Difficulty: Intermediate | Prerequisites: Innate immunity basics, haematopoiesis, inflammation.
Neutrophils are the most abundant white blood cells and the first responders to bacterial infection. The complement system is a cascade of plasma proteins that tags pathogens for destruction and directly kills some bacteria. These two topics overlap heavily: complement activation recruits neutrophils to infection sites, and complement proteins (especially C3b) coat bacteria so that neutrophils can phagocytose them more efficiently. This material is core innate immunity content and is tested through both descriptive questions ("describe the life of a neutrophil") and experimental design questions involving knockout mice. If you are unclear on the difference between innate and adaptive immunity or on what opsonisation means, review those first.
Neutrophils are born in the bone marrow, circulate in the blood, migrate to infected tissues when summoned by inflammatory signals, kill bacteria through phagocytosis and degranulation, and die within hours to days. The complement system amplifies this response: C3 sits at the centre of all three activation pathways (classical, lectin, alternative) and its cleavage products recruit neutrophils (C3a, C5a), opsonise bacteria (C3b), and assemble the membrane attack complex (C5b through C9) to lyse pathogens directly. Mice lacking C3 cannot do any of this and are highly susceptible to bacterial infections such as Neisseria meningitidis.
Neutrophil
The most abundant type of white blood cell, characterised by a multi-lobed nucleus and cytoplasmic granules filled with antimicrobial enzymes. In simple terms, these are the front-line soldiers of innate immunity.
Haematopoiesis
The process by which all blood cells, including neutrophils, are produced from haematopoietic stem cells in the bone marrow.
Phagocytosis
The process by which a cell engulfs and internalises a particle (such as a bacterium) into a phagosome, which then fuses with lysosomes for destruction. Think of it as the cell "eating" the pathogen.
Degranulation
The release of antimicrobial contents from a neutrophil's cytoplasmic granules into either the phagosome or the extracellular space. This includes enzymes like elastase and defensins.
NETosis (neutrophil extracellular trap formation)
A form of neutrophil death in which the cell releases its DNA and granule contents as a web-like structure that traps and kills bacteria.
Apoptosis
Programmed cell death. Neutrophils are short-lived and undergo apoptosis within 1 to 2 days unless their lifespan is extended by inflammatory signals.
Complement system
A group of roughly 30 plasma proteins that work in a cascade to tag pathogens for destruction, recruit immune cells, and directly lyse certain microbes. It is part of innate immunity but also bridges to adaptive immunity.
C3
The central protein of the complement cascade. All three activation pathways (classical, lectin, alternative) converge on C3, which is cleaved into C3a (an anaphylatoxin that promotes inflammation) and C3b (an opsonin that coats pathogens).
Membrane attack complex (MAC)
A pore-forming structure assembled from complement proteins C5b, C6, C7, C8, and multiple copies of C9. It inserts into the lipid bilayer of a target cell, causing osmotic lysis.
Opsonisation
The coating of a pathogen with molecules (such as C3b or antibodies) that make it easier for phagocytes to recognise and engulf it. Think of it as "seasoning" the pathogen so neutrophils and macrophages find it more appetising.
Anaphylatoxin
Small complement fragments (C3a, C5a) that promote inflammation by triggering mast cell degranulation, increasing vascular permeability, and recruiting leukocytes. C5a is the most potent.
Neisseria meningitidis
A Gram-negative bacterium that causes meningitis and septicaemia. It is particularly susceptible to complement-mediated killing because it has a thin cell wall with an outer membrane that the MAC can penetrate.
1. Place of birth: bone marrow
Neutrophils develop from haematopoietic stem cells in the bone marrow through a process called granulopoiesis. They pass through several precursor stages (myeloblast, promyelocyte, myelocyte, metamyelocyte, band cell) before maturing into segmented neutrophils. The bone marrow maintains a large reserve pool, releasing mature neutrophils into the blood as needed.
2. Places of life: blood and tissues
Once released, neutrophils circulate in the bloodstream. Some are marginated (loosely attached to blood vessel walls), and others flow freely. Their primary job in the blood is to be on standby for rapid deployment.
When infection or tissue damage occurs, inflammatory signals (chemokines, complement fragments like C5a, bacterial products) cause neutrophils to adhere to the endothelium (via selectins and integrins), roll along the vessel wall, and then squeeze through the vessel wall into the tissue (diapedesis/extravasation). Once in the tissue, they migrate toward the source of infection along a chemokine gradient.
