Antibodies & Complement (humoral) System

Question 1

What is the difference between humoral (antibody-mediated) and cell-mediated immune system?

Answer 1

Humoral immune system control of freely circulating EXTRACELLULAR pathogens, while cell-mediated immune system control of INTRACELLULAR pathogens.

Humoral: B cells
Cell-mediated: T cells

Question 2

Antibodies and complement are part of the humoral immune system, which can be transferred by _______.

Answer 2

plasma

Question 3

Which statements are true or false?

1. Antibodies are present 7-10 days after infection.
2. The humoral immune response is dominating compared to cell-mediate immune response.

Answer 3

Both are true!

Question 4

1) What are antibodies?

2) What is an antigen?

3) What synthesizes antibodies? What are their two forms?

Answer 4

1) circulating proteins that are produced in response to exposure to foreign structures (antigens).

2) any substance that may be bound by a B cell or a T cell receptor

3) Antibodies are synthesized only by B cells and they exist in two forms: MEMBRANE-BOUND antibodies on the surface of B cells and SECRETED antibodies

Question 5

T cells recognize _________, while antibodies recognize _______.

Both are very ________.

Antigen recognition is mediated by the _______ regions in both receptors.

Answer 5

peptides; anything!

diverse

variable

Question 6

1) What are the same basic characteristics shared by all antibody molecules?

2) Where do they display remarkable variability?

Answer 6

1) An antibody molecule has a SYMMETRIC CORE structure composed of TWO identical LIGHT chains and TWO identical heavy chains.

2) They display remarkable variability in the regions that bind antigens.

Question 7

Mark these statements as true or false:

1) Fab (fragment, antigen binding) is important for antigen recognition, while Fc (fragment, crystallizable) is important for effector function.

2) Membrane and secreted associated antibodies differ in the amino acid sequence of the amino terminal end of the heavy chain C region.

3) Antigens are also called gamma or immunoglobulin.

4) B-cell activation converts membrane IgM to secreted IgG.

Answer 7

1) True!

2) False. Membrane and secreted associated antibodies differ in the amino acid sequence of the CARBOXYL-terminal end of the heavy chain C region.

3. False. ANTIBODIES are also called gamma or immunoglobulin.
4. True!

Question 8

An immunoglobin (Ig) domain contains two layers of __________, like two pieces of bread (B sandwich), with each layer composed of three to five strands of _________ polypeptide chain.

The two layers are covalently linked by a _______ ____ between cysteines and adjacent strands of each sheet are connected by ______ ______.

Answer 8

B-pleated sheet; anti-parallel

Disulfide bridge; short loops

Question 9

(T/F) The amino acids in the disulfide bridge of an immunoglobin are the most variable and critical for antigen recognition.

Answer 9

False!

The amino acids in the short loops aka COMPLEMENTARITY DETERMINING REGION (CDR) loops that connect the adjacent strands of each b-sheet are the most variable and critical for antigen recognition.

Question 10

(T/F) All proteins that contain domains with an Ig fold structure belong in the Ig superfamily.

Answer 10

True!

Question 11

What is the difference between the variable and constant domains of an immunoglobin?

Answer 11

Variable domain is LARGER and contains EXTRA B-strands, called C’ and C’’.

The flexible loops formed between some of the B strands in the variable region contribute to the antigen-binding site.

Question 12

1) Both heavy and light chains consist of amino-terminal _________ regions that participate in antigen recognition and carboxy- terminal ______ regions that mediate effector functions.

2) In the heavy chains, the V is composed of ____ Ig domain(s), while the C is composed of _____ Ig domain(s).

3) Each light chain is composed of _____ V region Ig domain and ____ C region Ig domain.

4) The V region of one heavy chain and the adjoining V region of one light chain form an _______________ ____.

Answer 12

1) Variable; Constant

2) One; Three/Four

3) One; One

4) Antigen-binding site

Question 13

Why does an antibody molecule have at least TWO antigen-binding sites?

Answer 13

Because the core structural unit of each antibody molecule contains two heavy chains and two light chains.

*the variable region of one heavy and one light chain makes one antigen binding site

Question 14

IgG cleaved by proteolytic enzymes generate fragments with distinct structural and functional properties.

What is the difference between the enzymes PAPAIN and PEPSIN?

Answer 14

Enzyme PAPAIN acts on the hinge region and cleaves the IgG into three separate pieces; 2 Fab and 1 Fc.

Enzyme PEPSIN acts on distal to the hinge region and generates a F(ab)’2 fragment of IgG with the hinge and the interchain disulfide bonds and two antigen-binding sites identical to the intact antibody, but without inducing the effector functions.

