Ch3 antigens

3.0 Antigens

Prepared by Pratheep SandrasaigaranLecturer at Manipal International University

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By the end of this chapter you should be able to understand:

1. Overview on antigen-receptor2. Structure of antigens and receptors3. Genetic control of immune

response4. Antigens processing by antigen

presenting cell5. Role of MHC and accessory

molecules6. Antigen-antibody interactions

Prepared by Pratheep Sandrasaigaran

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Diagram Adopted from Internet

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3.1 Overview on antigen-receptor


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Immune responses initiated by the ligand-receptor protein interactions


4If the binding is sufficient, the receptor is able to provide a signal to the cellDiagram Adopted from Lippincott's Illustrated Reviews: Immunology, 2nd ED



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• Ligand-receptor protein interactions trigger the activation of leukocytes or white blood cells.

• Ligands may be expressed by• Cells as cell-surface molecules (e.g. on

microbes)• Soluble molecules (e.g. the secreted products

of cells).



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• Factors influence the binding of a ligand to a cell-surface receptor:

• The shape and charge affect binding affinity• The collective affinities where multiple

receptors may be involved (avidity)• The intracellular signals that are triggered • The presence of other receptors

• Cells receive signals can influence whether they respond to those signals.



7Diagram Adopted from Lippincott's Illustrated Reviews: Immunology, 2nd ED




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• Ligands recognized by cells of both the innate and adaptive immune systems are collectively known as antigens.

• The smallest individually identifiable part of an antigen that is bound by a receptor is known as an epitope.

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• The innate immune system employs a limited set of receptors to recognize epitopes expressed by a wide range of microorganisms.

• The adaptive immune system, on the other hand, generates a vast number of epitope-specific lymphocyte receptors.


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Innate AdaptivePrepared by Pratheep Sandrasaigaran

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Types of immunity

Diagram Adopted from Lippincott'sIllustrated Reviews: Immunology, 2nd ED

TEST YOUR KNOWLEDGE 1


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1. Ligand and receptor has to bound tightly to induce strong reactions?

2. All ligand automatically will trigger an immune reactions.

3. The intracellular signals of immune cell will determine the affinity of ligand-receptor interactions.

4. All antigens will trigger immune response.

5. All immune cell use the same mechanism during immune response

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1. Define ligand and receptor. What is the relationship of these two structure?

2. What is a antigen and epitope?

3. How does an immune response is triggered in leukocytes?

4. How innate immunity antigen recognition varied to adaptive immunity?

5. Does the priming of antigen/ epitope always result in immune response? Explain your answer.


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Why you

don’t get

chicken pox

two times..?

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3.2 Structure of antigens and receptors


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Antigens


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Antigens


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• What is an antigen?

• An organism

• A molecule

• Part of a molecule

• Antigens should be able to be recognized by the immune system.


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Antigens


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• What is antigen may made (molecule) of?

• May come in simple or complex

• Protein

• Carbohydrate

• Synthetic in origin molecules

a. Epitopes: The basic recognition unit


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• Epitopes, the smallest part of an antigen that is "seen" by somatically generated B- and T-cell receptors.

• Different lymphocytes, each with a unique set of receptors, may recognize different epitopes on the same antigen.

• Antigens/epitopes are divided into three broad functional types:• Immunogens• Haptens• Tolerogens

a. Epitopes: The basic recognition unit


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• Complex antigens may contain large numbers of different epitopes.

Diagram Adopted from Lippincott'sIllustrated Reviews: Immunology, 2nd ED

b. lmmunogens


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• Immunogen is a substance or antigen that evokes a specific, positive immune response

• Antigen mean a molecule or cell recognized by the immune system.

• Some nonimmunogenic molecules (e.g . haptens) can be bound to an immunogen and in this context, the immunogen is referred to as a carrier.

c. Haptens


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• Haptens are small , non-immunogenic molecules, usually of non-biologic origin, that behave like synthetic epitopes.

• Haptens are antigens and can bind to immune receptors but cannot induce a specific immune response by themselves; hence are not immunogenic.

• Therefore hapten has to be chemically bound to an immunogen (carrier).

• Immune responses may be generated against both the hapten and the epitopes on the immunogen.


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Table adopted from Lippincott'sIllustrated Reviews: Immunology, 2nd ED

d. Tolerogens


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• During development of the immune repertoire tolerance to self molecules and cells develops first.

• Non-self antigens are subsequently recognized as foreign.

• Tolerance can also develop later in life, for example, to antigens that are administered orally.

• Hence tolerogens induce adaptive immune un-responsiveness; diminishes immune response rather than an enhanced one.

Immunogen and lmmunogenicity


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• Knowing only Immunogens induces immune response, how do you determine whether a substance is an antigen?• Immunogen• Hepten • Tolerogen

1Size 2Complexity3Conformation

and accessibility

4Chemical properties



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• Size: Proteins greater than 10 kDa are usually more immunogenic.

