The Complement System

Complement Proteins

It is a series of more than 50 soluble and cell bound proteins that interact with the innate and adaptive immune systems to enhance host defenses against infection.

CSF and urine

Complement constitutes 5% of normal serum protein, heat labile that are present in normal human serum and all tissue fluids except?

C3

It is the key mediator in all pathways

C1r

It activates C1s

1. Lysis of cells (bacteria, allografts, tumor cells)

2. Generation of mediators of inflammation (activation of inflammatory response)

3. Opsonization

4. Clearance of immune complex

Main effects/functions of complement:

• Recognition Phase

• Activation Phase

• Membrane Attack Phase

The three phases of the Classical Pathway:

Classical Pathway

It is initiated by formation of antigen-antibody complex

Alternative Pathway

• It is initiated by foreign cell surface

• Antibody independent

• Part of innate immunity

• Components include:

  • C3

  • Factor B

  • Factor D

  • Properdin

Lectin Pathway

It is activated by direct recognition of surface moieties that are found on pathogens.

Lectin

It is a protein that binds to carbohydrates

Mannose-binding Lectin (MBL)

It is considered an acute-phase protein because it is produced in the liver and is normally present in the serum but increases during an initial inflammatory response

S Protein

• a.k.a Vitronectin

• Binds to C5b67

• Inhibits formation of the MAC

• Prevents insertion into cell membranes near the site where the complement cascade was initiated

HRF or CD59

• Binds to C8

• Inhibits C9 binding

Factor H

• It is the principal soluble regulator of the alternative pathway

• Binds to C3b

• Facilitates binding of Factor I

  • cleaves C3b to inactive iC3b

  • cleaves C4b to inactive fragments

C1q

• It is composed of six globular heads with a collagen-like tail portion.

• It binds to the Fc region of IgM and IgG

C4

It is the second most abundant complement protein, with a serum concentration of approximately 600 µg/mL

C4a

C1s cleaves C4 to release a 77-amino acid fragment called?

C6

Binds to C5b in MAC

C7

Binds to C5bC6 in MAC

C8

It starts pore formation on membrane

C9

It polymerizes to cause cell lysis

C3 convertase

The combination of C4b and C2a is known as?

C3

• It is the major and central constituent of the complement system

• Activation of the alternative pathway begins with this

• It serves as the pivotal point of convergence of all three complement pathways.

• It is present in the plasma in a concentration of 1 mg/mL to 1.5 mg/mL.

C5

• The most potent molecule, it is at least 200 times more powerful than C3a

• Initiates membrane attack complex

• It consists of two polypeptide chains, α and β, which are linked by disulfide bonds to form a molecule with a molecular weight of about 190 kDa.

C5b

It is extremely labile and is rapidly inactivated unless binding to C6 occurs.

Properdin

• It is a protein in normal serum with a concentration of approximately 5 to 15 µg/mL

• The alternative pathway was originally named for this protein

Factor D

• It is a plasma protein that goes through a conformational change when it binds to Factor B.

• It cleaves Factor B

• In the plasma, it has the lowest concentration of all the complement proteins, approximately 2 µg/mL.

C1 inhibitor (C1-INH)

It inhibits activation at the first stages of both the classical and lectin pathways.

Its main role is to inactivate C1 by binding to the active sites of C1r and C1s.

Immune adherence

The ability of cells to bind complement coated particles is referred to as?

MCP or CD46

• It has a molecular weight of between 50 kDa and 70 kDa and is found on the cell membrane of virtually all epithelial and endothelial cells except erythrocytes.

• It is the most efficient cofactor for Factor I mediated cleavage of C3b.

Collectin receptors

They bind the collagen portion of C1q and generally enhance the binding of C1q to Fc receptors.

Anaphylatoxin

It is a small proinflammatory peptide that causes increased vascular permeability, contraction of smooth muscle, and release of histamine from basophils and mast cells.

C5a

It serves as a chemotaxin for neutrophils, basophils, eosinophils, mast cells, monocytes, and dendritic cells.

Paroxysmal nocturnal hemoglobinuria (PNH)

A prime example of a disease caused by a missing or defective regulatory component is?