*antigen recognition functions and effector functions are spatially separated.

Question 15

Where can we find the most of the sequence differences and variability among different antibodies?

Answer 15

In the hypervariable regions or complementarity-determining regions (CDR1, 2, & 3).

The three protruding loops connecting the adjacent strands of the b-sheets that make up the VARIABLE domains of Ig HEAVY and LIGHT chain proteins.

*2 variable light domains
*2 variable heavy domains
*each domain has its three CDRs; total of 12 CDRs

Question 16

(T/F) In a complete antibody molecule, the pairing of a heavy chain and a light chain brings together the hypervariable loops from each chain to create a single hypervariable surface, which forms the antigen-binding site at the tip of each arm.

Answer 16

True!

*three fingers from the variable heavy chain and three fingers from the variable light chain in ONE antigen-binding site

Question 17

What does confinement of the sequence variability to three short stretches allow?

Answer 17

Allows the basic structure of all antibodies to be maintained despite the variability of specificities among different antibodies.

Question 18

Antibodies bind antigens via contacts in CDRS that are complementary to the ______ and _____ of the antigen.

What are the four different surfaces found on antibodies?

Answer 18

size; shape

1) pocket
2) groove
3) extended surface
4) protruding surface

Question 19

Which kinds of forces hold together the antigen:antibody complex?

Answer 19

Noncovalent forces!

• electrostatic
• hydrogen bonds
• van der waals
• hydrophobic
• cation-pi

Question 20

What is an epitope?

What are the two different types? Briefly describe each.

Answer 20

An epitope is any available shape or surface on a molecule that may be recognized by an antibody.

1) Discontinuous epitopes (conformational determinant): antibodies recognizes certain confirmations; if antigens denatures, no longer recognizable.

2) Linear epitopes (linear determinant): antibody can bind to determinant in both naive and denature protein OR antibody binds to denatured protein ONLY (neo-epitope).

Question 21

Antibody molecules can be divided into distinct classes and subclasses on the basis of differences in the structure of … ?

Answer 21

HEAVY chain CONSTANT regions

• their constant regions are critical for antibody functions
• different isotypes and subtypes of antibodies perform different effector functions

Question 22

Antibody molecules are flexible, permitting them to bind to different arrays of antigens.

What are the two angles found between the arms of antibodies?

Answer 22

60˚ (closely spaced cell surface determinants) and 90˚(widely spaced cell surface determinants)

Question 23

What is the difference between the affinity and the avidity of an antibody?

Answer 23

Affinity: the strength of binding between a single combining site of an antibody and an epitope of an antigen

Avidity: the overall strength of attachment (low in monovalent, high in bivalent, and very high in polyvalent)

*avidity is much greater than the affinity of any one antigen-binding site

Question 24

What are the three valency of interactions?

Answer 24

1. Monovalent: one antibody binds to one antigen
2. Bivalent: one antibody binds to two antigens
3. Polyvalent: one antibody binds to more than 2 antigens

Question 25

What are the three major effector mechanisms of humoral immunity (antibody)?

Answer 25

1) Neutralization of microbes and toxins

2) Opsonization and phagocytosis of microbes

3) Antibody-dependent cellular cytotoxicity

Question 26

Can humoral immunity be transferred with serum?

Answer 26

Yes!

Question 27

Match the three effector mechanisms of antibodies to their definitions:

1) Neutralization of microbes and toxins

2) Opsonization and phagocytosis of microbes

3) Antibody-dependent cellular cytotoxicity

A) NK cells and other leukocytes may bind to antibody-coated cells and destroy these cells. NK cells express an Fc receptor called FcyRIII. Cells infected with enveloped viruses express viral glycoproteins on their surface that may be recognized by antibodies

B) antibodies bind to and block, or neutralize, the infectivity of microbes and the interactions of microbial toxins with host cells.

C) antibodies (IgG) coat a microbe, which is followed by binding of the microbe to phagocyte Fc receptors (FcYRI) that activate the phagocyte to ingest + kill the coated microbe.

Answer 27

Neutralization of microbes and toxins: antibodies bind to and block, or neutralize, the infectivity of microbes and the interactions of microbial toxins with host cells.

Opsonization and phagocytosis of microbes: antibodies (IgG) coat a microbe, which is followed by binding of the microbe to phagocyte Fc receptors (FcYRI) that activate the phagocyte to ingest + kill the coated microbe.

Antibody-dependent cellular cytotoxicity: NK cells and other leukocytes may bind to antibody-coated cells and destroy these cells. NK cells express an Fc receptor called FcyRIII. Cells infected with enveloped viruses express viral glycoproteins on their surface that may be recognized by antibodies; Antibody Dependent Cellular Cytotoxicity.

