Effects of the Complement System: Videos & Practice Problems

The complement system activation leads to three key immune responses: opsonization, microbe cell lysis, and inflammation. Opsonization enhances phagocytosis by coating microbes with opsonins like C3b, facilitating their recognition by phagocytes. Microbe cell lysis occurs through membrane attack complexes (MACs), effective against gram-negative bacteria. The inflammatory response, driven by C3a and C5a, promotes blood vessel dilation and immune cell recruitment, characterized by swelling, redness, and pain. Understanding these mechanisms is crucial for grasping innate immunity and its role in pathogen elimination.

Effects of the Complement System

In this video, we're going to begin our lesson on the effects of the complement system. And so, recall from some of our previous lesson videos that activation of the complement system can result in three possible effects or immune responses. The first of those is going to be called opsonization. The second is going to be lysis of the microbe or microbe cell lysis. And the third is going to be the inflammatory response leading to inflammation. If you take a look at our image down below, notice that we're showing you the complement system, which we know from our previous lesson videos can be activated by three different pathways. All three of these pathways will ultimately lead to the formation of the enzyme C3 convertase. And the formation of this enzyme C3 convertase ultimately will lead to these immune responses, either opsonization, microbe cell lysis, or the inflammatory response. Really, these are the effects of the activation of the complement system. As we move forward in our course, we'll be able to talk some more details about each of these effects of the complement system. This here concludes our brief introduction to the effects of the complement system, and I'll see you all in our next lesson video to talk more details about each of these three effects.

1) Opsonization

This video, we're going to talk more details about opsonization, which is one of the three effects of complement system activation. And so, opsonization is really just a process that makes microbes easier to bind and engulf during the process of phagocytosis, which recall is cellular eating. Now during the process of opsonization, microbes are going to be coated with these molecules called opsonins. And so opsonins are really just these small molecules that will bind to the cell surface of microbes, and they can be recognized by cell surface receptors of phagocytic cells. And so for example, C₃B is an opsonin. And so C₃B as an opsonin, it will bind to the surface of microbes. And by binding to the surface of microbes, C₃B can make phagocytosis much easier and more effective for phagocytic cells. And so if we take a look at our image down below, notice at the top we're showing you a map of our lesson that starts with C3 convertase. And we know that activation of the complement system through either of the 3 pathways will ultimately lead to C₃ convertase. And C₃ convertase will ultimately lead to one of these three effects of the complement system activation. And here in this video, we're focused on optimization. Now down below over here on the left-hand side, we're showing you a little cartoon. If there is no optimization, then notice that the macrophage, which is our phagocytic cell, is gonna have a lot more trouble, phagocytosing the microbe. And so notice here the microbe is saying can't catch me, and the macrophage is asking why can't I grab you? And so phagocytosis is much more difficult if there is no opsonization. Now through opsonization, which recall is one of the effects of complement system activation, Then these opsinin molecules, which are these little blue circles will bind to the surface of the microbes. And so those little opsonins somewhat act as like little grips that makes it a lot easier for the phagocytosed, for the phagocyte to bind and phagocytosed the microbe. And so notice that these gloves here are representing opsonin cell surface receptors that allow for the phagocyte to bind a lot easier to the microbe. And so notice that the macrophage is saying, gotcha now. And the microbe here is not very happy saying, oh, no. And so basically what we're saying here is that opsonization makes phagocytosis way easier for the phagocyte. Now, up here at the top right, we're showing you a similar image, but we've got our, macrophage here in blue, and it is phagocytosing this microbe that you see right here. And if we zoom in, what you'll notice is that the microbe is going to be covered with C₃B, which is an example of an opsonin. And the Opsonin C₃B binds to the surface of the microbe, and so the cell surface receptors of the phagocyte can bind to the C₃B and it makes it a lot easier for the, phagocyte to phagocytose, the microbe. And so ultimately what we're saying here is that optimization is a process that makes phagocytosis much easier and much more effective and efficient. And so, this here concludes our brief lesson on optimization, and we'll be able to get some practice applying these concepts as well as move forward and talk about the other effects of complement system activation including microbe sole lysis and the inflammatory response. So I'll see you all in our next video.

