Immunology, Exam 2, 2023

Haptens

Haptens

• Low molecular weight substances

• Not immunogenic by itself

• Couple to a larger carrier molecule to become immunogenic

• The smallest part of an antigen that is recognized by B and T cell receptors

Immunogen

• An antigen that induces an immune response

• Contain epitopes

• All immunogens are antigens, but not all antigens are immunogens

• Foreign, large, and chemically complex

Epitope

• Portion of an antigen that is recognized and bound by an antibody/T-cell receptor

• Antigenic determinant

• Immunologically active regions of an antigen that bind to receptors on lymphocytes/antibodies.

• B cells and T cells recognize different types on the same antigenic molecule.

Antigen

• Any substance that binds to a B cell receptor, antibody or T cell receptor.

• Combine specifically with antibodies produced by B-lymphocytes or sensitized T-lymphocytes

Multivalent Antigen

Many epitopes of the same specificity

Polyvalent Antigen

Many epitopes of different specificities

Transplantation antigens

• Poteins expressed on the donor tissue that have the capacity to initiate an immune reaction against the allograft.

• Blood group antigens

• Histocompatibility antigens

Blood group antigens

• A

• B

• Rh antigens

Histocompatibility antigens

• Glycoprotein molecules on all nucleotide cells

• MHC and HLA

• Most potent are class I and class II molecules of the MHC

MHC

• Cell surface proteins

• Adaptive immune system

• Recognition of foreign molecules

• Determines histocompartibility

• Helper T-cells recognize foreign antigens on surface of APCs when antigens are presented in the groove of MHC II

• Cytotoxic T-cells recognize antigens, on the surfaces of cells when antigens are presented in the groove of MCH I

• Poolymorphism

• Capability to react with an antigen preexists

Exogenous Antigens

• Come from foreign substances that enter the body through cuts, nose, skin ,mouth etc

• Bacterial antigens

• Viral antigens

• Superantigens

Antigens related to bacterial cells

• Somatic antigen: part of cell wall gm –ve bacter.

• Capsular antigen: usually polysaccharide

• Flagellar Ag: a protein made of flagellin

• Fimbrial Ag: surface antigens in fimbriated bacilli

Antigen secreted by bacteria

• Exotoxins

• Enzymes

Viral antigens

• protein coat viral antigens

• Soluble antigens

Superantigens

• They activate multiple clones of T-lymphocytes

• Bacterial toxins

• They have the ability to bind both class II MHC molecules and TCR β chain

• They act as a clamp providing a signal for T-cell activation

• Active at low concentration and release of large amounts of cytokines

• Not specific for the pathogen

• No memory

Autoantigens

• Capable of immunizing the host from which they are obtained.

• Self antigens are non-antigenic

• Modifications of self-antigens are capable of eliciting an immune response

Endogenous Antigens

• Antigens found within the cytosol of cells

• viral proteins

• tumor proteins

• Proteins from intracellular bacteria

• They come from the body’s cell

Immunogencity

is the ability to induce a humoral and/or cell-mediated immune response

Factors influencing Immunogenicty

• influence the potency and diversity of host immune responses to antigens

• High molecular weight and complexity increase immunogenicty

• Parenteral routes are more immunogenic to oral route

• Substance when injected with an antigen enhance immunogenicty

Alloantigen

Any antigen that stimulates the production of antibodies in those that lack it (an isoantigen)

Zenoantigen

an antigen that occurs in organisms of more than one species

Zenograft

a graft of tissue taken from a donor of one species and grafted into a recipient of another species

How antigen degradability influences immunogenicity

• T lymphocytes respond only to processed protein antigens

• Antigen-presenting cells

• Process and present antigenic peptides to T cells

• Activate T cells

Antigens are recognized by and bind to

• BCR

• TCR

• MHC

B-cell receptors (BCR)

• These are membrane-bound immunoglobulins on B-cells

• Can be secreted in plasma as antibodies

T-cell receptors (TCR)

