Immuno Chemistry 2011

8/3/2019 Immuno Chemistry 2011

http://slidepdf.com/reader/full/immuno-chemistry-2011 1/26

I-Year MBBS IMMUNO CHEMISTRY

Mr. Pradeep Kumar.G 1

IMMUNO CHEMISTRY

Immunity: The state of body that gives protection against pathogens

Immunology: Study of immunity is immunology

1. Types of Immunity:

Immunity is majorly divided in to two types

I) Innate immunity II) Adaptive immunity or Acquired immunity

I) innate immunity:

Developed since birth (before the entry of the pathogen for the first)

Is not specific against the pathogen, but acts very fast (within hours of infection)

Innate immunity contains four types of components

a. Anatomic barriers

Skin

Mucous membrane

b. Physiologic barriers

Temperature

Low pH

Chemical mediators

c. Phagocytic/ endocytic barriers

d. Inflammatory barriers





a. Anatomic barriers

Skin:

Physically prevents the entry of microorganisms

Skin secretes lactic acid and fatty acids, hence the pH of the skin is acidic (3-5 pH)

Acidic pH of the skin do not allow the growth of microorganisms

Mucosal lining:

Is present in conjunctivae and the alimentary, respiratory, and uro-genital tracts

Mucous entraps the pathogens

Cilia present in respiratory tract propel mucous covered pathogens out.

Mucous secretions contains antibacterial and antiviral substances that kills the pathogens

Normal flora attached to the mucous lining prevents the attachment and growth of the

pathogens

b. Physiologic barriers

Temperature

Normal body temperature prevents the growth of many pathogens

Increased body temperature during fever prevents the growth of pathogens

Low pH

High acidic pH of the gastric juice prevents the growth of the pathogenic microbes

The acidic pH (3-5) of the skin also prevents the growth of the pathogens

Chemical mediators:

Lysozyme – secreted in tears, mucous etc, cleaves the bacterial cell wall.

Interferon – gives antiviral protection for the uninfected cells

Complement molecules – facilitate phagocytosis

Collectins – disrupt cell wall of bacteria







d. Inflammatory barriers

Tissue damage caused by wound or infection induces inflammation

Vasodilatation occurs at the site of infection

Permeability of the blood capillaries will increase

Fluid moves from the blood to the tissue termed as exudate

Phagocytic cells attaches to the inner walls of the blood capillaries, called margination

Marginated phagocytic cells passes between the blood capillary cells and enters in tissue

called extravasation

Phagocytic cells entered in to tissues moves towards site of infection by chemotaxis

Phagocytic cells clear the pathogens and dead cells

Then regeneration of tissue occurs





II. Adaptive Immunity (Acquired Immunity)

Adaptive immunity develops after the entry of the pathogen for the first time

It will take time to develop against infection for the first time

But acts very fast against pathogen entering second time and so on

Adaptive immunity shows four different properties

Antigenic specificity: Acts more specifically on antigens (pathogens)

Diversity: Recognizes and acts on different kinds of antigens (pathogens)

Immunologic memory: Keeps the memory of antigenic structure of pathogens

Self/non self recognition: distinguish between body cells and foreign cells or molecules.

Adaptive immune response is divided into two types

a. Cell mediated response b. Humoral response

a. Cell mediated response: Shown mainly by T-lymyphocytes (TC & TH cells)

Cell mediated response given by TC-cells

Kills the pathogens present inside the cell (endo pathogens) ,

Destroys cancerous or viral infected cells

Antigens (viral /cancer peptides) are attached to class-I MHC receptors and presented on

cell surface

TC-cells recognizes antigens presented with class-I MHC and gets activated

Activated TC-cells differentiates into effector cells and memory cells

Effector cells release cytotoxins that kill specifically the cancerous or viral infected cells





Cell mediated response given by TH-cells:

Pathogen present in blood (exo pathogens) are engulfed by the antigen presenting cells

Antigen presenting cells (B-cells, macrophages etc) destroy pathogens

Pathogen antigens are attached to class-II MHC

Class-II MHC presents antigens on the surface of antigen presenting cells

TH-cells interacts with the antigens presented with class-II MHC molecules and activated

Activated TH cells divides into effector cells and memory cells

Effector TH-cells produces cytokines

Cytokines produced by activated TH – cells show two actions

T H 1 response:

Cytokines activates macrophages and TC-cells to kill the infected cells

T H 2 response:

Cytokines activates B-Cells to show humeral response (production of antibodies)

b. Humoral response

B-lymphocytes (B-cells) shows humeral response

kills exo pathogens (pathogens present in blood)

