The Complement System (Part I) - Nomenclature of Complement Proteins (FL-Immuno/13)

Summary

The complement system is a crucial part of the immune response, composed of plasma proteins that assist immune cells in eliminating pathogens. These proteins are inactive in the bloodstream but become active upon infection, triggering a cascade of reactions. The system's three major functions are enhancing phagocytosis, inflammation, and pathogen elimination. There are three distinct activation pathways—classical, alternative, and lectin—which differ in their initiation and protein involvement. The classical pathway's proteins are numbered C1 to C9, with unique naming conventions for cleaved fragments. Understanding the complement system's structure and function is key to appreciating its role in immune defense.

Takeaways

• 😀 The complement system is a group of plasma proteins that assist the immune cells in destroying pathogens.
• 😀 These complement proteins are mainly proteolytic enzymes that circulate in inactive forms and become active during infection, a process called complement activation.
• 😀 The complement system's activation triggers a cascade of reactions, where one reaction stimulates the next, amplifying the immune response.
• 😀 Complement activation leads to three major consequences that help eliminate pathogens: enhanced phagocytosis, inflammation, and cell lysis.
• 😀 There are three main complement pathways: the alternative, lectin, and classical pathways, which differ in initiation and involved proteins.
• 😀 The classical pathway's proteins were identified first, and are designated with 'C' followed by numbers (C1 to C9), which indicate their order of discovery.
• 😀 When complement proteins are cleaved, the resulting fragments are labeled with 'a' or 'b' for smaller or larger fragments, respectively (e.g., C3 cleaves into C3a and C3b).
• 😀 The larger fragment of cleaved complement proteins typically contributes to enzymatic activity in the cascade, such as C3b in this case.
• 😀 Complement protein C2 is an exception, where the larger fragment is C2a and the smaller fragment is C2b.
• 😀 Complement protein C1 is a complex of three proteins: C1q, C1r, and C1s, rather than a single protein.
• 😀 Proteins of the alternative pathway are termed factors (e.g., Factor B, Factor D), and they are designated with capital letters.

Q & A

What is the complement system?

The complement system is a group of plasma proteins that assist the immune cells in destroying pathogens. These proteins are made in the liver and are present in blood and extracellular fluids.

Why is the complement system called the 'complement' system?

It is called the complement system because the proteins complement the immune system by aiding immune cells in pathogen destruction.

What is complement activation?

Complement activation refers to the process where inactive complement proteins become activated during an infection, helping to fight the infection.

What happens during complement activation?

During complement activation, the proteins undergo a cascade, where one reaction triggers another, leading to three major consequences that help eliminate pathogens.

What are the three major consequences of complement activation?

The three major consequences of complement activation are enhanced phagocytosis, inflammation, and catalyzing cell lysis.

How many complement pathways are there, and what are they?

There are three complement pathways: the classical pathway, the alternative pathway, and the lectin pathway.

What differentiates the three complement pathways?

The pathways differ in how they are initiated and which proteins are involved in each pathway.

What is the general structure of complement proteins?

Complement proteins are named with a 'C' followed by a number, which indicates the order of their discovery. These proteins are often cleaved into two fragments: a larger fragment (e.g., C3b) and a smaller fragment (e.g., C3a).

How are the complement proteins designated when cleaved?

When complement proteins are cleaved, the larger fragment receives a 'b' suffix (e.g., C3b), and the smaller fragment receives an 'a' suffix (e.g., C3a). The larger fragment is generally the one contributing to the enzymatic activity in the cascade.

What is the exception in the cleavage of complement proteins?

An exception is complement protein C2. When cleaved, the larger fragment is C2a, and the smaller fragment is C2b.

What is unique about complement protein C1?

Complement protein C1 is not a single protein but a complex of three proteins: C1q, C1r, and C1s.

What are the proteins in the alternative pathway called?

Proteins in the alternative pathway are referred to as factors and are designated by capital letters, such as factor B and factor D.