1. Complement System 2. Antigen Specific Receptors K.J. Goodrum - - PDF document

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• 1. Complement System
• 2. Antigen Specific Receptors

K.J. Goodrum Department of Biomedical Sciences 2005

Topic Outline

• Complement System

– Activation pathways – Bioactive complement peptides & functions – Complement regulatory proteins

• Lymphocyte receptors for antigens

– B cell receptor (immunoglobulin) and T cell receptor (TcR) structure

Complement System

Composition: proenzyme enzyme plasma protein cascade system with >25 proteins (enzymes, receptors, and complement inhibitors) Synthetic source: liver, macrophages Function: mediates inflammation by increasing vascular permeability, attracting phagocytes, enhancing phagocytosis, and causing microbial lysis

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Complement Activation Pathways, their activating signals, and common immune functions

Classical Pathway Ag:Ab complex Lectin Pathway MBL:microorganism complex Alternative Pathway C3 C1q C1r C1s C1 C4 C14b C2 C14b2b MASP1 MASP2 MBL-MASP1-MASP2 C4 MC4b C2 MC4b2b H2O C3H2O B D C3(H2O)Bb C3 Target bound C3b B D C3bBb P C3bBbP C4a C4a C2a C2a Ba C3a Ba

C3 C3a + C3b C3 C3a

C14b2b MC4b2b C3bBbP

C3 convertases C14b2b3b or MC4b2b3b or (C3b)nBbP C5 convertases

C5 C5a C5b C5b67

C5b678(9)n Membrane attack complex

C6 +C7 C8 +C9 Terminal complement pathway common to all activation pathways

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Bioactive Complement Peptides

• Plasma protease cascade generates peptides

active in inflammation and innate immunity

– C3a & C5a anaphylatoxins release histamine from mast cells vascular permeability – C5a chemotaxin, recruits phagocytes – C3b & C3bi opsonins (coats microbes to enhance recognition & uptake by phagocytes) – C5b6789n membrane attack complex (lyses microbial cell membranes) Classical Complement Pathway Classical Complement Pathway- continued

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Alternative Complement Pathway -1 Alternative Complement Pathway -2 Alternative Complement Pathway -3

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Classical and Alternate Pathway formation of C5 convertase and cleavage of C5

Terminal Complement Pathway and formation of the Membrane Attack Complex

Cells detect and respond to complement peptides via cell-membrane receptors for complement peptides.

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Spontaneous or chronic activation of complement is prevented by inhibitory proteins, both soluble and membrane associated.

Antigen Recognition

• B cells recognize antigens via membrane

associated immunoglobulin molecules (antibodies)

– Antigen-activated B cells synthesize and secrete a soluble form of this receptor (plasma antibody) which accumulates in fluids and specifically binds and eliminates the stimulating antigen.

• T cells recognize antigens via a membrane associated

heterodimeric protein, the T cell receptor or TcR

– TcR are not secreted

Antibody Activity

• Antibodies specifically recognize and bind

to the inducing antigen

• Humoral immunity: antibody-mediated

immunity can be transferred from an immune to a non-immune person by transfer of serum

• The gamma-globulin protein fraction of

serum contains the antibody activity

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Basic antibody structure = 4 covalently attached polypeptide chains with 2 identical heavy (H) and 2 identical light chains (L) Each H and L chain have structurally constant (C) and structurally variable (V) domains. The Fab fragment

• f antibody

molecules contains the antigen binding

• region. The Fc

fragment determines antibody class and effector functions (complement interaction,

• psonic activity)

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The “immunoglobulin fold” refers to the characteristic 3D structure of V and C region domains. Hypervariable amino acid residues in the H and L chain V regions physically bind the antigen and determine the specificity of the antigen receptor. Hypervariable amino acid residues are located at the exposed ends

• f V domain

loops.

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VH and VL domains fold to form antigen binding sites that resemble pockets, grooves, open faces, or extrusions. The TcR binding site for antigen (one site per receptor) is also formed by the variable domains

• f an alpha and

beta chain polypeptide. The structure of the T cell receptor (TcR) resembles that of antibody but consists

• f only 2 polypeptide

chains with a single binding site. Each chain has a constant and variable structural

• domain. An αβTcR

and a γδTcr have been described.

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Antigen interaction with receptors is noncovalent and reversible.

There are 5 immunoglobulin classes based on structural similarity of the H chain C domains. IgM and IgA are secreted as pentamers and dimers respectively. The J chain protein promotes polymerization.

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Nomenclature for structural differences between any 2 antibodies.

Antigen-Antibody Reactions, Definitions and Characteristics to Know

• Size of Epitope/antigenic determinant
• specific,noncovalent,reversible binding
• affinity vs. avidity
• valency, crosslinking,

agglutination,precipitation

• multideterminant antigens,
• heterogeneous antibody responses
• cross reactions

Fig A.9 Soluble antigens mixed with equivalent amounts

• f soluble specific

antibody can bind to form lattices (antigen-antibody complexes) large enough to become insoluble and

• precipitate. Insoluble

(particulate) antigens + antibody form clumps (agglutinate).

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Protective functions of antibody reactions with antigens. Protective functions of antibody interactions with

• antigen. -continued.

Protective functions of antibody interactions with antigen. -continued.