B-cells bind the antigen (pathogen) with membrane bound antibodies

Antigen is taken inside the B-cell, processed and attached to the class-II MHC receptors

Class-II MHC receptors express the antigens on the surface of the B-cells

T-helper cells (TH-cells) recognizes the antigen presented by class-II MHC receptors

TH-cells gets activated

Activated TH-cells secretes cytokines

Cytokines secreted by TH-cells activates B-cells termed as TH2 response

Activated B-cells divides in to plasma cells (effector cells) and memory cells

Plasma cells secretes antibodies specific to the antigen (pathogen)





Antibodies bind to the pathogen in blood and initiate the killing

Hence TH-cells activates both humoral and cell mediated responses

Cell mediated and humoral responses are interlinked.





2. Types of adaptive immunity

Adaptive immunity is divided into two types

i) Active adaptive immunity ii) Passive adaptive immunity

Active adaptive immunity:

The adaptive immunity developed in our own body is termed as active immunity

It is again divided in to two types

Active natural: the adaptive immunity developed against a normal infection

Active artificial: the adaptive immunity developed against a vaccine

Passive adaptive immunity:

The immunity given by antibodies synthesized out of our body (other source) is termed as

passive adaptive immunity

It is again two types

Passive natural: immunity given by antibodies transferred from mother to fetus or child

Passive artificial: Immunity given by antibodies produced in laboratory

eg: antiserum, or monoclonal antibodies given treat snake poisoning





3. Components of immune system

a. Antibodies:

Antibodies are antigen binding glycoproteins secreted by B-cells (plasma cells)

They corresponds to globulins of serum proteins, hence also called as immuno globulins

b. Haptanes:

Haptanes are low molecular weight molecules that cannot show immune response

Heptanes attached to any other large molecular weight substance can show the immune

response

c. Antigen:

A molecule that can bind to antibodies or activated TH or TC cells

d. Immunogen: A molecule that can show immune response (production of antibodies,

activation of TH or TC cells)

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

e. Complement system:

Compliment system contains a group of nearly 30 different proteins

Complement proteins are membrane bound and soluble

Most of the complement proteins are synthesized by Liver

Blood monocytes, tissue macrophages, and epithelial cells of the gastrointestinal and

genitor-urinary tracts synthesize complement proteins.

Compliment system helps the antibodies for killing the microbes (bacteria, viruses etc)

Helps the phagocytosis of particulate antigens by phagocytic cells by opsonization

Helps to clear the antigen-antibody complexes by phagocytic cells

Involves in inflammatory response





f. Opsonization:

The molecule that binds both antigen and phagocytic cell to enhance the phagocytosis is

termed as an opsonin

Antibodies and complement proteins can act as opsonins

Opsonin binds the antigen and also binds the opsonin receptor on phagocytic cell

Binding of antigen attached opsonin to the phagocytic cell increases the phagocytic

capacity

g. Major Histocompatibility complex (MHC) molecules

MHC are also termed as HLA (human leukocyte antigens) in human

MHC molecules are synthesized by a group of genes present on chromosome 6

MHC molecules are glycoproteins

Three major classes (types) of MHC molecules are present

Class-I-MHC:

Synthesized in all nucleated cells

Are membrane bound glycoproteins

Antigens are attached to the MHC molecules and they are expressed on the cell surface istermed as antigen presentation

Antigens presented by the class-I-MHC are recognized by cytotoxic T-cells

Class-II-MHC:

Produced only in antigen presenting cells (APCs), macrophages, B-cells, dendritic cells

Are membrane bound glycoproteins

Presents antigens on the surface of antigen presenting cells

Antigens presented by the class-II-MHC are recognized by T-helper cells





Class-III-MHC:

Not involved in antigen presentation

They are secreted glycoproteins

They help the compliment system and inflammation.

h. Vaccine:

Killed or inactivated pathogen which do not causes pathogenesis but shows

immunological response





4. Cells of Immune system:

Hematopoisis:

The formation of Red & White blood cells in human is termed as hematopoisis.

All blood cells differentiates from the Hematopoietic stem cell (HSC)

Before birth hematopoisis takes place in yolk sac, fetal liver and spleen

After birth hematopoisis takes place in Bone marrow.

Hematopoietic stem cells shows pluripotency or multipotency

Pluripotency or multipotency is the ability to differentiate in to different types of cells

Pluripotent hematopoietic stem cells differentiate in to two progenitor cells,

Myeloid progenitor cell

Differentiates into erythrocytes, platelets, granulocytes, macrophages and dendritic cells

Lymphoid progenitor cell

Differentiates into B-lymphocytes, T-lymphocytes, Natural Killer cells





Hematopoisis is stimulated by hematopoietic growth factors (cytokines)

During infection, macrophages and T-cells are activated and produce cytokines

Lymphoid cells:

20 % - 40 % of total body white blood cells are lymphoid cells

Divided in to three types, B-cells, T-cells and natural killer cells

Circulates both in lymph and blood and can migrate in to tissues and lymphoid organs

Lymphoid cells that are not interacted with any antigen are termed as naive cells

/unprimed cells /resting cells.

