Immune Response Kim

7/27/2019 Immune Response Kim

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SPECIFIC HOST

DEFENSES: THEIMMUNE RESPONSE

Shalimar Kim P. Inay


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The Immune Response

Immunity - Ability of an organism to recognizeand defend itself against specificpathogens or

antigens.

Immune Response - it involves production ofantibodies and generation of specialized

lymphocytes against specific antigens.

Antigen - Molecules from a pathogen or foreign

organism that provoke a specific immuneresponse.


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Defense against Pathogens in the

body

Nonspecific Resistance

External barriers

Leukocytes and macrophages

Antimicrobial proteins

Fever

Specific Immunity

General aspectCellular immunity (Recognition, Attack, Memory)

Humoral immunity (Recognition, Attack, Memory)


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Immune System

Humoral (Antibody-Mediated) Immunity

Involves production of antibodies against foreignantigens.

Antibodies are produced by a subset of lymphocytescalled B cells.

B cells that are stimulated will actively secreteantibodies and are calledplasma cells.

Antibodies are found in extracellular fluids (blood

plasma, lymph, mucus, etc.) and the surface of Bcells.

Defense against bacteria, bacterial toxins, and virusesthat circulate freely in body fluids, before they entercells.

Also cause certain reactions against transplanted


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Antigens

Most are proteins or large polysaccharides from a

foreign organism.

Microbes: Capsules, cell walls, toxins, viral capsids,

flagella, etc. Nonmicrobes: Pollen, egg white , red blood cell surface

molecules, serum proteins, and surface molecules from

transplanted tissue.

Lipids and nucleic acids are only antigenic whencombined with proteins or polysaccharides.

Hapten: Small foreign molecule that is not antigenic. Must becoupled to a carrier molecule to be antigenic. Once antibodies areformed they will recognize hapten.


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Antigens

Epitope

Small part of an antigen that interacts with an

antibody.

Any given antigen may have several epitopes.

Each epitope is recognized by a different

antibody.


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Epitopes: Antigen Regions that

Interact with Antibodies


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Antibodies

Proteins that recognize and bind to aparticular antigen with very high specificity.

Made in response to exposure to the antigen.

One virus or microbe may have severalantigenic determinant sites, to which differentantibodies may bind.

Each antibody has at least two identical sites

that bind antigen: Antigen binding sites. Belong to a group of serum proteins called

immunoglobulins (Igs).


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Antibody Structure

Monomer: A flexible Y-shaped molecule with fourprotein chains: 2 identical lightchains

2 identical heavychains

Variable Regions: Two sections at the end of Ysarms. Contain the antigen binding sites (Fab).Identical on the same antibody, but vary from oneantibody to another.

Constant Regions: Stem of monomer and lowerparts of Y arms.

Fc region: Stem of monomer only. Importantbecause they can bind to complement or cells.


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Antibody Structure


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Immunoglobulin Classes

IgG

Most abundant, about 80% of all antibodies in the

blood; found in blood, lymph, and intestines;

monomer (one unit) structure. Protects againstbacteria and viruses by enhancing phagocytosis,

neutralizing toxins and trigerring the complement

system. It is the only class of antibody to cross

placental from mother to fetus; conferringconsiderable immune protection in newborns


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IgA

Found mainly in sweat, tears, saliva, mucus,

breast milk, and gastrointestinal secretions.

Smaller quantities are present in the blood andlymph. Makes up 10-15% of all antibodies in the

blood. Occurs as monomers and dimers (two

units). Levels decrease during stress, lowering

resistance to infection. Provides localizedprotection of mucuos membranes against

bacteria and viruses


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IgM

Makes up 5-10% of all antibodies in the blood; also

found in lymph. Occurs as pentamers (five units);

first antibody class to be secreted by plasma cellsafter an initial exposure to any antigen. Activates

complement and causes agglutination and lysis of

microbes. Also present as monomers on the

surfaces of B cells, where they serve as antigen

receptors. In blood plasma, the anti-A and anti-B

antibodies of the ABO blood group, which bind to A

and B antigens during incompatible blood

transfusions are also IgM antibodies.


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IgD

Mainly found on the surfaces of B cells as antigen

receptors, where it occurs as monomers, involved

in activation of B cells. About 0.2% of allantibodies in the blood

IgE

Less than 0.1% of all antibodies in the blood.

