Question 1

Which of the following complement fixation pathways can be initiated by a soluble C3 convertase?

Accepted Answer

The alternative pathway can be initiated by a soluble C3 convertase, which is formed spontaneously and can activate the pathway without the need for antibodies or lectins.

Question 2

On a holistic scale, the complement system plays a role in

Accepted Answer

The complement system is a complex network of proteins that work together to defend the body against infection. It is activated by the presence of pathogens, and it can kill pathogens directly, or it can help other immune cells to kill them. The complement system is both specific and nonspecific, meaning that it can recognize and kill a wide range of pathogens.

Question 3

Complement components can be classified into seven functional categories, including

Accepted Answer

Complement components indeed serve various functions, including acting as enzymatic mediators that catalyze the activation of other complement proteins, inflammatory mediators that recruit immune cells to sites of infection, membrane attack proteins that form pores in the membranes of pathogens, and regulatory components that control the complement system to prevent damage to host tissues.

Question 4

Of the molecules listed, which can be considered the key molecule in complement pathways?

Accepted Answer

C5 convertase is a key molecule in the complement pathways as it cleaves C5 into C5a and C5b, leading to the formation of the membrane attack complex.

Question 5

In considering the following list of molecules, which can act as a bridge to connect complement labeled pathogens to effector immune cells?

Accepted Answer

C1R and C3aR are receptors for complement proteins C1q and C3a, respectively. They are expressed on effector immune cells and can bind to complement-labeled pathogens, thereby bridging the gap between the complement system and the effector immune response.

Question 6

Which of the following provides serine protease activity in the classical complement activation pathway?

Accepted Answer

Both C1r and C1s are serine proteases that act in the classical complement activation pathway. C1r is activated first and then activates C1s, which then cleaves C4 and C2 to propagate the pathway.

Question 7

Which of the following is the MOST potent anaphylatoxin produced during the complement cascade?

Accepted Answer

C5a is recognized as the most potent anaphylatoxin produced during the complement cascade, playing a significant role in inflammation and attracting neutrophils and other immune cells to the site of infection or injury.

Question 8

To which of the following is MBL MOST structurally similar?

Accepted Answer

MBL (Mannose-binding lectin) is most structurally similar to C1q. Both are part of the lectin pathway of the complement system and share a similar collagen-like structure, allowing them to bind to pathogens and initiate the complement cascade.

Question 9

Which of the following is TRUE about the C5 convertase?

Accepted Answer

C5 convertase is crucial in the complement system, performing the specific function of cleaving C5 into C5a and C5b, where C5b initiates the formation of the membrane attack complex (MAC). The description of C5 convertase components in option A, although not standard nomenclature, implies its complex nature involving multiple components. C5a acts as a potent proinflammatory chemotactic factor, and the formation of MAC is a direct consequence of C5 convertase activity, making all provided statements true regarding its function and importance.

Question 10

Which of the following BEST explains the timing of the classical pathway of complement fixation, relative to the other pathways of complement fixation, against a pathogen in an individual who has never been exposed to said pathogen?

Accepted Answer

The classical pathway requires antibodies to bind to the pathogen, which are not present in an individual who has never been exposed to the pathogen. Therefore, there is no target for C1qrs in the earliest stages.

Question 11

Which of the following binds to surface-bound C3b during formation of the alternative C3 convertase?

Accepted Answer

The answer of Which of the following binds to surface-bound...

Question 12

Which of the following can be directly activated as a soluble protein in the microenvironment surrounding a pathogenic cell?

Accepted Answer

C3 is the only protein that can be directly activated as a soluble protein in the microenvironment surrounding a pathogenic cell.

Question 13

Which of the following acts as a protease that is required to generate the alternative pathway C3 convertase?

Accepted Answer

Factor D is a protease that cleaves Factor B when it is bound to C3b, leading to the formation of the alternative pathway C3 convertase (C3bBb).

Question 14

Which of the following is the first complement factor to be activated, resulting in insertion of one of its subunits into the pathogen surface in two of the three complement fixation pathways?

Accepted Answer

C4 is the first complement factor to be activated, resulting in insertion of one of its subunits into the pathogen surface in two of the three complement fixation pathways.

Question 15

Which of the following, when activated itself, acts as a protease that converts soluble C3 into C3a and C3b in the alternative pathway?

Accepted Answer

Factor B, when cleaved by Factor D, forms Bb, which acts as a protease to convert C3 into C3a and C3b in the alternative pathway.

Question 16

Which of the following, when activated, utilizes its "a" component instead of the "b" component in the formation of the C3 convertase?

Accepted Answer

The answer of Which of the following, when activated, utilizes...

Question 17

Which of the following immunoglobulin isotypes is the MOST efficient at initiating a complement fixation cascade?

Accepted Answer

IgM is the most efficient at initiating a complement fixation cascade due to its pentameric structure, which provides multiple binding sites for the complement system, leading to a rapid and effective activation of the cascade.

Question 18

Addition of which of the following components to any C3 convertase leads to the formation of the C5 convertase?

Accepted Answer

C3b binds to C3 convertase to form C5 convertase, which then cleaves C5 into C5a and C5b, initiating the formation of the membrane attack complex.

Question 19

Which form of IgM is MOST readily recognized by C1q?

Accepted Answer

"C"Staple-form" pentameric IgM is the form most readily recognized by C1q. This is because the staple form exposes the Fc regions in a way that is optimal for C1q binding, which is essential for the activation of the classical complement pathway.

Question 20

Which of the following acts as a positive regulator of complement fixation via stabilization of C3bBb?

Accepted Answer

Properdin acts as a positive regulator of the complement system by stabilizing the C3 convertase enzyme complex (C3bBb), thereby promoting complement fixation and enhancing the immune response.

Quiz 5: The Complement System