Lecture 6 - Antigen Recognition by Lymphocytes

Adaptive immunity is based on

Specificity and diversity

3 aspects to Adaptive Immunity

Recognize very specific foreign molecules → Limitless in the diversity of molecules recognized → Combat new and previously encountered pathogens

3 things that Innate Immunity lacks compared to Adaptive

Lack of specific receptors, Not capable of recognizing diverse types of pathogens, Not sufficient to halt an infection

Why isn’t Adaptive the first to act

It takes a week for it to learn and act

Clonal deletion

Removal of potentially self-reactive immature lymphocytes

In a pool of mature naive lymphocytes, how many have the right receptor

Only ONE has the right receptor for a specific antigen as it is educated to recognize that one

What happens to the lymphocyte with the correct receptor

Proliferation and differentiation of activated specific lymphocytes to form a clone of effector cells

4 basic principles of clonal selection

1. Each lymphocyte bears a single type of receptor with unique specificity

2. Interaction between a foreign molecule and a lymphocyte receptor capable of binding that molecule with high affinity leads to lymphocyte activation

3. Differentiated effector cells derived from an activated lymphocyte will bear receptors of identical specificity to those of the parent cell from which the lymphocyte was derived

4. Lymphocytes bearing receptors specific for ubiquitous self molecules are deleted at an early stage of development and are absent from mature lymphocytes

Where do B cells develop and mature

Both in the Bone Marrow

Where do T cells develop and mature

Develop in the Bone Marrow, mature in the Thymus

Plasma cells

Anti-body secreting B cells

Helper T cells - CD4

Helps cytotoxic T cells and B cells in their immune functions

Cytotoxic T cells - CD8

Kills virus-infected and damaged cells

Which are Cell-mediated Immunity and which are Humoral Immunity

T cells are Cell-mediated, B cells are Humoral

What are the primary lymphoid organs - where lymphocytes mature and develop

Bone marrow and thymus

What are the peripheral (secondary) lymphoid organs - where mature lymphocytes encounter antigen and become activated

Adenoid, tonsil, lymph node, appendix, Peyer’s patch in small intestine, spleen

Mature lymphocytes circulate continuously between

Blood, lymph, and secondary lymphoid organs

Recognition of antigen can trigger

T-cell activation and expansion

Dendritic cells reside in peripheral tissues and migrate via lymphatic vessels to lymph nodes to do what

Mature dendritic cells activate naive T cells in lymphoid organs such as lymph nodes

What do lymph nodes bring together

Antigen and rare antigen-specific lymphocytes together

What happens to lymphocytes that do not encounter their antigen

Leave and continue circulating

Naive lymphocytes circulate through secondary lymhpoid organs, what do antigens from infected cells do

Drain to nearby lymph nodes through lymphatic vessels

Antigen

A molecule that is recognized by the body as foreign, stimulates an immune response, can be foreign or self

Foreign antigen

Microbial (cell wall, nucleic acid, bacterial toxins) or non-microbial (pollen, egg white, transplanted tissue, transfused blood)

Self antigen

Molecules normal found on or within our own cells

Epitope

Specific part of an antigen recognized by an antibody or antigen receptor

How many epitopes can a single antigen contain

Multiple, recognized by specific antibodies/BCRs/TCRs

B-cell receptor (BCR)

Highly specialized recognition proteins of B cells

Each B cell produces Igs of ___ -specificity

Single-specificity

Membrane-bound form on B-cell surface

BCR, functions as the cell’s receptor for antigens

Secreted form of B-cell receptor

Antibody produced by terminally differentiated B cells

Antibodies (secreted B cells) binds what in the extracellular spaces

Pathogens or their toxic products, recruits other cells and molecules to destroy the pathogen

What pathway is used for antibodies to recruit complement proteins to destroy the pathogen it has bound

Classical pathway

Each B cell only makes ___ type of antibody

One

Plasma cells are

Specialized effector B cells that produce soluble antibodies (Igs)

Shape of an antibody (Ig) molecule

Y-shaped

Chains of an antibody (Ig) molecule

Two heavy (H) chains and two light (L) chains

What joins the heavy chain to a light chain and the two heavy chains together in an antibody

Disulfide bonds

Avidity

Total strength of the interaction between an antibody and antigen

Affinity

Strength of the interaction between a single antigen-binding site and antigen

Heavy or light chains at amino terminal and carboxyl terminal

Heavy + light at amino, heavy at carboxyl

How many antigen binding sites on an antibody

Heavy + light = 1, so 2 total

What is the role of the carboxyl terminal

Does not participate in antigen binding, decides the Isotype

How many Ig domains do L and H chain have

L has 2, H can have 4 or 5

What does of Ig domains foes the N-terminus of the antibody have

Variable Ig domains, determine antigen-binding specificity

What region comes below the Variable region

Constant Ig domains, distinguishes different classes (IgM, IgG, IgD, IgA, IgE)

