁺)
Function	Surveillance for intracellular infection / malignancy	Activation of adaptive immunity to extracellular pathogens

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Step 2 — Recognition (extended)

Recognition is more nuanced than a single receptor system. Phagocytes carry an entire toolkit of receptors:

• Toll-like receptors (TLRs) — a family of ten human transmembrane PRRs that recognise different PAMPs. TLR4 binds bacterial lipopolysaccharide (LPS); TLR2 binds peptidoglycan and lipoteichoic acid; TLR5 binds flagellin; TLR3 binds double-stranded RNA (viral); TLR9 binds unmethylated bacterial CpG DNA. Each receptor activates a signalling cascade culminating in NF-κB activation and pro-inflammatory gene transcription.
• Mannose receptor — binds the mannose-rich glycans on many fungal and bacterial surfaces; widely expressed on macrophages.
• Dectin-1 — recognises β-glucans of fungal cell walls; crucial for anti-fungal immunity.
• Complement receptor 3 (CR3, Mac-1) — binds C3b-coated targets.
• Fc receptors (FcγRI, FcγRIIA, FcγRIIIA) — bind the constant (Fc) regions of antibody-coated pathogens.

Most phagocytes integrate signals from several of these receptors at once, with engagement strength determining whether the pathogen is engulfed and how aggressively the phagocyte responds. The redundancy is part of why broad-spectrum pathogen detection is so robust.

Antigen Presentation — The Bridge to Adaptive Immunity

Antigen presentation is the link between innate and adaptive immunity. The molecular logic was definitively established by Peter Doherty and Rolf Zinkernagel in their 1974 work on MHC restriction (Nobel 1996): T cells recognise antigens only when those antigens are presented on the cell's own MHC molecules, which is why MHC matching matters in organ transplantation and why intracellular pathogens such as viruses and M. tuberculosis can still be detected.

An activated macrophage carrying pathogen peptides on MHC-II encounters circulating T helper lymphocytes (CD4⁺). If one of the T cell's somatically rearranged T-cell receptors (TCRs) has the correct shape to bind the peptide-MHC-II complex, the T cell undergoes activation, clonal expansion and differentiation into effector helper T cells — beginning the adaptive immune response of Lesson 8.

The professional antigen-presenting cells are:

• Dendritic cells — the most efficient APCs. They sample antigen at body surfaces, migrate to lymph nodes, and prime naïve T cells. Ralph Steinman's discovery of dendritic cells earned him a posthumous 2011 Nobel Prize.
• Macrophages — tissue-resident, antigen-presenting and effector cells. They both engulf pathogens and activate T cells.
• B cells — specifically present antigens that their surface immunoglobulin has captured.

Neutrophils, by contrast, are primarily rapid first-responder killers; they die quickly after engulfing a few pathogens and contribute little to antigen presentation.