Question 28

(T/F) Complement system is part of the adaptive immunity.

Answer 28

False!

it is part of the innate immunity

Question 29

1) Define complement

2) What is the complement system activated by?

3) What happens to the product of complement activation?

Answer 29

1) Complement: another component in serum that assists, or complements, the lytic function of antibodies

2) Complement system is activated by microbes and by antibodies that are attached to microbes. Activation of the complement involves the SEQUENTIAL CLEAVAGES OF PROTEINS to generate enzymes complexes with proteolytic activity.

3) The products of complement activation become COVALENTLY attached to microbial cell surfaces or to antibodies bound to microbes.

Question 30

How many molecules are there in the complement system?

Answer 30

C1-C9

Question 31

Complement activation depends on the generation of two proteolytic complexes: the ____________, which cleaves the C3 into C3a and C3b, and the _____________, which cleaves C5 into C5a and C5b.

Answer 31

C3 convertase; C5 convertase

*binding -> formation of c3 -> cleavage of c3 -> formation of c5 -> cleavage of c5

Question 32

What are the three pathways of complement activation?

How do they differ?

Answer 32

Classical, alternative, and lectin

They differ in how C3b is produced but follow a common sequence of reactions after the cleavage of C5. Classical and lectin pathways are similar and need C2 and C4 but alternative pathway does not need C2 and C4.

Question 33

Match the following pathways to their definitions:

1) The alternative pathway

2) The classical pathway

3) The lectin pathway

A) activated by the MANNOSE-binding protein that binds to surface carbohydrates on microbes

B) activated on microbial cell surfaces in the ABSENCE of antibodies

C) activated by antibodies bound to antigens

Answer 33

The alternative pathway: activated on microbial cell surfaces in the ABSENCE of antibodies

The classical pathway: activated by antibodies bound to antigens

The lectin pathway: activated by the MANNOSE-binding protein that binds to surface carbohydrates on microbes

Question 34

What is the central event in complement activation:

Answer 34

The proteolysis of the complement protein C3 to generate biologically active products and the subsequent covalent attachment of a product of C3, called C3b, to microbial cell surfaces.

*mutations in C3: high susceptible to infections

Question 35

1) How is the classical pathway initiated?

2) What is C1?

3) what can initiate classical pathway activation?

4) What must happen to each C1 complex molecule to be activated?

Answer 35

1) Initiated by binding of complement protein C1 to the CONSTANT region of the HEAVY chain of 2 antibodies that have bound antigens

2) C1 is a large, multimeric protein complex composed of C1q, r, s subunits. C1q binds to the antibody while C1r and C1s are proteases.

3) Only antibodies bounds to antigens

4) Each C1 complex must bind at least two Ig heavy chains to be activated

Question 36

What is the membrane attack complex (MAC)?

Answer 36

MAC, formed by the complement system on the microbial membrane, forms pores that create channels that allow free movement of water and ions, disrupting osmotic integrity, and resulting in CELL DEATH.

Question 37

(T/F) Activation of the complement system occurs on the surface.

Answer 37

True.

Question 38

What are the three major effector functions of complement?

Answer 38

1) Phagocytosis of microbes opsonized with complement fragments (e.g., C3b)

2) Inflammation

3) Lysis of microbes

Question 39

Match the effector functions of complement to their descriptions:

1) Phagocytosis of microbes opsonized with complement fragments (e.g., C3b)

2) Inflammation

3) Lysis of microbes

A) most pathogens have evolved thick cell walls or capsules that impede access of the MAC to their cell membranes. Complement-mediated lysis appears to be critical for defense against only a few pathogens that are unable to resist MAC insertion, such as bacteria of the genus Neisseria, which have very thin walls.

B) microbes on which complement is activated become coated with C3b, and are phagocytosed by the binding of these proteins to specific receptors on macrophages and neutrophils.

C) the proteolytic complement fragments C5a, C4a, and C3a induce acute inflammation by activating mast cells, neutrophils, and endothelial cells.

Answer 39

Phagocytosis of microbes opsonized with complement fragments: microbes on which complement is activated become coated with C3b, and are phagocytosed by the binding of these proteins to specific receptors on macrophages and neutrophils.

2) Inflammation: the proteolytic complement fragments C5a, C4a, and C3a induce acute inflammation by activating mast cells, neutrophils, and endothelial cells.

3) Lysis of microbes (cytolysis): most pathogens have evolved thick cell walls or capsules that impede access of the MAC to their cell membranes. Complement-mediated lysis appears to be critical for defense against only a few pathogens that are unable to resist MAC insertion, such as bacteria of the genus Neisseria, which have very thin walls.