Beyond Antibodies —
The Complement System Nobody Talks About

Ask most people how immunity works and you'll hear one word: antibodies. You catch something or get vaccinated, you make antibodies, and you're protected. That isn't wrong, but it leaves out much of the story.

One of the biggest missing pieces is a system most people have never heard of. It runs through everything from gut health to autoimmunity to eye disease. It's called complement, because it was first discovered as something that complemented antibodies, finishing the job they started. We now know it does far more than that.

What Complement Actually Is

Complement is a family of roughly 30 proteins circulating in your blood, most of them made by your liver. They sit inactive until triggered, then activate each other in a rapid chain reaction, a bit like a row of dominoes.

There are three ways to start the chain:

• The classical pathway is triggered when antibodies (the right kinds) bind to a target. This is the link to antibodies.
• The lectin pathway is triggered by sugar patterns on the surface of microbes. It doesn't need antibodies at all.
• The alternative pathway is always ticking over at a low level. It acts as a surveillance system that amplifies any activation it finds.

Once activated, complement does four main jobs. It tags invaders so immune cells can find and swallow them. It calls in reinforcements through the inflammatory signals C3a and C5a. It kills directly by assembling the membrane attack complex, which punches holes in bacterial membranes. And it clears debris, dead cells and antibody-antigen clumps, keeping the system clean.

Because it's so powerful, complement is tightly regulated. Your own cells carry protective proteins, such as CD59 and factor H, that stop complement attacking them. Most complement-related disease comes down to one of two problems: not enough complement, or not enough control over it.

Where Antibodies and Complement Meet

IgG isn't one thing, and its four subclasses behave very differently when it comes to complement. This is where it gets interesting for anyone who has had an IgG food test.

IgG4 can even swap halves with other IgG4 molecules, which makes it poor at forming the immune complexes that trigger complement (van der Neut Kolfschoten et al., Science 2007). So the subclass matters as much as the number. A raised IgG4 to a food you eat every day means something quite different from a raised IgG1 or IgG3.

There is a catch worth knowing if you have had an IgG food test, including the one I use. Standard IgG food panels measure total IgG to each food. They don’t separate the subclasses, so a raised result can’t tell you whether you’re looking at a complement-activating IgG1/IgG3 response or a tolerance-type IgG4 one. That’s one reason I read an IgG food result as a pointer to test with elimination and reintroduction, not as a verdict on its own.

The same principle has come up in COVID vaccine research. A German study found the IgG4 share of spike antibodies rose on average from 0.04% shortly after the second mRNA dose to 19.27% late after the third. Those antibodies were also less able to drive complement deposition (Irrgang et al., Science Immunology 2023).

What this means clinically is still argued over. A 2026 Swedish study linked the IgG4 shift to a higher breakthrough infection risk, though in a subgroup of only 41 people, and its authors say it does not call vaccine efficacy or safety into question (Pongrácz et al., Sci Rep 2026). A smaller US study found the IgG4 rise did not reduce neutralisation and breakthrough rates were comparable (Berber et al., Hum Vaccin Immunother 2025). Either way, it shows why "more antibodies" is too simple a measure of immunity.

When Complement Goes Wrong

Complement problems fall into two camps: too little of it, or too little control over it.

The nervous system examples deserve a closer look. In NMOSD, antibodies bind to astrocytes and trigger complement, which then injures oligodendrocytes (the cells that make myelin), endothelial cells and neurons (Targeting complement in NMOSD). In MS, the complement proteins C1q and C3 are elevated in animal models, though the human picture is still developing (C1q and oligodendrocyte progenitors).

The symptoms range from recurrent infections and unexplained swelling to fatigue, kidney problems and neurological changes. That variety is why complement is rarely the first thing anyone thinks of.

