Understanding the Complement System: Key Factors & Pathways

Understanding the Complement System

Among the factors that may act on both the Alternative Complement pathways and lectins are:

• C3 Factor I
• C2 C4

The Complement system factors that can act with serine protease activity are:

• Bb / Factor I
• MBL / C2

Complement Factors and Their Derivatives

Regarding complement factors or their derivatives:

• C3b can act as an opsonin
• Anaphylatoxin C4a
• C5a is a chemotactic agent
• C2a is an inflammatory agent
• C3b helps elimination of immune complexes
• C1r is a serine protease

Opsonization

Opsonization is a phenomenon that may be immune mediated by:

• C4b
• MBL
• IgG
• C3b

Classical Pathway of Complement Activation

Key aspects of the classical pathway of complement activation:

• It is associated with the adaptive immune response.
• It is activated by an antigen bound to any class of immunoglobulin.
• Evolutionarily, it is the oldest.
• Properdin is required to stabilize the C3 convertase.

Complement System Activation

The complement system can be activated by any of the pathways, induced by:

• Membrane molecules of pathogens
• Collectins
• IgM and IgA

Consequences of Complement System Activation

Activation of the complement system leads to:

• Stimulation of phagocytosis
• Inflammation
• Chemotaxis of phagocytes
• Elimination of circulating immune complexes

Characteristics of the Alternative Pathway

Among the characteristics of the alternative pathway are:

• Involving two C3 convertases
• Properdin protein participation
• The attack phase begins after membrane rupture C5

An antigenic stimulus needs to activate the complement system.

Cofactors in Factor I

In the complement system, cofactors in factor I are:

• Factor H
• C4bp

• C1 inhibitor
• Factor DAF

Cell Complement Production

The main factor producing cell complement is the macrophage.

Complement Regulatory Molecules

Complement regulatory molecules can act on:

• Factor H in via AC
• C4bp in via classical
• CD 59 in all avenues

• C1 inhibitor in the road of Lectins

General Characteristics of Complement System Molecules

Regarding the molecules of the complement system in general:

• They normally circulate in plasma in an inactive form.

IgM as a Complement Activator

IgM is a better complement activator than IgG because:

• It is a pentamer.

MHC Class 1a Molecules

About MHC Class 1a molecules:

• They are associated with the antigenic peptide in the endoplasmic reticulum.
• They are expressed in skin cells.
• They are highly polymorphic.
• Both chains of the molecule are encoded in the MHC.

Anchor Residues

Anchor residues are amino acids present in:

• The peptide to be presented.

More on Class 1a Molecules

In relation to Class 1a molecules:

• The β chain is not polymorphic.
• Domain α3 is the region recognized by CD8.
• The peptide binding site is given by α1 and α2 domains.

They only present foreign peptides.

Shared Characteristics of MHC Class 1a and 1b

Among the characteristics shared by the MHC Class 1a and 1b:

• They consist of an α string and β chain.

Professional Antigen-Presenting Cells

Cells considered professional antigen-presenting cells:

• Macrophages
• Dendritic Cells
• Lymphocytes B
• Hepatocytes

B Cell Receptor Reprogramming

Editing or Receiver Reprogramming means when B cells:

• Change the specificity of the autoreactive BCR.

• Via classic: adaptive immune response
• Via alternative: innate immune response
• Via lectins

• Activation of collectins (MBL): RI innate
• RI collectins IgA and adaptive (both use MBL and MASP)