• Complexity: Complex proteins with numerous, diverse epitopes are more likely to induce an immune response than are simple peptides that contain only one or a few epitopes.

• Conformation and accessibility: Epitopes must be "seen by" and be accessible to the immune system.



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• Chemical properties: A protein immunogen has to be enzymatically cleavable by phagocytes.

• For example, L-amino acid-containing polypeptides are generally good immunogens, whereas D-amino acid containing polypeptides are poor immunogens as proteolytic enzymes only able to cleave the L-amino acids.

• Many carbohydrates, steroids, and lipids tend to be poor immunegens.



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1. Antigen and epitope are no different. Both induce immune response.

2. Non-organic molecule can act as an antigen.

3. All Immunogens, haptens and tolerogens may induce immune reaction.

4. An antigen’s complexity, accessibility and chemical property are very crucial in order for it act as an immunogen.

5. Laboratory synthesized amino acids may spontaneously elicit an immune response when come in encounter with the immune cells.


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1. What could be an antigen?

2. What could be an epitope?

3. Distinguish between Immunogens, haptens and tolerogens.

4. What is the difference between antigen and immunogens?

5. Why most of the food that we ate does not prompt immune response?

6. What would be possibly a substance can be an antigen?

Receptors


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Receptors


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• The engagement of receptors provides to epitope/ antigen will initiate an event that can lead to a wide variety of immune activities.

• Three functions of the receptors:• Bind to molecules that then generate signals

between cells.• Sample the environment to detect the presence of

intruders.• Examine their neighbors to be sure that they

belong to self and do not present a threat.

Receptors


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a. Preformed receptors

b. Somatically generated receptors

Preformed receptors


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• Known as receptors of innate immune system.

• Pattern recognition receptors.• Toll - like receptors• Killer activation receptors• Killer inhibition receptors• Complement receptors• Fc receptors: Immunoglobulins Diagram Adopted from Internet

a. Pattern recognition receptors


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• Receptors of the innate immune system recognize broad structural motifs presented by microbes.

• Pattern recognition receptors (PRRs), are present in soluble forms (complement proteins) or on host cell surfaces.

• They recognize pathogen-associated molecular patterns (PAMPs) associated with microbes:• Sugars• Some proteins• Lipids• Nucleic acids

a. Pattern recognition receptors


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• PRR binding to PAMPs triggers various forms of inflammation intended to destroy the pathogens.

Diagram Adopted from Lippincott's Illustrated Reviews: Immunology, 2nd ED

b. Toll - like receptors


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• In humans, PRRs also include toll - like receptors (TLRs) that are present on various host cells.

• When triggered by binding to a PAMPs on an infectious organism, TLRs mediate the generation of defensive responses that include• Transcriptional activation• Synthesis of cytokines• Secretion of cytokines

• Cytokines are needed for promote inflammation, and the attraction of immune cells to the site of infections.


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Cytokines

Macrophages

Neutrophils

Natural killer (NK) cells

Dendritic cells

c. Killer activation receptors


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• NK cells are part of the lymphocyte lineage that do not express the extremely variability like B cells and T cells.

• Instead, their receptors able to detect any alterations in host cells that have been infected by pathogens, particularly viruses.

• Killer activation receptors (KARs) on NK cells allow them to recognize the presence of stress-related molecules.

• Two stress-related molecules found in human; MICA and MICB, expressed by host cells that are unhealthy or abnormal cells

c. Killer activation receptors


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• Binding of MICA or MICB molecules by the NK cell's KARs induces the NK cell to attach and destroy the targeted (infected) host cell.


d. Killer inhibition receptors


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• NK cells also uses the killer inhibition receptors (KIRs), to monitor the major histocompatibility complex (MHC) class I molecules.

• MHC-I is normally displayed on the cell surfaces of all nucleated cells in the body.

• By scrutinizing MHC class I molecules, NK cells determine the normality of host cells.

• Many processes, including some cancers and some types of viral infection, decrease the number of MHC class I molecules displayed on the surface of the affected cell.

d. Killer inhibition receptors


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• Once bound to a target cell via its KARs, the NK cells use their KIRs to assess the expression of MHC class I molecules on that cell.

• If NK cells determine that the level is subnormal, they proceed to kill the target cell.

• If they determine that normal levels are present, the killing process is terminated and the target cell is released unharmed.

e. Complement receptors


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• The complement system is a complex set of soluble molecules that generate various reactions that attract immune cells to the site of infection and lead to destruction of microbes.

• Some of these activities are accomplished by the binding of certain complement components or their fragments to microbial surfaces.

• This enables "tagging" for the microbes for destruction by other elements of the immune system.

e. Complement receptors


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• Cell-surface bound complement receptors on phagocytic cells and B cells will recognize these bound complement fragments.