Hereditary angioedema (HAE)

• It is characterized by recurrent attacks of swelling that affect the extremities, the skin, the gastrointestinal tract, and other mucosal surfaces.

• This disease is caused by a deficiency or lack of C1-INH.

Hemolytic uremic syndrome (HUS)

It is one of the most common causes of acute renal failure in children worldwide and is characterized by hemolytic anemia, low platelet count, and acute renal failure

C3 Glomerulopathy (C3G)

It is a disease that causes inflammation of the glomeruli of the kidneys, leading to hematuria, proteinuria, hypertension, and gradual decline of kidney function.

Nephelometry and Immunoturbidimetry

The most common methods for measuring individual complement proteins are?

Nephelometry

It measures the concentration of an individual complement protein according to the amount of light scattered by the antigen–antibody mixture

Immunoturbidimetry

It is based on the reduction in light transmission resulting from immune complex formation

Hemolytic titration (CH50) assay

This assay measures the amount of patient serum required to lyse 50% of a standardized concentration of antibody-sensitized sheep erythrocytes.

• MBL

• MASP-1

• MASP-2

• MASP-3

Proteins distinct to the lectin pathway include:

Membrane Attack Complex (MAC)

All three pathways terminate with formation of the?

• C1-INH

• C4BP

• Factor H

• Factor I

• S protein

Soluble regulators include:

• CR1

• MCP

• DAF

Examples of cell-bound regulators are:

Chemotaxins

They attract phagocytic cells to a specific area

Opsonins

They coat damaged or foreign cells to enhance phagocytosis.

aHUS

It is a thrombotic microvascular disease that is rooted in improper control of the alternative pathway of complement caused by genetic mutations in the complement system.

• Hemolytic CH50

• Automated Liposomal CH50

• ELISA assays

The three main methods for testing classical pathway function:

Antigen–antibody complexes.

REVIEW QUESTION:

The classical complement pathway is activated primarily by:

Normally present in serum

REVIEW QUESTION:

Which of the following is characteristic of complement components?

C1q becomes an active esterase.

REVIEW QUESTION

All of the following are true of the recognition unit except

• C4bC2a

• C3bBb

REVIEW QUESTION

Which of the following is referred to as C3 convertase?

• C1qrs

• C4bC2a

• C3bBb

MASP-2

It cleaves C4 and C2

C1q

REVIEW QUESTION

Mannose-binding protein in the lectin pathway is most similar to which classical pathway component?

Stabilization of C3 convertase

REVIEW QUESTION

Which of the following describes the role of properdin in the alternative pathway?

C9 polymerizes to form the transmembrane channel.

REVIEW QUESTION

Which best characterizes the MAC?

Decreased clearance of antigen–antibody complexes.

REVIEW QUESTION

All of the following represent functions of the complement system except:

All of the above

REVIEW QUESTION

Which of the following are diseases associated with complement deficiencies or improper control?

• Age-related macular degeneration (AMD)

• Paroxysmal nocturnal hemoglobinuria (PNH)

• Atypical hemolytic uremic syndrome

• C3 glomerulopathy

Increasing amounts of C1qrs are produced.

REVIEW QUESTION

Which of the following is not true of the amplification loop in complement activation?

Factor B

REVIEW QUESTION

Factor H acts by competing with which of the following for the same binding site?

Acts as co-receptor on B cells for antigen

REVIEW QUESTION

Which best describes the role of CR2 on cell membranes?

• Low CH50

• Low AH50

REVIEW QUESTION

Which of the following would be expected to be true for the testing of a patient who is on a complement C5 inhibitor?

It is a common cause of renal failure in children

REVIEW QUESTION

Which of the following best characterizes HUS?

Patient serum required to lyse 50% of sensitized sheep RBCs

REVIEW QUESTION

The CH50 test measures which of the following?

Decrease in components of the classical pathway only

REVIEW QUESTION

A decreased CH50 level and a normal AH50 level indicate which deficiency?

Increases permeability of blood vessels

REVIEW QUESTION

Which best describes the role of an anaphylatoxin?

C3bBbC3b

REVIEW QUESTION

Which of the following is not a cofactor for Factor I?

A CH50 level would be falsely low

REVIEW QUESTION

If a specimen for complement testing was left on a laboratory bench overnight and not properly frozen, which of the following would be true?