Phagocytosis: Mechanism and Steps

Some children suffer repeated, severe infections with bacteria and fungi that should be easy to clear. Their phagocytes find the microbes, surround them, and swallow them normally. The problem comes at the very last step: the cell cannot produce the burst of reactive oxygen that kills what it has eaten. The microbe sits alive inside the cell.

This condition, chronic granulomatous disease, is a natural experiment that shows why each step of phagocytosis matters, and why swallowing a microbe is not the same as killing it. This article walks through those steps in order, and each one becomes easier to remember when you can see what goes wrong if it fails.

Phagocytosis is the ingestion of extracellular particulate material such as invading pathogens or dead/dying cells by phagocytic cells and is one of the important innate defense mechanisms. It is primarily conducted by specialized cells, such as macrophages, neutrophils, and dendritic cells.

Phagocytosis is one type of endocytosis, others are, receptor-mediated endocytosis and pinocytosis.

Phagocytes and the other cells that carry out innate defense are introduced in the article on cells of the immune system.

The steps of phagocytosis at a glance

Phagocytosis can be broken into a clear sequence. Different textbooks group them as anywhere from four to seven steps, but the underlying events are the same:

1. Chemotaxis: the phagocyte is drawn to the site.
2. Recognition and adherence: the phagocyte binds the microbe, often with the help of opsonins.
3. Ingestion: pseudopods engulf the microbe into a phagosome.
4. Phagolysosome formation: the phagosome fuses with a lysosome.
5. Killing and digestion: oxygen-dependent and oxygen-independent mechanisms destroy the microbe.
6. Elimination: waste is expelled by exocytosis.

The rest of this article takes each step in turn.

Step 1: Activation of Phagocytic cells and Chemotaxis

In the first step of phagocytosis, phagocytes are attracted by and move toward a variety of substances generated in the immune response; this process is called chemotaxis.

Resting phagocytes are activated by inflammatory mediators (bacterial products, cytokines, prostaglandins, and complement proteins). Activation increases their metabolic and microbicidal activity. Activated cells also express more glycoprotein receptors which help them to reach the site of infections as well as to bind firmly with microorganisms. Neutrophils are the first to appear and are later replaced by macrophages.

Figure-1: PRR binding with PAMPs (Image source: Gary E. Kaiser)

Step 2: Recognition of invading microbes

The next step in phagocytosis is the adherence of the antigen to the cell membrane of the phagocytic cells. Adherence induces membrane protrusions, called pseudopodia, to extend around the attached material and to ingest them.

Phagocytic cells contain various receptors which help them to attach with bacteria or viruses. Some of these receptors are:

• Pattern recognition receptors (PRR): Pattern recognition receptors recognize pathogen-associated molecular patterns (PAMPs). These include bacterial molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, flagellin, pilin, and bacterial DNA. For example, scavenger receptors and toll-like receptors bind and internalize gram-positive and gram-negative bacteria after binding with PAMPs.

Besides pattern recognition receptors, phagocytes carry Fc receptors and complement receptors, which recognize microbes that have been coated by opsonins. These are covered next.

Figure-2: Fc receptor-mediated Opsonization (Image source: philpoteducation)

Figure-3: Complement mediated phagocytosis

Opsonins: the two that matter

An opsonin is a molecule that coats a microbe and makes it far easier to phagocytose, like a handle the phagocyte can grab. The word comes from the Greek for "to prepare for eating." Two opsonins account for most of what you need to know, and each has its own receptor:

IgG antibody is recognized by Fc receptors on the phagocyte. The antibody binds the microbe with its arms, and its stem (the Fc portion) is grabbed by the phagocyte. This links a specific, adaptive response to phagocytosis. IgM does not act through Fc receptors here; it promotes phagocytosis indirectly by activating complement and depositing C3b.

C3b, a complement fragment, is recognized by complement receptors such as CR1. C3b coats the microbe during complement activation, and the phagocyte binds it directly. This works without antibody, as part of the innate response.

A microbe coated with these opsonins is engulfed much more efficiently than a bare one. This is why encapsulated bacteria, which resist opsonization, are so dangerous: without opsonins, the phagocyte struggles to grip them. It is also why the spleen, a major site of opsonized-particle clearance, matters so much in defense against encapsulated organisms.

Complement activation, which deposits the opsonin C3b, is covered in a separate article on the complement system.

Step 3: Ingestion and formation of phagosomes

Figure-4: Formation of phagosome (Image source: Gary E. Kaiser)

Following attachment, polymerization and then depolymerization of actin filaments send pseudopods out to engulf the microbe.  Fusion of the pseudopodia encloses the material within an endocytic vesicle called a phagosome, which then enters the endocytic processing pathway.

Step 4: Formation of phagolysosome

In this pathway, a phagosome moves toward the cell interior, where it fuses with a lysosome to form a phagolysosome.

Step 5: Microbial killing and formation of residual bodies

Lysosomes contain lysozyme and a variety of antimicrobial and cytotoxic substances that can destroy phagocytosed microorganisms and cells. Microorganisms are killed either by oxygen-dependent or by oxygen-independent mechanisms.