• This enable and facilitate the binding, ingestion, and internal degradation of the tagged microbes.


f. Fc receptors: Immunoglobulins


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• These are including epitope-binding immunoglobulins termed antibodies such as lgA , lgD, lgE, lgG, and lgM.

• Epitope binding by lgA, lgG, or lgM antibodies triggers a conformational change in the ''tail" or Fc portion of the antibody.



46Diagram Adopted from Internet



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• Fc receptors (FcRs) are expressed on the surfaces of phagocytic cells.

• Phagocytic cells recognize and bind epitope-engaged antibodies. How?

• They are able to recognize epitope-engaged antibodies when altered conformation of the Fc region happens.

• This leads to the phagocytosis of the epitope-antibody-FcR complex.



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• Antibodies that have not bound one or more epitopes do not bind to FcRs, and in this way, an antibody that has not bound to an epitope remains in circulation.

• The Fc receptor that binds lgE has some exception.

• lgE molecules will bind to the Fc receptors prior to epitopes encounter.

• However intracellular signaling does not occur until the lgE antibody binds the appropriate antigen later.



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1. Preformed and the somatically generated receptors are the benchmark of adaptive immunity.

2. Toll-like receptor are also the affiliate of PRRs.

3. When NK-cell encounter the stress related molecules, they spontaneously elicit an immune response.

4. Complements act as a ‘tagging’ for the phagocytes to internalize the pathogens that they binds to.

5. Every Ig will undergo Fc region modification before binding to the Fc receptors of immune cells


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1. What are the function of receptors in leukocytes?

2. What is the different between preformed receptor and somatically generated receptors?

3. Can you identify what are mechanism of preformed receptor?

4. Can you identify the difference of pattern recognition receptors and pathogen-associated molecular patterns.

5. What are the function of cytokines?


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6. What are the two stress-related molecules found in human that activate NK cells?

7. Differentiate Killer activation receptors and Killer inhibitor receptor receptors and how does this two receptors function in NK cells?

8. How complement receptors function in prompting immune response in immune cells?

9. What is a Fc region and Fc receptors. Where do Fc receptors found?

10. Explain the mechanism of immune response triggered by the Fc receptor: immunoglobulin.

Receptors


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a. Preformed receptors

b. Somatically generated receptors

3.3 Genetic control of immune response


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Somatically generated receptors


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• The preformed receptors of the innate immune system are encoded in the germline and passed on intact from one generation to the next.

• In contrast, the specialized receptors of B cells and T cells are regenerated anew in the lymphocytes of each individual through random somatic chromosomal rearrangements and mutations.

Somatically generated receptors


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• What is advantage of somatically generated receptors?

• Vast array of receptors specific for precise molecular details found in unique epitopes that may be encountered in the future.

• Somatically generated receptors:• B-cell receptors• T-cell receptors

a. B-cell receptors


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• B-cell receptors (BCRs) are cell-surface bound monomeric immunoglobulin associated with disulfide-linked heterodimers called lgα and lgβ.

• When a BCR binds an epitope, the specialized cytoplasmic tails of lgα and lgβ initiate an intracellular signaling cascade that may lead to B-cell activation.

• In addition, some activated B cells terminally differentiate into plasma cells, which secrete immunoglobulins that have the same epitope-binding specificity as their BCR


b. T-cell receptors


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• Structurally similar to immunoglobulin molecules, T-cell receptors (TCRs) are heterodimers, consisting of either an αβ or a γδ chain pair.

• TCRs are always membrane bound and recognize antigen combined with MHC molecules.

• They are associated with the cluster of differentiation 3 or CD3 complex of transmembrane surface molecules.


b. T-cell receptors


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• The CD3 complex functions much like the lgα and lgβ of BCRs.

• The CD3 complex links the TCR with intracellular signaling molecules.

• An additional accessory molecule (CD4 or CD8) is also present to serve as a type of co-receptor for the TCR.




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1. The somatically generated receptors enables lymphocytes to excel specific and precise molecular details found in unique epitopes.

2. The CD 3 complex elicit the cascades of cell activation in T cells.

3. CD 4 and CD 8 are the accessory molecules that has minimal functions to T cells.

4. B cells does not go through the ‘advance education’ as the T cells as they only develop in the Bone marrow.


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1. How does the Somatically generated receptors are generated in compression to the preformed receptors?

2. What is advantage of somatically generated receptors?

3. What are the constituent of somatically generated receptors?

4. Can you draw and identify the B-cell receptors (BCR)?

5. How do you distinguish BCR and antibody?

6. Can you draw and identify the T-cell receptors (TCR)?

7. Cluster of differentiation 3 or CD3. What do you understand from the statement.

3.4 Antigens processing by antigen presenting cell


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Antigen processingis necessary for TH-cell activation


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• TH cells functions to secretes the cytokines for the activation of both the humoral and cell-mediated immune cells.

