The complement system is a network of over 30 proteins circulating in your blood that serve as one of the body’s first lines of defense against infection. These proteins work in a chain reaction, each one activating the next, to identify and destroy bacteria, viruses, and other invaders. It’s part of the innate immune system, meaning you’re born with it rather than developing it through exposure to specific germs. When functioning properly, complement proteins can kill pathogens directly, flag them for destruction by immune cells, and trigger inflammation to bring reinforcements to the site of infection.
Three Pathways, One Outcome
The complement system can be switched on through three different routes, all of which converge on the same end result: destroying the threat. What differs is how each pathway recognizes that a threat exists in the first place.
The classical pathway is the most targeted. It typically activates when antibodies latch onto a pathogen, forming a complex that attracts a complement protein called C1q. C1q can also bind directly to certain pathogen surfaces without antibodies, but the antibody route is particularly important because it links complement activity to the adaptive immune system, the branch of immunity that “remembers” specific infections.
The lectin pathway starts when a blood protein called mannan-binding lectin recognizes specific sugar molecules (mannose) on the surface of bacteria or viruses. Human cells don’t display these sugar patterns, so this pathway selectively targets foreign organisms.
The alternative pathway is always slightly active. One of the key complement proteins, C3, spontaneously breaks apart at a low rate in the bloodstream. If a fragment of C3 lands on a pathogen surface, it triggers a rapid chain reaction. If it lands on one of your own cells, regulatory proteins shut it down before any damage occurs. This constant, low-level surveillance means the alternative pathway can respond to threats almost immediately, without waiting for antibodies or sugar-pattern recognition.
All three pathways ultimately generate an enzyme called C3 convertase, which cleaves massive amounts of C3 into active fragments. This is the amplification step. Once C3 convertase forms, the system rapidly escalates its response regardless of which pathway started the process.
How Complement Destroys Pathogens
The complement system eliminates threats through three main mechanisms, often running simultaneously.
- Tagging for destruction (opsonization): Fragments of C3 coat the surface of a pathogen, essentially painting a “eat me” signal on it. Immune cells like macrophages and neutrophils have receptors that recognize these tags, making them far more efficient at engulfing and digesting the invader.
- Recruiting immune cells: When C3 and another protein called C5 are cleaved during activation, smaller fragments called C3a and C5a are released into the surrounding fluid. These fragments act as chemical alarms. They widen blood vessels, increase the permeability of vessel walls so immune cells can squeeze through, and create a chemical trail that draws neutrophils, macrophages, and other immune cells toward the infection. C5a is one of the most potent signals the immune system has for pulling white blood cells to a specific location. C3a and C5a also cause mast cells and basophils to release histamine, amplifying the inflammatory response.
- Punching holes in cells: The most dramatic weapon in the complement arsenal is the membrane attack complex (MAC). Five complement proteins, C5b through C9, assemble sequentially on the surface of a target cell and form a physical pore through its membrane. Water and ions rush in through the channel, and the cell swells and bursts. This is particularly effective against certain bacteria like those that cause meningitis.
How Your Cells Avoid Friendly Fire
A system this aggressive needs tight controls, and the body uses both soluble blood proteins and proteins anchored directly on cell surfaces to keep complement in check. Factor H, one of the key soluble regulators, recognizes chemical markers on your own cells, particularly clusters of sialic acid and other sugar-based molecules that pathogens lack. When Factor H detects these markers, it breaks down C3 convertase and blocks further complement activation on that surface. It binds C3b on host cells with roughly ten times the affinity it shows for C3b on foreign surfaces, creating a strong bias toward protecting your own tissue.
Cells also carry their own protective molecules. DAF (decay-accelerating factor) speeds the breakdown of C3 convertase that forms on cell surfaces, while CD59 blocks the final assembly of the membrane attack complex. Together, these regulators create a system where complement activation proceeds aggressively on anything that looks foreign but is quickly shut down on anything that looks like “self.”
What Happens When Complement Goes Wrong
Deficiencies in complement proteins cause real, sometimes serious disease. The specific problems depend on which part of the system is affected.
People missing early classical pathway components (C1, C2, or C4) are more prone to autoimmune diseases, especially systemic lupus erythematosus. This happens because complement normally helps clear dead cells and immune complexes from the body. Without that cleanup function, debris accumulates and triggers immune reactions against the body’s own tissues.
Deficiencies in the late complement proteins, those that form the membrane attack complex, leave people vulnerable to severe infections with bacteria like meningococcus and pneumococcus. These infections can be fatal without prompt treatment.
Two well-known diseases involve complement regulatory failures rather than complement deficiencies. In paroxysmal nocturnal hemoglobinuria (PNH), red blood cells lack the surface proteins that normally protect them from complement. The result is that complement attacks the body’s own red blood cells, causing episodes of severe anemia and dark-colored urine. In hereditary angioedema, a missing regulator in the classical pathway leads to sudden, painful swelling episodes in the face, throat, hands, or abdomen.
Overactive complement also plays a role in atypical hemolytic uremic syndrome, a condition where uncontrolled complement damages the small blood vessels in the kidneys, and in some forms of age-related macular degeneration, where complement-driven inflammation damages the retina over time.
Complement-Targeting Treatments
Understanding complement biology has led to drugs that block specific steps in the cascade. The first major breakthrough was eculizumab, a lab-made antibody that binds to C5 and prevents it from being cleaved. This stops both the release of the inflammatory signal C5a and the formation of the membrane attack complex. It’s approved for PNH, atypical hemolytic uremic syndrome, generalized myasthenia gravis, and neuromyelitis optica spectrum disorder. All four conditions involve complement-mediated tissue damage, and blocking C5 can dramatically reduce that damage.
Because blocking complement at C5 leaves patients unable to form the membrane attack complex, people on these drugs face a higher risk of meningococcal infection and are vaccinated against it before starting treatment. This trade-off illustrates a central reality of complement biology: the same destructive power that protects you from bacteria can, when misdirected, destroy your own tissues. The challenge in treating complement-related diseases is dampening the damage without eliminating the protection entirely.