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Membrane Attack Complex

The membrane attack complex, or MAC, is the final complement structure that forms pores in a target cell membrane and can lyse pathogens. In Immunobiology, it is part of the terminal complement pathway.

Last updated July 2026•Fiveable Content Team

What is the Membrane Attack Complex?

The membrane attack complex is the terminal product of the complement cascade in Immunobiology. It forms when complement proteins assemble on a target surface and build a pore-like structure that damages the membrane enough to kill the cell.

You can think of it as the complement system’s direct killing step. Earlier complement proteins tag microbes, amplify the signal, and recruit inflammation, but MAC is the part that actually punctures the membrane. That makes it especially useful against microbes that are exposed in blood or tissue fluids, where complement proteins can reach them quickly.

Mechanistically, MAC forms after C5 convertase cleaves C5 into C5a and C5b. C5b then recruits C6, C7, C8, and multiple C9 molecules. Those components insert into the lipid bilayer and polymerize into a channel, often described as C5b-9. Once enough pores form, the membrane loses integrity, ions leak, water moves in, and the target cell can lyse.

This is not random damage. The complement system is aiming at surfaces it recognizes as foreign or insufficiently protected by host regulatory proteins. That is why MAC is part of innate immunity even though it can also be triggered by the classical and lectin pathways upstream. The terminal step is the same no matter how the cascade started.

A useful way to remember it is that complement does three big things: opsonize, inflame, and lyse. MAC is the lysis branch. If a bacterium survives the early tagging steps, MAC gives the immune system one more way to remove it by physically breaking its membrane.

Not every microbe is equally vulnerable. Some bacteria resist MAC by altering their surface, and host cells are normally protected by complement regulatory proteins. That balance is why problems in the complement system can lead to recurrent infections or, on the other side, unwanted tissue damage when complement is activated in the wrong place.

Why the Membrane Attack Complex matters in IMMUNOBIOLOGY

Membrane Attack Complex shows how innate immunity can kill directly, not just signal for help. In Immunobiology, that matters because the complement system is more than inflammation markers and antibody support. MAC is the end of the line for one branch of complement activation, so if you can trace MAC formation, you can also trace the logic of the whole cascade.

It also connects several course ideas at once: enzyme-like amplification, surface recognition, membrane biology, and host protection. The same system that can lyse a pathogen has to avoid attacking your own cells, which is why complement regulatory proteins matter so much. When those controls fail, you get infection risk or immune damage, depending on what is broken.

You’ll also see MAC show up when a question asks you to connect upstream steps to a downstream outcome. If a prompt gives you C3 convertase or C5 convertase, MAC is often the final consequence to identify. If a case mentions a pathogen that is especially sensitive to complement, MAC is usually part of the explanation.

Keep studying IMMUNOBIOLOGY Unit 1

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How the Membrane Attack Complex connects across the course

Complement System

MAC is one output of the complement system, which is the broader protein cascade that tags microbes, recruits inflammation, and can directly lyse targets. If you understand where MAC fits, you can place it at the end of the complement story instead of treating it like a separate immune trick.

C3 Convertase

C3 convertase comes earlier in the cascade and sets up amplification by cleaving C3. Without that step, the pathway cannot efficiently build toward the terminal proteins that form MAC. So when you trace complement, C3 convertase is one of the main checkpoints before pore formation happens.

C5 convertase

C5 convertase is the immediate upstream enzyme complex that starts the terminal pathway. It cleaves C5 into C5a and C5b, and C5b is the piece that begins MAC assembly. If you remember one sequence, it is C5 convertase first, then C5b-9 formation after that.

Complement Regulatory Proteins

These proteins stop complement from injuring host cells, which is essential because MAC can lyse membranes if it is not controlled. They explain why your own cells are usually protected even though the complement system is active in your body all the time.

Is the Membrane Attack Complex on the IMMUNOBIOLOGY exam?

A quiz item or lab question usually asks you to trace the complement pathway from activation to cell damage. If you see a diagram with C5 being split, the next move is to identify C5b and recognize that it recruits the proteins that build C5b-9. If a case describes a pathogen cell bursting after complement activation, MAC is the structure behind that lysis.

In short-answer or discussion prompts, you may need to explain why complement can kill some microbes directly while also warning that host cells need regulatory proteins to stay safe. If the question gives a defect in complement components, connect it to weaker membrane pore formation and a higher risk of infection. On visuals, look for the terminal complex inserted into a membrane and name the outcome as pore formation and lysis.

Key things to remember about the Membrane Attack Complex

  • The membrane attack complex is the terminal complement structure that forms pores in a target membrane.

  • It is built from C5b, C6, C7, C8, and many C9 molecules, often written as C5b-9.

  • MAC kills by disrupting membrane integrity, which can cause water influx, ion imbalance, and lysis.

  • It sits at the end of the complement cascade, after C5 convertase has already generated C5b.

  • Host cells are normally protected by complement regulatory proteins, so MAC is aimed at foreign surfaces or poorly protected cells.

Frequently asked questions about the Membrane Attack Complex

What is Membrane Attack Complex in Immunobiology?

The membrane attack complex is the final protein assembly in the complement cascade that forms a pore in a target cell membrane. In Immunobiology, it represents the direct killing arm of innate immunity. Once the pore forms, the membrane leaks and the target cell can lyse.

How does the membrane attack complex kill cells?

MAC kills by inserting into the lipid bilayer and creating a channel through the membrane. That disrupts ion balance and membrane stability, which can lead to cell lysis. It is a physical membrane injury, not a signaling event.

What comes before MAC in the complement pathway?

C5 convertase comes right before MAC formation. It cleaves C5 into C5a and C5b, and C5b starts assembling the terminal complex with C6, C7, C8, and C9. If you know that sequence, the pathway becomes much easier to trace.

Is MAC the same thing as complement?

No. MAC is only one part of complement, specifically the terminal pore-forming step. The complement system also includes upstream activation, amplification, opsonization, and inflammatory signaling. MAC is the part that directly helps lyse a target cell.