• Why such system has to be employed?

• An inappropriate T-cell response to self-components can have fatal autoimmune consequences.

• How to ensure that they can be prompt to respond only to the non-self?



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• TH cells can only recognize antigen that is displayed together with class MHC II molecules on the surface of antigen-presenting cells (APCs).

Table Adopted from Kuby Immunology by Kindt Thomas J., W.H. Greeman and Company, 2007



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• Characteristics of APCs:• Express class II MHC molecules on their membranes.• Deliver a co-stimulatory signal for TH-cell activation.

• Antigen-presenting cells first has to internalize antigen before processing via:• Phagocytosis• Endocytosis



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Diagram adopted from Nature Reviews: Immunology



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• Once the antigen is lysed, a part of the antigen is displayed on their membrane bound to a class II MHC molecule.

• TH cell able to recognize and interacts with the antigen–class II MHC molecule complex on the membrane of the APCs.

• An additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the TH cell.



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• Electron micrograph of an antigen-presenting macrophage (right) associating with a T lymphocyte

Diagram Adopted from Kuby Immunology by Kindt Thomas J., W.H. Greeman and Company, 2007



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• Experiment by K. Ziegler and E. R. Unanue

• Experiment by K. Ziegler and E. R. Unanue R. P. Shimonkevitz

Table Adopted from Kuby Immunology by Kindt Thomas J., W.H. Greeman and Company, 2007




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1. CTL secretes cytokines to that has to be monitored to avoid autoimmune response.

2. Dendritic cells, macrophages and B cells are the professional APCs.

3. Phagocytosis and endocytosis is part of internalization process of the APCs.

4. When peptides are presented by the APCs to the TH cells, this will immediately prompt the TH cell activations.

3.5 Role of MHC and accessory molecules


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Major Histocompatibility Molecules


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• The major histocompatibility complex (MHC) is a large genetic complex with multiple loci.

• MHC loci encode two major classes of membrane-bound glycoproteins: class I and class II MHC molecules.

• MHC molecules function as antigen-recognition molecules, but they do not possess it.



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MHC Class I

MHC Class II

MHC Class I



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• For a foreign protein antigen to be recognized by a T cell, it must be degraded into small peptides that form complexes with class I or class II MHC molecules.

• The MHC-associated peptide fragments is called antigen processing and presentation.

• Question: Which antigen to be presented by MHC class I or II and what determines this?



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• Determined by the route that the antigen takes to enter a cell:

• Exogenous antigen

• Endogenous antigen

Exogenous antigen lead to MHC class II pathway


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• Exogenous antigen is produced outside of the host cell and enters the cell by endocytosis or phagocytosis.

• APCs degrade ingested exogenous antigen into peptide fragments within the endocytic processing pathway.

• Peptides produced by degradation of antigen in this pathway bind to the cleft within the class II MHC molecules.

Exogenous antigen lead to MHC class II pathway


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• Presentation of exogenous peptide– class II MHC complexes is limited to APCs only.


Endogenous antigen lead to MHC class I pathway


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• Endogenous antigen is produced within the host cell itself.

• Two common examples:• Viral proteins synthesized within virus-infected host

cells• Unique proteins synthesized by cancerous cells.

• Endogenous antigens are degraded into peptide fragments that bind to class I MHC molecules within the endoplasmic reticulum.



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• The peptide–class I MHC complex is then transported to the cell membrane.

• All cells producing endogenous antigen use this route to process the antigen… WHY?

• T cells displaying CD8 recognize antigen associated with class I MHC molecules.

• Cytotoxic T cells attack and kill cells displaying the antigen–MHC class I complexes



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• Presentation of endogenous peptide– class I MHC complexes is by all nucleated cells.


Internalized antigen digested by cells

Altered self-cells

T cell receptors recognize antigen bound to MHC molecules

Binding antigen-MHC activates T cells

TH cells secretes cytokines and activates T cells, B cells and innate cells

Activated TC kill altered cells

Antibody binds antigen during humoral immune reaction

B cells differentiate into plasma cell when reacted with antigen



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Summary

3.5 Antigen-antibody interactions


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Antigen-antibody interactions


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• The antigen-antibody interaction is a bio-molecular association similar to an enzyme-substrate interaction.

• However it does not lead to an irreversible chemical alteration in either the antibody or the antigen.

• The contact between antibody-antigen involves various non-covalent interactions between epitope of antigen and the variable-region (VH/VL) domain of the antibody molecule.

Antigen-antibody interactions


87Diagram Adopted from Kuby Immunology by Kindt Thomas J.,

W.H. Greeman and Company, 2007

Education

Ch3 antigens