• α and β chains anchored to T-cells

• There is a groove which binds small peptides

• presented by MHC on surface of APCs

Cross-reacting Antigens

• Antigens that share identical or very similar epitope

• It is a measure of relatedness between two different antigens

• Heterophile Antigens

• Cross-Protection

Antigen binding molecules

• Immunoglobulin

• T cell receptor

• MHC molecules

Carrier proteins produces

Anticarrier antibody

Hapten-carrier conjugate produces

• Anticarrier antibody

• Antihapten antibody

Autocoupling Haptens

• Haptens that spontaneously form covalent bonds with host cell proteins or polysaccharides

• Usually results in allergic phenomena

Adjuvants

• molecules which interact with the immune response to make a molecule that is not antigenic more antigenic

• Adjuvants enhance the immune response

• Commonly used in vaccines

The Instructional Theory

This was the concept that a par-ticular antigen would serve as a template around which antibody would fold

The Selective Theory

• This was the concept that cells possessed antigen receptors that could react with specific infectious agents

• The specificity of the receptor was determined prior to antigen exposure and that antigen selected the cell with the appropriate receptor

Immunoglobulins

• immunoglobulins are proteins that specifically react with the antigens that triggered their production.

• They make up 20% of all proteins in the blood serum.

• Antibody fractions are found in both serum and plasma.

• Antibodies can exist either in a soluble form or attached to the surface of B cells.

• Heavy and light chains chains linked by disulfide bonds

B cell receptor

• carry receptors on their surfaces called BCRs

• Are membrane-bound immunoglobulin

• May be monomeric IgM or IgD

• Function to recognize and bind specific antigen

• Are associated with accessory molecules that aid in signal transduction

• Are sulphide-linked heterodimers

• Contain ITAMs

Fab fragment

• Immunoglobulin subunits

• Binds with antigen (variable)

• Detects antigen

• Precipitate antigen

• Block the active sites of toxins or pathogen-associated molecules

• Block interactions between host and pathogen-associated molecules

Fc fragment

• Immunologic trigger (non-variable)

• Binds complement and receptors

• Fc structure is common to all specificities of antibody within an ISOTYPE

• The structure acts as a receptor for complement proteins and a ligand for cellular binding sites

Hypervariable region

• Found on both heavy and light chain of the immunoglobulin

• Responsible for specific binding to antigen

• Comprises one domain

• Interaction of VH and VL

Constant region

• Responsible for triggering

• Comprises three domains

Antibody isotypes

• IgM, IgG, IgA, IgE, IgD

• Differences lie in heavy chain types

• Kappa or Lambda light chains with any of the classes

Allotypes

Genetically determined differences in antibodies between different animals

Idiotypes

Antibodies that recognize different specific epitopes

Heavy chain

• Determines the immunoglobulin class the receptor becomes or the antibody the cell will secrete.

• V, D, J segments

Light chain

• λ (lambda) or κ (kappa)

• Each antibody molecule contains either λ or κ, but not both

• V and J segments

IgM

• Present as surface B cell receptor or pentamer

• Can fix / activate complement

• Can cause agglutination of large particles

• First ab secreted after activation little in quantity but with 10 ag.

• Binding sites, good for agglutination and complement fixing

• Cμ - constant portion

pentamer

• secreted IgM form in blood

• secreted only during primary immune response to an antigen

• 5 units joined by a J chain

IgG

• Most prevalent

• Contains gamma heavy chain

• Is a monomer

• Four subclasses (IgG1-4)

• Longest half-life

• Activate complement

• Can cross the placenta and confers immunity to developing fetus

• Bind to & coat large antigens marking them for phagocytosis (opsonins)

• Can bind to Fc receptors on macrophages & neutrophils and facilitate phagocytosis

• Cγ- constant portion

IgA

• Monomeric is found in serum

• Dimeric is found in secretions

• Joined by a J chain

• Accompanied by a secretory component which aids in transfer across epithelial cells

• Four binding sites

• Important in mucosal regions

• Cα - constant portion

IgE

• The primary trigger of “allergic” reactions

• A trigger for mast cells and basophils

• Stimulates release of cytokines that mediate allergy

• Is the primary defense against multicellular parasites (Bind to antigens on the surface of the worm then binds to eosinophils causing them to degranulate)