Occurs as monomers, located on mast cells andbasophils. Involved inn allergic and

hypersensitivity reactions. Provides protection

against parasitic worms


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Antigen(Ag)- Antibody(Ab)

reactions

It is the reaction between an antigen and an

antibody when they come in contact.

This occurs when antigens enter the body.

When antigens enter the body, signals are

sent out and in response to that antibodies are

made specifically to destroy the antigens.

It is part of the Adaptive(Specific) immunity


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B cells and Antibodies

When a B cell encounters its triggering antigen(alongwith collaborating T cells and accessory cells), it givesrise to many large plasma cells. Every plasma cell isessentially a factory for producing antibody. Each ofthe plasma cells descended from a given B cells

manufactures millions of identical antibody moleculesand pours them into the bloodstream.

A given antibody matches an antigen much as a keymatches a lock. The fit varies: sometimes it is veryprecise, while at other times it is little better than thatof a skeleton key. To some degree, however, theantibody interlocks with the antigen and therebymarks it for destruction.


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How Do B Cells Produce Antibodies?

B cells develop from stem cells in the bone

marrow of adults (liver of fetuses).

After maturation B cells migrate to lymphoid

organs (lymph node or spleen).Clonal Selection: When a B cell encounters an

antigen it recognizes, it is stimulated and divides

into many clones called plasma cells, which

actively secrete antibodies.


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Humoral Immunity

Clonal Selection

Clonal Selection: B cells (and T cells) that

encounter stimulating antigen will proliferate into

a large group of cells.Clonal Deletion: B and T cells that react against

selfantigens appear to be destroyed during fetal

development. Process is poorly understood.


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Antigen-Antibody Binding

Consequenses

Antigen-Antibody Complex: Formed when anantibody binds to an antigen it recognizes.

Agglutination: Antibodies cause antigens(microbes) to clump together.

IgM (decavalent) is more effective that IgG (bivalent).

Hemagglutination: Agglutination of red blood cells.Used to determine ABO blood types and to detectinfluenza and measles viruses.

Opsonization: Antigen (microbe) is covered withantibodies that enhances its ingestion and lysis byphagocytic cells.


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Antigen-Antibody Binding

Consequenses

Neutralization: IgG inactivates viruses by binding

to their surface and neutralize toxins by blocking

their active sites.

Antibody-dependent cell-mediated cytotoxicity:Used to destroy large organisms (e.g.: worms).

Target organism is coated with antibodies and

bombarded with chemicals from nonspecific

immune cells. Complement Activation: Both IgG and IgM trigger

the complement system which results in cell lysis

and inflammation.


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Immunological Memory

Antibody Titer: Amount of antibody in theserum

Pattern of Antibody Levels during infection

Primary ResponseAfterinitialexposure to antigen, no antibodies are

found in serum for several days.

A gradual increase in titer, first of IgM and then of

IgG is observed.Most B cells become plasma cells, but some B

cells become long living memory cells.

Gradual decline of antibodies follows.


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Immunological Memory

Secondary Response

Subsequent exposure to the same antigen

displays a faster and more intense antibody

response. Increased antibody response is due to the

existence of memory cells, which rapidly produce

plasma cells upon antigen stimulation.

R l ti hi B t C ll


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Relationship Between Cell-

Mediated and Humoral Immunity

Antibody Production

T-Dependent Antigens: Antibody production requires assistance from T helper cells.

A macrophage cells ingest antigen and presents it to TH

cell.

TH cell stimulates B cells specific for antigen to become plasmacells.

Antigens are mainly proteins on viruses, bacteria, foreign redblood cells, and hapten-carrier molecules.

T-Independent Antigens: Antibody production does not require assistance from T cells.

Antigens are mainly polysaccharides or lipopolysaccharides withrepeating subunits (bacterial capsules).

Weaker immune response than for T-dependent antigens.


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Relationship Between Cell-

Mediated and Humoral Immunity

Antibody Dependent Cell Mediated

Cytotoxicity Target cell is covered with antibodies, leaving Fc

portion sticking outwards. Natural killer and other nonspecific cells that have

receptors for Fc region are stimulated to kill targeted

cells.

Target organism is lysed by substances secreted byattacking cells.

Used to destroy large organisms that cannot be

phagocytosed.

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Immune Response Kim