Question

How does complement kill bacteria?

Short answer

The complement system activates proteins that form a complex to punch holes in bacterial membranes, causing lysis.

Step-by-step solution

Understand the Role of Complement

The complement system is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear pathogens from an organism. It includes a set of proteins that circulate in an inactive form in the blood until they are activated.

Activation of the Complement System

The complement system can be activated through three different pathways: the classical pathway, the lectin pathway, and the alternative pathway. Activation usually involves recognition of foreign pathogens by antibodies or other molecules, leading to a cascade of reactions that activate the complement components.

Formation of Membrane Attack Complex (MAC)

When activated, the complement proteins form complexes that assemble into a structure known as the Membrane Attack Complex (MAC). The MAC forms a pore in the membrane of the target bacterial cell.

Lysis of Bacterial Cell Membrane

The formation of the MAC results in punching holes in the bacterial cell membrane. This disrupts the integrity of the membrane, causing an influx of water and ions.

Bacteria Cell Death

The influx of water and ions into the bacterial cell leads to swelling and eventual rupture of the cell membrane. This results in lysis, effectively killing the bacterial cell.

Key concepts

Immune Response

The immune response is the body's defense mechanism against harmful pathogens like bacteria and viruses. When a pathogen such as a bacterial invader breaches the body's initial defenses, the immune system kicks into gear.
A key component of this response is the complement system. This is a series of proteins in the blood that, when activated, work in tandem with antibodies and other immune cells to manage and destroy the pathogen.
The complement system helps in several ways:

• Tagging pathogens for destruction by phagocytes
• Recruiting inflammatory cells to the site of infection
• Punching holes in bacterial cell walls

Each of these actions is crucial for an efficient immune response, ensuring pathogens are swiftly and effectively neutralized.

Bacterial Cell Lysis

Bacterial cell lysis refers to the process where a bacterial cell is destroyed by rupturing its cell membrane. The complement system plays a pivotal role in this process.
Upon activation, the complement proteins initiate a sequence of events leading to the formation of the Membrane Attack Complex (MAC). This complex is essentially a multi-protein unit that directly interacts with the bacterial cell membrane.
Once the MAC is positioned on the surface of the bacterial cell:

• It creates a pore by drilling through the membrane
• This pore compromises the cell's structural integrity
• Allows water and ions to rush inside

Eventually, the swelling caused by the water influx makes the bacterial cell burst open, a process known as lysis, thereby leading to the cell's death.

Membrane Attack Complex

The Membrane Attack Complex, commonly abbreviated as MAC, is the final product in the activation sequence of the complement system. This complex is crucial in the direct elimination of pathogens.
The complement proteins, when activated, come together to form this complex which is shaped like a tube. When the MAC is assembled on the microbial membrane:

• It inserts itself like a cylinder composed of several protein units
• This assembly forms a hole, or pore, on the bacterial cell membrane
• The integrity of the membrane is massively disrupted

This disruption is like opening the floodgates, causing uncontrollable flow of substances in and out of the cell, leading to the bacterial cell's eventual bursting. The MAC is an essential part of the swift immune response that prevents the further spread of infections by effectively destroying bacterial cells.