The complement system is a network of more than 20 proteins circulating in your blood and tissue fluids that work together to identify and destroy pathogens. Most of these proteins sit inactive until they detect a threat, at which point they activate one another in a rapid chain reaction, like a row of dominoes. The system is one of the oldest parts of immune defense, bridging the gap between the fast-acting innate immune system you’re born with and the slower, more targeted adaptive immune system that learns from past infections.
Its three main jobs are tagging invaders so immune cells can eat them, punching holes in bacterial membranes to kill them directly, and triggering inflammation to recruit more immune cells to the site of infection.
How Activation Works
The complement cascade can start through three different entry points, but all three converge on the same core machinery. Think of them as three different alarm systems that all call the same fire department.
The classical pathway is the most targeted. It kicks in when antibodies (proteins your adaptive immune system makes after encountering a specific pathogen) latch onto a microbe. The antibody-pathogen complex binds to a protein called C1, which triggers the rest of the chain. This pathway can also be activated by dying cells and certain other molecular signals, not just antibodies.
The lectin pathway responds to sugar molecules, specifically mannose, that are commonly found on the surfaces of bacteria and fungi but not on healthy human cells. A recognition protein binds to these sugars and activates its own set of enzymes to start the cascade.
The alternative pathway is the most ancient and the least picky. It’s always running at a low level, spontaneously activating in the blood. On healthy human cells, regulatory proteins shut it down immediately. On foreign surfaces that lack those protections, the cascade proceeds unchecked. This pathway acts as a constant surveillance system, ready to escalate the moment it lands on something that doesn’t belong.
The Amplification Loop
Regardless of which pathway starts the process, all three produce a critical enzyme called C3 convertase. This enzyme splits a protein called C3 into two fragments: C3a, which drifts away to trigger inflammation, and C3b, which sticks to the surface of whatever activated the cascade.
Here’s where the system gets powerful. Each C3b molecule deposited on a pathogen’s surface can recruit more proteins to form additional C3 convertase, which splits more C3, which deposits more C3b. This feedback loop means a small initial signal can rapidly coat an invader in thousands of C3b molecules. The amplification becomes independent of whatever originally triggered it, driven almost entirely by the alternative pathway’s machinery. A stabilizing protein called properdin helps lock these enzyme complexes in place, making the loop even more efficient. The process only stops when regulatory proteins break apart the convertase and inactivate C3b.
Tagging Pathogens for Destruction
Once C3b coats a pathogen, immune cells recognize it as something to eat. White blood cells like macrophages and neutrophils carry surface receptors that bind specifically to C3b. When one of these immune cells encounters a C3b-coated microbe, it engulfs and digests it in a process called phagocytosis. This tagging function, called opsonization, is one of the complement system’s most important contributions. Many bacteria have slippery outer coats that make them difficult for immune cells to grab. A thick layer of C3b essentially turns a Teflon surface into Velcro.
Triggering Inflammation
The small fragments released during the cascade, particularly C3a and C5a, act as powerful chemical alarms. These molecules bind to receptors on mast cells and other immune cells, causing them to release histamine and other inflammatory signals. The result is increased blood flow to the area, swelling that allows immune proteins to leak out of blood vessels and into tissue, and smooth muscle contraction. C3a and C5a also function as chemical beacons that attract white blood cells toward the site of infection, pulling in reinforcements from the bloodstream. C5a is the more potent of the two and plays a major role in directing neutrophils to where they’re needed.
Killing Cells Directly
The most dramatic weapon in the complement arsenal is the membrane attack complex, or MAC. When the cascade progresses far enough, C5 is split into C5a (the inflammatory signal) and C5b, which begins assembling a molecular drill. C5b rapidly binds to C6, then C7 joins and makes the growing complex attracted to fatty membranes. C8 attaches next and physically penetrates the target cell’s outer membrane. Finally, multiple copies of C9 are recruited and polymerize into a ring, forming a pore roughly 10 nanometers wide.
This pore allows water and ions to rush freely in and out of the cell, destroying the balance the cell needs to survive. The target swells and bursts. The MAC is particularly effective against certain bacteria, especially species of Neisseria, which cause meningitis and gonorrhea.
How Your Cells Stay Safe
With such a destructive system constantly circulating, your own cells need protection. Healthy human cells display several regulatory proteins on their surfaces that act as “don’t eat me” signals to the complement cascade.
CD55, also called decay accelerating factor, works by breaking apart C3 and C5 convertases before they can do damage. It physically pulls the enzyme complexes apart on the cell surface. CD59, sometimes called protectin, blocks the final step of the MAC by preventing C9 from assembling into a pore. CD46 appears on virtually all nucleated human cells and helps degrade C3b that lands on healthy tissue. Another protein, CD35, both breaks down C3b and accelerates the decay of convertases.
In the fluid phase, soluble regulators like Factor H and Factor I patrol the bloodstream, inactivating C3b before it can trigger amplification on the wrong surfaces. Together, these proteins ensure complement activation stays focused on foreign targets.
Clearing Dead and Damaged Cells
The complement system does more than fight infection. It also helps clean up your body’s own dead and dying cells. When a cell undergoes programmed death (apoptosis), classical pathway proteins recognize it and tag it with complement fragments. Macrophages then clear these tagged cells quietly, without triggering a major inflammatory response. This housekeeping function prevents dead cell debris from accumulating in tissues, which could otherwise provoke autoimmune reactions.
What Happens When Complement Goes Wrong
Deficiencies in complement proteins lead to predictable problems depending on which part of the system is affected. People missing early classical pathway components (like C1, C2, or C4) are prone to autoimmune conditions, particularly lupus, likely because their bodies can’t efficiently clear immune complexes and dying cells. People missing late components (C5 through C9) can’t form the membrane attack complex and are highly susceptible to infections with encapsulated bacteria, especially Neisseria meningitidis, as well as Streptococcus pneumoniae and Haemophilus influenzae.
On the other side, overactive or poorly regulated complement damages healthy tissue. When regulatory proteins fail, complement attacks the body’s own cells, contributing to conditions like a rare form of anemia where red blood cells are destroyed (paroxysmal nocturnal hemoglobinuria, or PNH) and certain kidney diseases.
Medications That Target Complement
Understanding the complement system has led to drugs that deliberately block parts of the cascade. The most established approach targets C5, preventing it from being split and thereby stopping both inflammation (from C5a) and cell killing (from the MAC). Eculizumab was the first such drug, initially approved for PNH. A longer-acting version, ravulizumab, now treats PNH, myasthenia gravis (a condition where complement damages the connections between nerves and muscles), and a rare neurological condition called neuromyelitis optica. Ravulizumab is given intravenously every eight weeks after initial dosing.
A newer option, zilucoplan, is a small self-injected daily medication that also blocks C5 and was approved in 2023 for certain patients with myasthenia gravis. These drugs represent a growing class of therapies, and complement-targeted treatments are being explored across a range of inflammatory and autoimmune diseases.