When naive lymphocytes interact with antigen they convert in to lymphoblasts

Lymphoblasts differentiate and divide in to memory cells and effector cells.

Effector cells shows immune response (kills and destroys pathogens).

Memory cells remember the antigenic structure of pathogen for future immune response.

Lymphocytes contains special membrane bound molecules called as CD markers

(cluster differentiation markers)

CD markers are grouped based on their ability to bind monoclonal antibodies.

B-Lymphocytes (B-Cells):

B-cells mature in bone marrow in human,

bursa of Fabricius in birds.

Contains membrane bound antibodies

Membrane bound antibodies binds the antigens

Contains MHC-II antigen presenting receptors

Contains CD 32, CD 35, CD 40, CD80, CD 86.

Naive B-cells are activated by the interaction of antigens with membrane bound

antibodies, and by cytokines secreted by macrophages and T-Cells.





Activated naive B- cells differentiate in to effector cells (plasma cells) and memory cells.

Plasma cells secretes antibodies, and shows humoral immune response

T- Lymphocytes (T-Cells):

T-cells mature in thymus

Involved in cell mediated immunity

Contain membrane bound receptors called as T-cell receptors (TCR)

TCR s binds antigens (pathogens)

TCR cannot bind free antigens, binds to the antigens attached to MHC molecules only

T-cell membrane contains different CD markers

There are two types of T-lymphocytes

i) T-helper cells (TH), ii) T cytotoxic cells (TC cells)

TH-cells:

Contains CD4 marker on the membrane

Can bind to the antigens attached to the MHC-II on anantigen presenting cell

TH-cells are activated when attached to antigen - MHC-II

complex

Activated TH-cells divide into effector cells and memory cells

Effector TH-cells secretes cytokines.

Cytokines from effector TH cells shows two different responses (functions)

TH1-response:

Cytokines produced by the TH-cells activates TC-cells, macrophages and inflammation

TH2-response:

Cytokines produced by the TH-cells activates B-cells





TC-cells:

Contains CD8 marker on the membrane

Binds to the antigens attached to the MHC-I on viral

infected cells or cancerous cells and becomes activated

Activated TC-cell differentiates in to cytotoxic T

lymphocyte (CTL), and memory cells

Cytotoxic T lymphocyte (CTL) then secretes cytotoxic

chemicals that kill the infected cells

T-suppressor cells (TS-cells):

Suppress the humoral and cell mediated immune responses

But presence of separate Ts-cells in the body is not clearly known

Natural Killer cells (NK-cells):

NK-cells are large granular lymphocytes

Kills cells infected with some viruses and cancerous cells

The cell membrane molecules present on B & T cells are not found on NK-cells

NK-cells contain special membrane molecules and CD markers

NK1-T cell:

Shows some properties of NK-cells and some properties of T-cells

Contains T cell receptors (TCR)

TCR of NK1-T cell interact with MHC like molecule called as CD1

Macrophages

Circulating monocytes enters in to the tissues and differentiates into macrophages

Macrophages are large cells containing single nucleus and granular cytoplasm

Macrophages do phagocytosis and antigen presentation

Some macrophages are free or motile which travel in tissues





Some macrophages are tissue specific

eg: Alveolar macrophages in the lung

Histiocytes in connective tissues

Kupffer cells in the liver

Mesangial cells in the kidneyMicroglial cells in the brain

Osteoclasts in bone

Macrophages are activated by

pathogens (particulate antigens),

Cytokinins secreted by activated TH cells

mediators of inflammatory response

components of bacterial cell wall

Activated macrophages secretes different compounds

Immunological role of compounds secreted by activated macrophages

Compound secreted Immunological roleMHC (class II) Antigen presentationInterleukin 1 (IL-1), Interleukin 6 (IL-6) Promotes inflammatory response and feverComplement proteins inflammatory response and kills theHydrolytic enzymes Promote inflammatory response Interferon alpha gives antiviral protection to the cellTumor necrosis factor Kills tumor cells

Neutrophils

50 -70 % of total circulating WBC are neutrophils

Contains multi lobed nucleus and two types of cytoplasmic granules

Which containing myeloperoxidase, defensins, lysozyme, lactoferrin, lysozyme, etc

Neutrophils are phagocytic cells





Eosinophils:

Eosinophils stains with acidic dyes like eosin

Nucleus is bilobed, Cytoplasm is granular

Eosinophils are phagocytic cells, but phagocytic role is less important

Eosinophils kill the parasites, by secreting basic protein.