Variable region interacts with constant region to decide

Isotype

What is the name of the region that joins the two arms of the antibody to the trunk

The hinge region, allows flexibility in binding to multiple antigens, differs between isotypes

Fab - Fragement antigen binding

Antigen-binding activity

Fc - Fragment cytallizable

Does not interact with antigen, differs between H chain isotypes

What does Papain do

Cleave at hinge and release 3 functionally distinct fragments, 2 Fab, 1 Fc

What structure of an antibody determines its class/isotype

The CH regions

Five main heavy-chain isotypes

IgM, IgG, IgA, IgD, IgE

Distinct characteristics in the Constant region

Number and location of disulphide bonds, number of attached carbohydrate groups, number of C domains, length of hinge regions

IgG 4 subclasses

IgG1 > IgG2 > IgG3 > IgG4

IgA 2 subclasses

IgA1 and IgA2

What is the most abundant Ig in serum

IgG

What is the first Ig produced after B-cell activation

IgM

What is the form of IgM and where is it normally present

Pentamer, in blood not tissues, too big for tissues and needs to be converted to IgG

Pentamer of IgM increase ___ of IgM for antigens

Avidity

Which Igs interact with C1 to kickstart the Complement pathway

IgM and IgG

Which Ig is transferred from mom to fetus

IgG

Which Igs do not activate complement

IgA and IgE

Which Ig has the highest molecular mass

IgM

IgA role

Acts at mucosal surfaces, secreted into the gut, respiratory tract, breast milk

IgE role

Parasite immunity and allergic reactions

What links the pentamer of IgM? What links the IgA dimer?

J chain

IgA molecules are found as dimers in mucous secretions and breastmilk but as monomers in

Plasma

How many antigen binding sites does IgM have

10

What makes up for IgM sites having low affinity due to being made early in immune response

Multisite binding makes up for this, improving overall functional binding strength

Hypervariable regions (HVRs)

Short regions within the variable domains of both H and L

What region on an antibody directly contacts the antigen and makes up the antigen binding site

HVRs

How many HVRs does each antibody have

6, 3 in the heavy and 3 in the light

What is the other name of HVRs

Complimentarity-determining regions (CDRs): CDR1, CDR2, CDR3

What is the most variable HVR

HVR3

What are framework regions

Regions between HV regions, there are 4 in each V domain, provide structural framework of the Ig domain

How is the immune system capable of generating antibodies of various specificities

Creating various combinations of H-chain and L-chain V regions

What process involves the formation of VH and Vl region genes from smaller DNA segments to create various combinations of H-chain and L-chain V regions

Combinatorial diversity

When does Combinatorial diversity occur

Development of B cells in the bone marrow

What protein chains do TCRs consist of

Alpha chain and beta chain, each one has a C and V region

TCR: What does the V region of alpha and beta chains make

Antigen-binding site

TCR: Differences in V regio allow TCR to

Recognize different antigens (diversity)

TCR: Each TCR can recognize how many antigens

One (specificity)

TCR: Each T cell expresses how many typesof TCR

One, each T cell offers protection against only one pathogen

(Can have multiple copies of a TCR but only 1 specificity)

TCR: Almost entirely extracellular structure

Membrane bound

Peptides must be presented on what for TCRs to recognize them

MHC

What is the MHC restriction

Antigen must be presented on the surface of an MHC or nothing will happen

Main differences between MHC class I and class II

B chain in MHC class I is soluble, no stalk, is floating

MHC class I has 3 alpha subunits and 1 beta subunits

MHC class II has 2 alpha subunits and 2 beta subunits

MHC class I: what part anchors MHC to plasma membrane and what form the peptide-binding groove

a chain

a1 and a2

MHC class I: B2 microglobulin function

Provide structural supports allong with a3, soluble

What does MHC class I mostly bind

Small peptides of 8-10 aa in length from intracellular proteins, viral infections

MHC class II: What anchors MHC to the plasma membrane

alpha chain

True or false: Both chains in MHC class II are transmembrane

True

MHC class II: What forms the peptide-binding groove

a1 and b1

What does MHC class II mostly bind

Larger peptides of 13-25 aa in length, generated from extracellular proteins

MHC molecule presenting the antigen binds to what and what

TCR and co-receptor expressed on T-cell surface

What are T cells two co-receptors and what do they target

CD4 binds to MHC-II, targets extracellular pathogens

CD8 binds to MHC-I, targets intracellular pathogens

TCR is not expressed without

CD3, it is required to bring TCR to surface

CD3 complex recruits signaling molecules that are activated upon what

TCR engagement, driving T cell activation