• Cε - constant portion

IgD

• The mystery immunoglobulin

• Exists on the surface of B cells as a receptor

• Small amount of IgD is found free in the serum

• Cδ - constant portion

Agglutination

• IgM, IgG, IgA

• Enhances phagocytosis and reduces number of infectious units

Opsonization

• IgG

• Coating antigen with antibody to enhance phagocytosis

Toxin Neutralization

• IgG, IgA, IgM

• Antibody blocks binding of toxin

Complement Activation

• IgM, IgG

• Cell lysis

Antigen-Antibody Interaction

• Electrostatic interactions

• Hydrogen bonds

• Van der Waals forces

• Hydrophobic interactions

• Similar to enzyme-substrate, receptor-ligand

• Antigen-antibody interaction is extremely specific

Antigen-Antibody Good Fit Interactions

high attraction and low repulsion

Antigen-Antibody Poor Fit Interactions

high repulsion and low attraction

Cross-reactivity and antigen binding

• if the antibody can bind similar, but different antigens

• Avidity will always be higher for the original antigen

• If there is competition, the original antigen will win

During B cell development

• L, J, D segments are rearranged and assembled into functional genes

• functional genes are then transcribed and translated into the peptide chains of the immunoglobulin/surface B cell receptor

• this step does NOT require the presence of antigen

Somatic Hypermutation

• Occurs in stimulated proliferating B cells

• Point mutations in genes encoding the Variable region

• Results in progeny B cells producing BCRs/antibodies with different affinities for the same antigen

• Responsible for Affinity maturation

Affinity maturation

B cells with receptors having better affinitiy for antigen are more likely to proliferate, since they bind to antigen more competitively

Class Switching

• A plasma cell may switch/change the heavy chain of the antibody it secretes

• The effector-end of the antibody molecule is different

• The specificity for antigen is unaltered

• Occurs In Memory B Cells

• The light chains do not change, just the constant heavy chain

Polyclonal antibodies

Refers to the different antibodies produced against various antigens of a single infectious agent

Monoclonal antibodies

• Refers to antibodies derived from a single clone of B cells

• All the antibodies derived will have specificity for a single antigen

• Used in many tests

Membrane-bound antibody

• Serve as receptors on B cells – can detect presence of antigen and generate signals

• Membrane bound antibodies have an additional anchoring segment

Secreted antibodies

• Serve to execute certain functions

• Initiation of the complement cascade

• Signal certain cells to kill

• Directly kill/neutralize

B cell recognition

• Directly recognize and bind antigen

• Can recognize diverse antigens

T cell recognition

• Can ONLY recognize antigens when presented by MHC molecules

• Recognize only peptide antigens

• Heterodimer (2 αβ chains embedded in the T cell membrane)

• Has C and V domains

• No class switching

• No somatic hypermutation

TCR Accessory molecules

• Associated closely with TCR

• Needed for signal transduction

• CD4 (helper T) or CD8 (cytotoxic T)

• CD3 complex

• Zeta-zeta dimer

• LFA-1 (CD11)

TCR C domains

close to the membrane

V domains

• Distant from the membrane

• Have 3 hypervariable regions arranged in a flat surface

• Are specific for a single sequence of amino acids AND the amino acids present on an MHC molecule

• Has a peptige (antigen) binding groove

CD4 (helper T) or CD8 (cytotoxic T)

Co-receptor, senses MHC molecule attached to antigen

CD3 complex

Four peptide chains which transduce signals

Zeta-zeta dimer

Have ITAMS for conveying signals to the interior of the T cell to the nucleus so transcription and translation can occur

LFA-1 (CD11)

is an integrin that docks the T cell to the APC

ITAMs

• Immunoreceptor tyrosine-based activating motifs

• Activated by phosphorylation

• Are BCR & TCR accessory proteins, and several other receptor proteins

• Essential for signal transduction

γδ T Cells

• Mysterious

• Derived from a separate cell lineage

• May be 1st line defense at epithelial tissues

• NOT MHC restricted

• May recognize carbohydrate antigens presented by a CD1 molecule (similar to an MHC)