The mechanism of killing the parasites is Antibody dependent cell mediated

cytotoxicity

Basophils:

Stains with basic dyes (methyline blue)

non phagocytic cells, Secretes pharmacologically active substances

Involves in allergic response along with mast cells

Mast cells:

Contains large number of cytoplasmic granules

Mast cells are present in Skin, connective tissues, mucosal epithelial tissues of

respiratory, genitourinary and digestive tracts.

Mast cells along with basophiles develop allergic response.

Dendritic cells:

Contains large membrane extensions that resembles dendrites of nerve cell

Four major types of dendritic cells are there

i) Langerhans cells, ii) Interstitial dendritic cells,

iii) Myeloid dendritic cells, iv) Lymphoid dendritic cells

Dendritic cells produce MHC (class-II) receptors

A dendritic cell presents antigens to T-helper cells

Dendritic cells are most powerful antigen presenting cells than macrophages and B-cells.





Follicular dendritic cells:

Follicular dendritic cells develop in Lymph follicles (not in bone marrow)

They are not antigen presenting cells (do not produce MHC-II), Activates the B-cells.

Antigen presenting cells (APCs):

B-cells, Macrophages and dendritic cells are antigen presenting cells

Dendritic cells are powerful antigen presenting cells than B-cells and macrophages

All APCs express class-II MHC receptors

APCs processes and present the antigens with MHC-II receptors

Antigens presented by the MHC-II are recognized by the T-helper cells

Then T-helper cells show the immunological response against the pathogen.





5. Antibody structure:

Antibody is a Y – shaped molecule

Contains two heavy chains (high molecular weight), two light chains (low M.wt)

All the four chains are interconnected through disulfide bonds.





Light Chains

Variable (VL) region of light chain: 100 to 110 Amino acids sequence at N-terminus of

the light chain is variable in different antibodies

Constant (CL) region of light chain

The amino acids present at the C-terminus are constant in all the antibodies

There are two different sequences of C-region, kappa (κ ) and lambda (λ )

Hence there are two types of light chains kappa (κ ) and lambda (λ )

In humans, 60% of the light chains are kappa (κ) and 40% are lambda (λ)

Lambda (λ) chains are sub classified into 4 types in human λ 1, λ 2, λ 3, λ 4.

λ 1, λ 2, λ 3, λ 4 subtypes will have minor amino acid differences

Heavy chains

variable (VH) region of heavy chain.

100-110 amino acid sequence present at N-terminus is variable in different antibodies

Constant (CH) region of heavy chain

Amino acid sequence present at C-terminus is constant in all the antibodies

There are five subtypes in C – regions μ, γ, α, δ, ε.

Hence there will be five different heavy chains μ, γ, α, δ, ε.

Each heavy chain type is called as an isotype.

The type of a heavy chain (isotype) determines the class of the antibody -

IgM(μ ), IgG(γ), IgA(α), IgD(δ), IgE(ε).

In humans, α heavy chains has two subtypes α1 and α2 (forms IgA1 and IgA2)

γ heavy chains has 4 subtypes γ1, γ2 , γ3 ,γ4 (forms IgG1, IgG2, IgG3, and IgG4)

A single antibody molecule contain identical Heavy chains and identical light chains





Hyper variable regions

Some amino acids in variable (V) region are more variable

Also called as complementarity determining regions (CDRs)

CDRs form the antigen binding site of the antibody molecule.

The less variable sequence in variable region called as framework regions (FRs).

Fab & Fc regions:

When antibody is cut by papain (protease)

it produces three fragments

Two N-terminal fragments – VL and VH

VL and VH can bind to the antigen

individually hence named as Antigen

binding fragment (Fab)

One C-terminal fragment – CH dimer

CH dimer do not bind the antigen and is

crystallized when kept in freezer, hencenamed as crystallizing fragment (Fc)

Hinge region:

The region that connects Fab and Fc

regions is termed as hinge region

Hinge region shows flexibility, useful for

the movement of Fabs





Multiple myeloma, is a cancer of antibody-producing plasma cells

Myeloma cells produce and secrete excess amount of antibodies

Excess light chains will be excreted in urine termed as Bence-Jones proteins,

6. Antibody Classes (Types)

Immunoglobulin G (IgG)

IgG, concentration in serum is more (about 80% of the total serum immunoglobulins)

The IgG molecule consists of two γ heavy chains and two κ or λ light chains.