What Testing Can and Can't Tell You

DUTCH, GI-MAP and OAT don’t measure complement. Some blood chemistry panels do: the fuller panel I use includes C3 and C4. The direct tests are:

• C3 and C4 levels. Low levels can mean complement is being used up, as in active lupus, or that there’s a deficiency. Raised levels are less specific: C3 rises with inflammation and tracks body fat (see below). There is no validated “optimal” band narrower than the lab range.
• CH50 and AH50. These check whether the classical and alternative pathways work end to end. Hospital tests, requested through your GP or a specialist.
• C1-inhibitor level and function. Used when angioedema is suspected. Also a specialist test.

The functional tests still add useful context around it:

• Blood chemistry. Liver markers and albumin reflect your capacity to make complement proteins at all. CRP is a separate marker of innate inflammation that often rises alongside complement activity.
• IgG food testing. Useful as a pointer, but, as above, a total-IgG panel can’t tell a complement-activating response from a tolerance-type one.
• GI-MAP. The lectin pathway exists to recognise microbial surfaces. Gut inflammation and barrier markers show how much immune pressure is coming from the gut.

The Medical Perspective

Conventional medicine takes complement very seriously in specific diseases. There are now licensed drugs that block it at different points, including eculizumab and ravulizumab (C5 inhibitors), C1-inhibitor concentrates for hereditary angioedema, and newer oral drugs.

The results can be striking. In NMOSD, eculizumab significantly reduced relapse risk compared with placebo in the phase III PREVENT trial (Eculizumab review, Drugs 2020).

These drugs also show what complement is for. Blocking it comes at a cost: C5 inhibitors markedly increase susceptibility to invasive meningococcal disease (C5 inhibitor study, 2025). Patients must be vaccinated against meningococcus before starting treatment and carry a safety card. Complement suppression is a serious medical intervention, not a wellness goal.

The Functional Perspective, and Where the Evidence Stands

There is no well-evidenced way to "balance" complement with supplements, and honesty matters here because the internet is full of those claims.

Supplements

Curcumin is the best-known example. In a test tube, curcumin (dissolved in sodium hydroxide, not as you would swallow it) inhibited the classical complement pathway with a half-maximal concentration of 404 micromolar, and also slowed the alternative pathway (Kulkarni et al., 2005). That’s far higher than the blood levels oral curcumin reaches, even in enhanced-absorption forms. In the same study a viral complement-control protein was nearly 3,000 times more potent.

Food and lifestyle

No specific food has been shown to tune complement directly. The strongest human evidence is about body fat. Adipose tissue, especially visceral fat, makes complement proteins, and serum C3 tracks closely with fat mass (Gabrielsson et al., 2003). In a randomised trial of 52 men with abdominal obesity, 8 weeks of diet-induced weight loss lowered plasma C3 by 0.15 g/L, mainly explained by visceral fat loss (Jin et al., 2022).

• Reduce excess body fat, particularly around the middle. This has the best human evidence of any lifestyle change.
• Eat enough. Starvation works the other way: in anorexia, alternative pathway proteins fall and recover with weight gain (Pomeroy et al., 1997).
• Keep the liver healthy, because it makes most complement proteins.
• Don't smoke. Smoking is a major risk factor for macular degeneration alongside factor H variants.
• Address chronic gut inflammation and infections that keep the immune system under constant pressure.

Things that suppress it

Complement-inhibiting drugs, heavy immunosuppression, significant liver disease and malnutrition all reduce complement capacity, and with it your defence against certain bacteria. Stacking supplements to deliberately suppress complement isn't wise even if they worked.

Complement is protective first and troublesome second. The goal is a well-regulated system, not a suppressed one.

A Current Question: Complement and Vaccine Reactions

Complement has recently entered public debate through a new door. Professor Sucharit Bhakdi, whose early research career included work on the membrane attack complex, has argued that dampening complement activity is key to treating people who became ill after mRNA vaccination, with particular concern for myelin and the oligodendrocytes that make it.

As a hypothesis, it isn't baseless. A major US evidence review noted that mRNA vaccine components may increase complement and inflammasome responses and activate mast cells (National Academies review). A 2026 review proposes that the lipid nanoparticles, not just the spike protein, drive the innate immune reaction behind vaccine-associated myocarditis, though it centres on inflammasome pathways rather than complement (Choi et al., Mediators Inflamm 2026).

