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Membrane attack complex (MAC)

The membrane attack complex (MAC) is the terminal group of complement proteins that forms a pore in a target membrane and can lyse microbes. In Microbiology, it is a classic innate immune mechanism for killing certain pathogens, especially Neisseria.

Last updated July 2026•Fiveable Content Team

What is the membrane attack complex (MAC)?

The membrane attack complex (MAC) is the last step of the complement system in Microbiology, where complement proteins assemble into a pore on a microbe's membrane. Once that pore opens, the target cell loses control of what enters and leaves, which can lead to lysis and death.

MAC formation starts after complement has already been activated through a pathway such as the alternative pathway or antibody-triggered pathways. That activation produces C5 convertase, which splits C5 into C5a and C5b. C5b then acts like a landing point for the rest of the complex, recruiting C6, C7, C8, and many C9 proteins.

The finished MAC is often described as C5b-9. C9 subunits polymerize into a ring-like channel, and that channel punches through the membrane. For bacteria, especially Gram-negative bacteria with an outer membrane, this can be deadly because the pore disrupts membrane integrity and the cell can no longer maintain its internal balance.

MAC is part of innate immunity, so it does not depend on prior exposure to a specific pathogen. That said, it can work alongside adaptive immunity when antibodies help trigger complement. This is why MAC sits at the intersection of nonspecific defense and antibody-driven defense in Microbiology.

Not every microbe is equally vulnerable. Enveloped organisms and some bacteria are easier targets than others, while microbes with thick protective walls, capsules, or complement evasion strategies may resist damage. A useful way to think about MAC is that it is not a general toxin, it is a membrane-damaging endpoint of a larger immune cascade.

Why the membrane attack complex (MAC) matters in MICROBIO

MAC shows up any time Microbiology turns from list-making to mechanism. If you are tracing how the immune system actually kills a pathogen, MAC is the final destructive step after recognition and complement activation.

It also connects several course ideas at once: innate immunity, antibody function, pathogen structure, and host defense defects. For example, if someone lacks a working terminal complement component, they can still make antibodies, but they lose an efficient way to punch holes in certain bacteria. That makes MAC a good example of why immune pathways are layered rather than redundant.

This term matters a lot for infection patterns too. Students often see MAC linked to Neisseria because those bacteria are especially sensitive to terminal complement defects. So when a case mentions repeated Neisseria infections, the MAC pathway is one of the first immune mechanisms to check.

MAC also helps you separate lysis from other immune outcomes. Some complement fragments attract inflammation, some tag microbes for phagocytosis, and MAC actually disrupts membranes. That distinction comes up in quizzes, case studies, and any question that asks you to match a complement component with its effect.

Keep studying MICROBIO Unit 18

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How the membrane attack complex (MAC) connects across the course

Complement System

MAC is the final product of the complement system. If you know the earlier steps, like activation and C5 convertase formation, MAC makes sense as the endpoint instead of a random protein complex. Many Microbiology questions ask you to trace the pathway from recognition to lysis.

Alternative Pathway

The alternative pathway is one of the routes that can lead to MAC formation. It matters because MAC does not only appear after antibodies bind a pathogen, it can also come from innate complement activation on microbial surfaces.

Neisseria

Neisseria species are the classic organisms associated with MAC susceptibility. If terminal complement components are missing, these bacteria can survive better, so recurrent Neisseria infection is a clue that the MAC branch of complement may be impaired.

Antibody-Dependent Cell-Mediated Cytotoxicity

This term is easy to confuse with MAC because both involve immune attack after antibody recognition. But ADCC uses immune cells such as NK cells to kill the target, while MAC kills by assembling a pore directly in the membrane.

Is the membrane attack complex (MAC) on the MICROBIO exam?

A quiz question may give you a complement diagram and ask which component creates membrane pores, so you would identify the C5b-9 complex and connect it to lysis. In a case study, you might explain why a patient with terminal complement deficiency gets repeated Neisseria infections. If the prompt asks for the effect of complement activation, MAC is the answer when the outcome is direct membrane disruption rather than opsonization or inflammation. In lab or problem-set questions, you may also be asked to compare intact and damaged cells under a membrane integrity assay and explain the result using pore formation.

The membrane attack complex (MAC) vs Antibody-Dependent Cell-Mediated Cytotoxicity

Both MAC and ADCC lead to target-cell death, but they do it in different ways. MAC is a complement pore that directly lyses the membrane, while ADCC depends on immune cells recognizing antibody-coated targets and then killing them. If the question mentions C5b-9 or membrane pores, think MAC; if it mentions NK cells and Fc receptors, think ADCC.

Key things to remember about the membrane attack complex (MAC)

  • The membrane attack complex is the pore-forming end stage of the complement cascade in Microbiology.

  • It is made from C5b, C6, C7, C8, and multiple C9 proteins, which assemble into a membrane channel.

  • MAC damages the target membrane directly, so the microbe can lose contents and die by lysis.

  • This mechanism is part of innate immunity, but antibodies can help trigger the complement pathway that leads to it.

  • Problems with terminal complement proteins are strongly linked to recurrent Neisseria infections.

Frequently asked questions about the membrane attack complex (MAC)

What is membrane attack complex (MAC) in Microbiology?

MAC is the terminal complement complex that forms a pore in a pathogen's membrane. In Microbiology, it is a direct killing mechanism that can lyse susceptible microbes, especially certain Gram-negative bacteria.

What proteins make up the membrane attack complex?

The MAC is built from complement proteins C5b, C6, C7, C8, and many C9 molecules. C5b starts the assembly, and C9 forms much of the final pore structure.

Why is MAC associated with Neisseria infections?

Neisseria are especially vulnerable to the terminal complement pathway. If MAC components are missing or defective, these bacteria are harder to kill, so recurrent Neisseria infection can point to a complement problem.

Does MAC come from innate or adaptive immunity?

MAC is part of innate immunity because it acts through the complement system without needing prior exposure to a specific microbe. Still, antibody binding can trigger complement, so adaptive immunity can set the process in motion.