MHC Class I

• Found on all nucleated cells

• heterodimers – α chain (contains groove) and β chain (does not insert inro cell membrane)

• presents endogenous (intracellular) peptides

• Presents antigen to cytotoxic T cells

• Bind to CD8

• The presence of foreign material targets cell for destruction

MHC Class II

• heterodimers – α chain and β chain that insert into the membrane

• peptide binding groove is located in the α chain AND the β chain

• groove is longer

• found on B cells, macrophages, and dendritic cells

• present exogenous peptides to T helper cells

• Presents antigen to helper T cells

• Bind to CD4

• The presence of foreign antigens induces antibody production and attracts immune cells

MHC Restriction

• T cells are specific for both antigen and MHC molecule

• The T cell can only react with its specific antigen if it is presented by a “self” MHC molecule

• TCR interaction with self MHC is learned in the thymus during development

Antibody diversity

• Gene rearrangement

• Somatic hypermutation (affinity maturation)

• Class switching

TCR / BCR diversity

gene rearrangement

Bone Marrow

• Primary and secondary lymphoid organ

• Site of early events in B cell development

• Site of long-lived plasma cells

RAG-1/RAG-2

Operate during gene rearrangement in BOTH T and B cells

Primary lymphoid organs

• Location where lymphocytes develop and mature to a stage where they can interact with antigen

• Thymus, Bursa of Fabricius, Bone marrow, and Peyer’s patches

Secondary lymphoid organs

• They are a site of further lymphocyte maturation,

• Trap antigens for exposure to T and B cells.

Functions of the Lymphoid tissues and organs

• the concentration of foreign antigen from all parts of the body into the lymphoid organs and tissues

• lymphocyte circulation to facilitate interaction between antigen and antigen-sensitive lym-phocytes

• carry products of the adaptive immune res-ponse [antibodies and effector cells] to the bloodstream and various organs and tissues

Lymphopoiesis

• To generate a diverse set of lymphocytes  each with a unique receptor.

• To get rid of lymphocytes that have receptors that react against self antigen

• To allow non-self reactive lymphocytes to continue  to mature and move to the peripheral lymphoid organs for activation

Hematopoietetic stem cell in the bone marrow

• matures to a common lymphocyte progenitor cell

• Can become either a B or T cell.

Common lymphocyte  progenitor

• To become a B cell, develops to immature B cell in the bone marrow

• Completes it maturation to antibody secreting B cell or plasma cell in lymph nodes and the spleen

• To become a T cell migrates to the thymus gland to become a thymocyte and completes its maturation to a T cell

IL-7

Stimulates growth and proliferation

Stromal cells provide developing B cells

• Constant interaction

• Adhesion molecules for attachment

• Growth factors like IL 7 for growth and proliferation

B cell development

• six stages

  • common lymphoid progenitor

  • Early pro-B

  • Late pro-B

  • Large B

  • Small B

  • Immature B

• permanent changes in its DNA

• receptor that binds to a specific foreign antigen but not self.

The B cell receptor Antigen biding site

• The specificity to a receptor is determined  by the shape of the variable region of the receptor

• made of 3 protein segments: Variable- V,  Diversity-D and Joining –J

• The region where the H and L chains come together

Deficiency of RAG genes causes

Severe Combined immunodeficiency or Omenn syndrome.

genetic defect in the recombinase complex

• Severe Combined Immunodeficiency (SCID).

• These animals are unable to make functional B cell and T cell receptors because they cannot successfully rearrange their DNA.

Autoimmune  Regulator gene (AIRE)

• Expressed by the primary lymphoid organ

• Allows them to express antigens normally expressed in other body tissues-liver brain, spleen etc. Hence serving as microcosm of the entire body

• B cell receptors are tested in the safe bone marrow environment

Negative selection

• Small Pre-B cell binds strongly to a self antigen

• APOPTOSIS

• Thymocytes whose TCRs react strongly with self antigen-MHC complexes; or cannot react with antigen-MHC complexes

Positive selection

Only Small Pre-B cells that do not recognize self antigens at all develop to immature B cells