A pathogenic bacterium that is able to avoid binding with the complement protein C3b would most likely protect itself from which of the following immune responses?

• A. Cell lysis of the bacterium.
• B. Opsonization.
• C. Triggering an inflammatory response.
• D. Antibodies binding to the bacterium.

Answer: (B)

2) Cell Lysis of Invading Microbes

This video, we're going to talk a little bit about cell lysis of invading microbes, which is one of the effects of complement system activation. Multiple complement system proteins can form a complex in cell membranes, and these complexes are referred to as membrane attack complexes. These membrane attack complexes are commonly abbreviated as MACs. These membrane attack complexes or MACs consist of multiple complement system proteins that create pores or holes in the cell. These pores or holes in the cell membrane can cause microbes to die by cell lysis. Now these membrane attack complexes or MACs are very effective against gram-negative bacteria, but they are not very effective against gram-positive bacteria because they have a thick peptidoglycan cell wall that surrounds their membrane. However, the gram-negative cells, recall, have an outer lipid membrane, and the membrane attack complex can affect that outer lipid membrane.

The components of the complement system that are involved in the MAC formation include the complement system proteins C5 through C9. If we take a look at our image down below here on the left-hand side, once again, we're showing you our map of the lesson here that starts with C3 convertase, which we know results from activation of the complement system. The formation of C3 convertase can ultimately lead to one of these three immune responses. Here in this video, we are focusing on microbe cell lysis. Cell lysis of invading microbes occurs when complement system proteins form a MAC, which is a membrane attack complex. Notice here, we're showing you a microbe. Zooming in to this microbe, notice that there is a pore being formed by this membrane attack complex or MAC. The membrane attack complex consists of multiple complement system proteins from C5 through C9, and you can see those complement system components right here. But ultimately, the formation of a membrane attack complex in these gram-negative bacteria can lead to cell lysis of the microbe, and cell lysis of the microbe can eliminate that microbe and prevent it from causing us harm.

This here concludes our brief lesson on this topic, and we'll be able to get some practice applying these concepts as we move forward. We'll also talk about the third effect of complement system activation, which is the inflammatory response. So, I'll see you all in our next video.

The complement system can lead to the removal or destruction of invading microbes and the inflammatory response. Which immune response of the complement system is not effective against gram-positive (+) pathogenic bacteria?

• A. Opsonization of invading microbe.
• B. Inflammatory response.
• C. Cell lysis of invading microbe.

Answer: (C)

How do membrane attack complexes cause cell lysis of invading microbes?

• A. The complexes attack the cell walls of the microbe causing it to be vulnerable to attack by immune cells.
• B. The complexes form pores in the cell membrane of the microbe causing the cytoplasmic contents to leak out.
• C. The complexes attack the cell surface receptors of the microbe triggering the cell membrane to degrade.

Answer: (B)

3) Inflammatory Response

In this video, we're going to briefly discuss the inflammatory response, which is one of the three effects of activation of the complement system. And so, in order to contain the site of damage, eliminate microbes, and restore tissue damage, the inflammatory response may occur. And again, the inflammatory response can be triggered by the activation of the complement system. First, we need to recall from some of our previous lesson videos that inflammation is an innate immune response. It is characterized by swelling, redness, heat, pain, and sometimes loss of function. Now, in this process of the inflammatory response, we're going to see that the complement proteins C_3a and C_5a are going to be the most important ones. These proteins will induce changes in the epithelial cells that are lining the blood vessels; C_3a and C_5a will cause those epithelial cells lining the blood vessels to dilate. Dilation just means to enlarge. The dilation of the epithelial cells lining the blood vessels allows for additional complement system proteins, as well as immune system cells, to leave the blood and enter into the infected site. This promotes inflammation.

Complement system proteins C_3a and C_5a will also cause mast cells to degranulate or release their granules into the environment, releasing pro-inflammatory cytokines that promote inflammation. Take a look at our image down below on the left-hand side, where we show you our map of the lesson starting with C3 convertase, which forms from the activation of the complement system. The formation of C3 convertase can lead to these three different effects: immune responses, either opsonization, microbe cell lysis, or the main focus of this video, the inflammatory response. Notice that it's going to be the complement system proteins C_3a and C_5a that are most important in the inflammatory response. C3 convertase cleaves the inactive protein C_3b into the active fragments C_3a and C_3b. C_3a is needed, and C_3b can combine with other molecules that ultimately leads to the cleavage of C₅ into C_5a and C_5b. C_5a and C_3a, the two we are mainly focused on here. When C_3a and C_5a bind to mast cells, which are very similar to basophils but exist in the tissues rather than circulating in the blood, it causes those mast cells to release their granules, to degranulate. When mast cells degranulate, they release histamine and other pro-inflammatory cytokines that promote inflammation.