Four human IgG subclasses are there IgG1, IgG2, IgG3, and IgG4

IgG1, IgG3, and IgG4 readily cross the placenta and protects developing fetus.

IgG3 activates compliment very effectively

IgG1 and IgG2 activates compliment weakly

IgG4 do not activate complement.

IgG1 and IgG3 bind with high affinity to Fc receptors on phagocytic cells during

opsonization.

IgG4 has an intermediate affinity for Fc receptors.

IgG2 has an very low affinity for Fc receptors.





Immunoglobulin M (IgM)

IgM accounts for 5 – 10% of the total serum immunoglobulin,

Monomeric IgM, is expressed as membrane-bound antibody on B cells.

IgM forms pentamer (five monomers joined together by disulphide bonds at Fc regions)

IgM pentamer is secreted by plasma cells.

IgM pentamer contains an additional polypeptide chain called the joining (J) chain.

J chain is linked to the pentamer at Fc regions through disulphide bonds

IgM is the first immunoglobulin class produced in a primary response to an antigen,

IgM is the first immunoglobulin to be synthesized by the neonate.

IgM pentamer can bind 10 antigens

IgM activates compliment more efficiently than IgG

Because of its large size, IgM does not diffuse well and therefore is found in very low

concentrations in the interstitial fluids.

IgM is secreted in to mucosal secretions by the mucosal epithelial cells.





Immunoglobulin A (IgA)

IgA present in the serum is called as serum IgA

Concentration of the serum IgA is 10% – 15% of the total immunoglobulin in serum.

IgA is secreted in external secretions (breast milk, saliva, tears, and mucus of the

bronchial, genitourinary, and digestive tracts) is termed as secretory IgA.

The concentration of secretory IgA is higher than any other Immunoglobulins

Serum, IgA exists as monomer, and polymeric forms (dimers, trimers, tetramers)

All polymeric IgA contains a J-chain

Secretory IgA, consists of a dimer or tetramer.

Secretory IgA contains J-chain and a polypeptide chain called secretory component

Secretory component is required for the transport of IgA across cell membranes

IgA binds to bacterial and viral surface antigens and prevents attachment of the

pathogens to the mucosal cells.

Complexes of secretory IgA and antigen are easily entrapped in mucus and then

eliminated by the ciliated epithelial cells of the respiratory tract or by peristalsis of the

gut.

Secretory IgA has been shown to provide an important line of defense against bacteria

such as Salmonella, Vibrio cholerae, and Neisseria gonorrhoeae and viruses such aspolio, influenza, and reovirus.

Breast milk contains secretory IgA and many other molecules that help protect the

newborn against infection during the first month of life.

Because the immune system of infants is not fully functional, breast-feeding plays an

important role in maintaining the health of newborns.





Immunoglobulin E (IgE)

The concentration of IgE in serum is very low (0.3 μg/ml)

IgE mediate the immediate hypersensitivity reactions

IgE binds to Fc receptors on the membranes of blood basophils and tissue mast cells.

Allergen (antigen) binds to the membrane bound IgE,

Then basophils or mast cells release pharmacologically active chemical mediators, this

process is termed as degranulation.

Pharmacologically active chemical mediators give the allergic response

IgE induces degranulation in mast cells to kill parasitic infections.

Immunoglobulin D (IgD)

IgD constitutes 0.2% of the total immunoglobulin in serum.

IgD, is also bound to the membrane of B-cells

Biological roles of IgD are still under study.





7. Monoclonal Antibodies:

A group of antibodies that can bind to only one type of epitope of an antigen are termed

as monoclonal antibodies.

A group of antibodies that can bind to different types of epitopes of an antigen are termedas polyclonal antibodies.

Serum containing polyclonal antibodies against an antigen is termed as antiserum.

Application of monoclonal antibodies

Diagnosis & imaging:

Monoclonal antibodies are used in diagnostic techniques like ELISA (Enzyme Linked

Immuno Sorbent Assay)

Radio labeled monoclonal antibodies that are specific to epitope of cancerous cells can be

used to detect the metastases of the cancer.

Immuno toxins:

Monoclonal antibodies specific to tumor cells are attached to toxins termed as immuno

toxins

Immuno toxins specifically bind tumor cells and kill them.

Abzymes:

Abzymes are monoclonal antibodies that catalyze the reactions like enzymes.

Abzymes specifically binds to the transitionary state of a molecule (antigen)

Binding of Abzymes decreases the activation energy for the reaction

Hence Abzymes can catalyze the reactions like enzymes

Documents

Immuno Chemistry 2011