Oxygen-Dependent Killing

Activated phagocytes produce a number of reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates that have potent antimicrobial activity. A metabolic process known as respiratory burst occurs in phagocytic cells that activate membrane-bound oxidase forming superoxide anion, hydroxyl radicals, and hydrogen peroxide. Other potent antimicrobial substances such as hypochlorite, nitric oxide, etc are also formed inside the phagolysosome. All of these substances showed marked antimicrobial activity against bacteria, fungi, parasitic worms, and protozoa.

The enzyme responsible for the respiratory burst is NADPH oxidase, assembled in the membrane of the phagosome. When it switches on, it rapidly consumes oxygen (this sudden spike in oxygen use is what "burst" refers to) and generates superoxide, which is converted into hydrogen peroxide and, with the enzyme myeloperoxidase, into hypochlorous acid, the active ingredient of household bleach. The phagocyte, in effect, kills the microbe with a controlled dose of bleach inside a sealed compartment.

The importance of this single enzyme is shown by what happens when it fails. In chronic granulomatous disease, NADPH oxidase is defective. Phagocytes still find, engulf, and trap microbes, but they cannot mount the respiratory burst, so they cannot kill catalase-positive organisms such as Staphylococcus aureus and Aspergillus. The result is repeated, serious infections despite phagocytes that look and behave normally up to the final step. This is the clearest demonstration that ingestion and killing are separate events.

Oxygen Independent Killing

Activated phagocytic cells also synthesize lysozyme and various hydrolytic enzymes (for example, cathepsin G, elastase, collagenase, cathelicidins and bactericidal permeability inducing protein) whose degradative activities do not require oxygen. In addition, activated macrophages produce a group of antimicrobial and cytotoxic peptides, commonly known as defensins. Defensins can kill a variety of bacteria, including Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and Haemophilus influenzae. Activated macrophages also secrete tumor necrosis factor α (TNF-α), a cytokine that has a variety of effects and is cytotoxic for some tumor cells.

Step 6: Elimination or exocytosis

The digested contents of the phagolysosome are then eliminated in a process called exocytosis.

How to remember

The step sequence: "Cops Ruin All Phagocyte-Killed Enemies." Chemotaxis, Recognition, Adherence/ingestion, Phagolysosome, Killing, Elimination. (Or simply: find it, grab it, swallow it, fuse it, kill it, dump it.)

Two opsonins, two receptors: IgG → Fc receptor; C3b → complement receptor. "Antibody grabs with Fc, complement grabs with C3b."

Respiratory burst = a bleach factory. NADPH oxidase turns oxygen into superoxide, then hydrogen peroxide, then hypochlorite (bleach). Kill the enzyme (CGD) and the microbe survives inside the cell.

Ingestion is not killing. The single most important takeaway: a phagocyte can swallow a microbe and still fail to kill it. CGD proves the two steps are separate.

Key exam facts in one table

Fact	Detail
Main phagocytes	Neutrophils, macrophages, dendritic cells
First to arrive	Neutrophils, then macrophages
Chemotaxis triggers	Bacterial products, cytokines, complement (C5a)
Opsonin 1	IgG → Fc receptor
Opsonin 2	C3b → complement receptor (CR1)
Vesicle after ingestion	Phagosome
After lysosome fusion	Phagolysosome
Oxygen-dependent killing enzyme	NADPH oxidase (respiratory burst)
Products of the burst	Superoxide, hydrogen peroxide, hypochlorite
Oxygen-independent killing	Lysozyme, defensins, hydrolytic enzymes
Failure of the burst causes	Chronic granulomatous disease (CGD)
CGD-vulnerable organisms	Catalase-positive (S. aureus, Aspergillus)

Where students get confused

"Swallowing the microbe means it is dead." No. This is the central misconception. Ingestion and killing are separate steps. In chronic granulomatous disease, phagocytes ingest microbes normally but cannot kill them, so the microbe survives inside the cell.

"Opsonins directly kill microbes." No. Opsonins do not kill. They coat the microbe so the phagocyte can grip and engulf it more easily. The killing happens later, inside the phagolysosome.

"The respiratory burst helps the cell breathe." No, the name is misleading. The "burst" is a sudden surge in oxygen consumption used to make toxic oxygen species that kill the microbe. It has nothing to do with respiration for energy.

"IgM is the main opsonin for Fc receptors." No. IgG is the antibody recognized by Fc receptors. IgM promotes phagocytosis indirectly, by activating complement and depositing C3b.

"Phagocytosis is part of adaptive immunity." Mostly no. Phagocytosis is a core innate mechanism. It connects to adaptive immunity when antibody (IgG) acts as an opsonin, but the process itself is innate and needs no prior exposure.

References and further readings

• Punt J, Stranford SA, Jones PP, Owen JA. Kuby Immunology. 8th ed. W.H. Freeman; 2019.
• Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2022.
• de Oliveira-Junior EB, Bustamante J, Newburger PE, Condino-Neto A. The human NADPH oxidase: primary and secondary defects impairing the respiratory burst function and the microbicidal ability of phagocytes. Scand J Immunol. 2011;73(5):420–427. https://doi.org/10.1111/j.1365-3083.2010.02501.x