Notice in the image down below that we have blood cells exiting the bloodstream and into the site of infection, leading to the events of inflammation. As we move forward in our course, we will talk even more about the details of the inflammatory response and inflammation. But for now, this concludes our brief lesson on how the activation of the complement system and the complement system proteins C_3a and C_5a can lead to an inflammatory response that helps to eliminate microbes. We will be able to get some practice applying these concepts, and then we will move on to the next topic. So, I'll see you all in our next video.

Which of the following cellular responses does not occur due to the inflammatory response?

• A. Mast cells degranulate releasing histamines.
• B. Epithelial cells change shape allow blood vessels to dilate.
• C. Macrophages pass through the epithelial cells of blood vessels to the site of injury.
• D. Neutrophils form membrane attack complexes to attack invading microbes.

Answer: (D)

The main complement proteins involved in the inflammatory response of the immune system are?

• A. C3a and C3b.
• B. C3a and C5a.
• C. C5a and C5b.
• D. C5a and C3b.

Answer: (B)

Here’s what students ask on this topic:

What are the three main effects of complement system activation?

The three main effects of complement system activation are opsonization, microbe cell lysis, and the inflammatory response. Opsonization enhances phagocytosis by coating microbes with opsonins like C3b, making them easier for phagocytes to recognize and engulf. Microbe cell lysis occurs through the formation of membrane attack complexes (MACs), which create pores in the cell membranes of microbes, leading to their destruction. This is particularly effective against gram-negative bacteria. The inflammatory response, driven by complement proteins C3a and C5a, promotes blood vessel dilation and the recruitment of immune cells to the site of infection, characterized by swelling, redness, heat, and pain.

How does opsonization enhance phagocytosis?

Opsonization enhances phagocytosis by coating microbes with molecules called opsonins, such as C3b. These opsonins bind to the surface of the microbes and are recognized by specific receptors on phagocytic cells like macrophages and neutrophils. This binding facilitates the attachment of the phagocyte to the microbe, making it easier for the phagocyte to engulf and digest the microbe. Essentially, opsonins act as 'handles' that improve the efficiency and effectiveness of the phagocytic process.

What is the role of membrane attack complexes (MACs) in the complement system?

Membrane attack complexes (MACs) play a crucial role in the complement system by forming pores in the cell membranes of invading microbes, leading to cell lysis and death. MACs are composed of multiple complement proteins, specifically C5 through C9. These complexes are particularly effective against gram-negative bacteria, which have an outer lipid membrane that is susceptible to pore formation. The formation of these pores disrupts the integrity of the microbial cell membrane, causing the cell to lyse and ultimately die, thereby eliminating the threat.

How do C3a and C5a contribute to the inflammatory response?

C3a and C5a are key complement proteins that contribute to the inflammatory response by inducing changes in the epithelial cells lining blood vessels, causing them to dilate. This dilation allows additional immune cells and complement proteins to exit the bloodstream and enter the site of infection. C3a and C5a also cause mast cells to degranulate, releasing histamine and other pro-inflammatory cytokines. These substances promote inflammation, characterized by swelling, redness, heat, and pain, which helps to contain the site of damage, eliminate microbes, and restore tissue function.

Why are membrane attack complexes (MACs) more effective against gram-negative bacteria than gram-positive bacteria?

Membrane attack complexes (MACs) are more effective against gram-negative bacteria because these bacteria have an outer lipid membrane that is susceptible to pore formation by MACs. In contrast, gram-positive bacteria have a thick peptidoglycan cell wall that surrounds their membrane, providing a protective barrier that MACs cannot easily penetrate. This structural difference makes gram-negative bacteria more vulnerable to the lytic action of MACs, leading to their destruction through cell lysis.