Lab work adds useful detail. In human serum, both Comirnaty and Spikevax activated complement mainly through the alternative pathway, driven by the lipid nanoparticles rather than the mRNA itself. The C5 blocker eculizumab and the C1-inhibitor Berinert both stopped that complement activation, yet neither stopped the release of inflammatory cytokines. The authors concluded that complement inhibitors alone are unlikely to control these reactions (Bakos et al., 2024).

There are also case reports of atypical HUS appearing after vaccination in people carrying complement gene variants, treated with eculizumab (Claes et al., 2022). These are individual cases in people with an underlying complement disorder, not evidence for treating post-vaccine illness in general.

But a plausible mechanism is not a proven treatment. I'm not aware of any controlled trial showing that complement suppression, with drugs or supplements, helps people with post-vaccine illness. The IgG4 shift after repeated doses also sits awkwardly with the theory, because IgG4 is the subclass least able to activate complement.

What the debate does usefully show is that immune reactions involve far more than antibody levels. If you're living with symptoms after vaccination, you deserve proper investigation, not a single-mechanism answer.

The Bottom Line

Complement is a reminder that immunity is a system of checks and balances, not a single number on a blood test. It clears infection and debris and links your innate and adaptive defences. When it's poorly regulated, it can damage kidneys, blood vessels, eyes and nerves.

Most people will never need to think about it. But if you have recurrent infections, unexplained swelling, autoimmune features or neurological symptoms, it's worth asking whether anyone has looked. Test, don't guess.

Sources

1. Irrgang P et al. Class switch toward noninflammatory, spike-specific IgG4 antibodies after repeated SARS-CoV-2 mRNA vaccination. Science Immunology 2023;8(79):eade2798. PubMed 36548397
2. van der Neut Kolfschoten M et al. Anti-inflammatory activity of human IgG4 antibodies by dynamic Fab arm exchange. Science 2007. PubMed 17872445
3. Pongrácz T et al. Vaccination-first immune priming shapes a sustained mRNA vaccine-induced IgG4 class switch that associates with SARS-CoV-2 breakthrough infection risk. Sci Rep 2026. doi:10.1038/s41598-026-64190-8
4. Berber E et al. Elicitation of neutralizing antibodies and IgG4 subclass switching following booster vaccination with ancestral COVID-19 mRNA vaccines does not reduce breakthrough infections. Hum Vaccin Immunother 2025. doi:10.1080/21645515.2025.2547517
5. Kulkarni A et al. Curcumin inhibits the classical and the alternate pathways of complement activation. Ann N Y Acad Sci, 2005. PubMed 16387680
6. Jin S et al. A randomized diet-induced weight-loss intervention reduces plasma complement C3. Obesity, 2022. doi:10.1002/oby.23467
7. Gabrielsson B et al. High expression of complement components in omental adipose tissue in obese men. Obesity Research, 2003. doi:10.1038/oby.2003.100
8. Pomeroy C et al. Effect of body weight and caloric restriction on serum complement proteins. Clin Exp Immunol, 1997. doi:10.1046/j.1365-2249.1997.3921287.x
9. Bakos T et al. mRNA-LNP COVID-19 vaccine lipids induce complement activation and production of proinflammatory cytokines. Int J Mol Sci, 2024. doi:10.3390/ijms25073595
10. Claes KJ et al. Atypical hemolytic uremic syndrome occurring after receipt of mRNA-1273 COVID-19 vaccine booster. Am J Kidney Dis, 2022. doi:10.1053/j.ajkd.2022.07.012
11. Choi K et al. Myocarditis after mRNA vaccination: a metabolic-innate immune cascade centered on lipid nanoparticles. Mediators Inflamm 2026. doi:10.1155/mi/8991922
12. National Academies. Evidence Review of the Adverse Effects of COVID-19 Vaccination. nationalacademies.org