At the site of infection, neutrophils kill pathogens through phagocytosis (engulfing bacteria into a phagosome, fusing it with granules containing reactive oxygen species and antimicrobial enzymes), degranulation (releasing granule contents into the extracellular space), and NETosis (extruding DNA and granule proteins as a web that traps bacteria).
3. Outcomes in life (ways to die)
Apoptosis: the default fate. After completing their job (or failing to encounter a pathogen), neutrophils undergo programmed cell death within 1 to 2 days. Apoptotic neutrophils are then cleared by macrophages, which helps resolve inflammation.
NETosis: a specialised form of death in which the neutrophil sacrifices itself to release its DNA and antimicrobial granule contents as a net-like trap.
Necrosis: if overwhelmed by a massive infection or toxic exposure, neutrophils may die in an uncontrolled manner, spilling their contents and potentially causing tissue damage.
Phagocytosis-induced cell death: in some cases, the act of engulfing a pathogen triggers the neutrophil's own death.
4. Lifespan duration
Neutrophils are short-lived. In the blood, their half-life is roughly 6 to 8 hours, though pro-inflammatory signals at infection sites can extend survival to 1 to 2 days. They are continuously replenished by the bone marrow, which produces approximately 10^11 neutrophils per day in a healthy adult.
Researchers generated C3 knockout mice and infected them alongside wild-type mice with N. meningitidis intravenously. The knockout mice had significantly higher bacterial numbers in the spleen, demonstrating that C3 is critical for controlling this infection.
Mechanism 1: C3 activation induces acute inflammation and recruits phagocytes
All three complement pathways (lectin, classical, alternative) converge on C3. Cleavage of C3 produces C3a and C3b. Further downstream, C5 is cleaved into C5a and C5b. C5a is a powerful anaphylatoxin that triggers mast cells to release histamine and other mediators, stimulates leukocyte adhesion to endothelium, and recruits neutrophils to the site of infection. C3b simultaneously coats the bacteria (opsonisation), making them far easier for neutrophils and macrophages to phagocytose.
Without C3, none of this happens. No C3a or C5a means no inflammatory recruitment. No C3b means no opsonisation. The bacteria proliferate unchecked.
Mechanism 2: Complement assembles the membrane attack complex (MAC)
C3 cleavage ultimately leads to C5 cleavage. C5b initiates the assembly of C6, C7, C8, and multiple C9 molecules into the MAC, a pore that inserts into the bacterial outer membrane. For Gram-negative bacteria like N. meningitidis, this pore causes osmotic lysis, killing the bacterium directly.
Without C3, no C5 cleavage occurs, so the MAC cannot form. The bacteria survive.
Why are C3 knockout mice more susceptible to infection?
C3 is the convergence point of all three complement activation pathways. Without C3, no downstream complement components are activated: no C3a or C5a (so no inflammatory cell recruitment), no C3b (so no opsonisation), and no C5b through C9 assembly (so no MAC formation). The entire complement cascade is effectively disabled. This leaves the mice dependent on complement-independent mechanisms alone, which are insufficient to control many bacterial infections.
Are there infections where C3 knockout mice would be as resistant as wild-type?
Yes. Infections caused by intracellular pathogens, such as viruses or intracellular bacteria (e.g. Listeria monocytogenes, Mycobacterium tuberculosis), would be handled primarily by cell-mediated immunity (T cells, NK cells, macrophage activation) rather than complement. Since these pathogens hide inside host cells, complement cannot reach them and the MAC cannot target them. For these infections, C3 knockout mice would perform similarly to wild-type mice because the critical defence mechanism (T cell-mediated killing of infected cells) does not depend on complement.
Similarly, infections where physical barriers (skin, mucus) or other innate mechanisms (antimicrobial peptides, type I interferons) provide the primary defence would not be significantly affected by the loss of C3.
Students often think neutrophils live for weeks. They do not. Their circulating half-life is roughly 6 to 8 hours, and even in inflamed tissue they rarely survive more than 1 to 2 days.
A common error is to say that C3 knockout mice have "no immune system." They still have functional T cells, B cells, NK cells, macrophages, and all other non-complement components. What they lack is the entire complement cascade.
Students sometimes list the three complement pathways as three separate mechanisms. The exam source explicitly states that listing different pathways of C3 activation counts as one mechanism, because they all converge on the same step (C3 cleavage).
Another mistake is confusing C3a/C5a (anaphylatoxins that recruit cells) with C3b (the opsonin that coats pathogens). They are different fragments with different functions.
The "life of a neutrophil" question is a classic exam essay. Know all four parts: birth (bone marrow), life (blood then tissues), death (apoptosis, NETosis, necrosis), and lifespan (hours to 1 to 2 days).
For complement questions, remember that listing the three activation pathways counts as one mechanism. You need two functionally distinct mechanisms: (1) inflammation/opsonisation via C3a/C3b/C5a, and (2) direct lysis via the MAC.
Be ready to explain why C3 knockout disables the entire cascade (it is the convergence point) and to identify infections where complement is not the primary defence (intracellular pathogens).
True or False: Neutrophils are produced in the thymus. (Answer: False. They are produced in the bone marrow.)
Fill in the blank: The complement fragment that coats bacteria to promote phagocytosis is called ______. (Answer: C3b)
True or False: C3 knockout mice have no functional immune system. (Answer: False. They retain all non-complement immune components.)
Fill in the blank: The pore-forming structure that lyses Gram-negative bacteria is called the ______. (Answer: membrane attack complex, or MAC)
True or False: Listing the classical, lectin, and alternative pathways counts as two mechanisms on this exam. (Answer: False. It counts as one mechanism because they all converge on C3.)
Q: Describe the life of a neutrophil, including place of birth, places of life, ways to die, and lifespan.
A: Neutrophils are born in the bone marrow from haematopoietic stem cells (granulopoiesis). They are released into the bloodstream, where they circulate or marginate along vessel walls, with a half-life of about 6 to 8 hours. When inflammatory signals (chemokines, C5a) are detected, neutrophils adhere to the endothelium, undergo diapedesis, and migrate to infected tissues. There they kill pathogens through phagocytosis, degranulation, and NETosis. Neutrophils die by apoptosis (the default, cleared by macrophages), NETosis (sacrificial DNA net formation), or necrosis (uncontrolled death in overwhelming infection). Total lifespan is typically less than 2 days.
Q: Describe two possible mechanisms of how C3 protein is integrated in the immune response to N. meningitidis. (Note: listing different pathways of C3 activation counts as one mechanism.)
A: (1) C3 activation produces C3a and C5a, which are anaphylatoxins that recruit neutrophils and other leukocytes to the infection site, and C3b, which opsonises bacteria for phagocytosis. This constitutes one mechanism: inflammation and opsonisation. (2) C3 cleavage leads to C5 cleavage, and C5b initiates assembly of the MAC (C5b through C9), which inserts into the Gram-negative outer membrane and causes osmotic lysis of the bacterium. This is a second, functionally distinct mechanism: direct killing.
Q: Are there infections where C3 knockout mice would be as resistant as wild-type mice? Justify.
A: Yes. Intracellular infections (e.g. viruses, Listeria, Mycobacterium tuberculosis) are controlled primarily by cell-mediated immunity (CD8 T cells, NK cells, activated macrophages), which does not depend on complement. The pathogen is inside host cells and cannot be reached by complement or the MAC. C3 knockout mice retain fully functional T cells and NK cells, so they would defend against these infections as effectively as wild-type mice.
Neutrophil biology connects to inflammation (neutrophils are the hallmark of acute inflammation) and to the lymphocyte trafficking topic (both involve leukocyte adhesion, diapedesis, and chemokine-directed migration). The complement system links to antibody function: the classical pathway is activated by antigen-antibody complexes, which bridges innate and adaptive immunity. C3 knockout experiments also connect to the broader concept of gene knockout studies in immunology, a methodology that appears across multiple exam questions in this course.
neutrophil, polymorphonuclear leukocyte, PMN, granulocyte, phagocyte, phagocytosis, degranulation, NETosis, neutrophil extracellular traps, apoptosis, necrosis, haematopoiesis, granulopoiesis, bone marrow, diapedesis, extravasation, chemotaxis, complement system, complement cascade, C3, C3a, C3b, C5, C5a, C5b, membrane attack complex, MAC, opsonisation, anaphylatoxin, classical pathway, lectin pathway, alternative pathway, Neisseria meningitidis, Gram-negative bacteria, C3 knockout, innate immunity, acute inflammation, intracellular pathogen