Antigen-specific

3rd line of defense

Provides adaptive protection against specific antigens which stimulate the immune response

Antigens are substances that are recognized by the immune system as non-self and provoke an immune response

Specificity – The size and shape of the antigen determines which lymphocyte population will respond

Systemic

Immunity can occur at other sites than just the site of invasion 

Memory component

After initial exposure to an antigen memory cells are produced that prompt a more vigorous response upon a second exposure to the same antigen

Involves two distinct populations of lymphocytes...

B cells - Humoral Immunity

T cells - Cell Mediated Immunity

Humoral Immunity

Antibody mediated immunity (AMI)

Works against antigens and extra cellular pathogens in body fluids

Most bacteria divide in the body fluids but rarely enter the cell

In most cases antibodies do not cross cell membranes 

Cell mediated immunity (CMI)

Particularly effective against intracellular pathogens such as viruses; some cancer cells; and tissue transplants

Do not respond to antigens in solution 

ANTIGENS

Substances that are recognized by the immune system and provoke immune responses.

Large complex molecules that are most often proteins

May also be lipoproteins, nucleoproteins, glycoproteins, or certain large polysaccharides

T-cells typically only respond to antigens that include protein

Exhibited on the cell surface membrane in association with MHC molecules

B-cells respond to antigenic proteins plus certain lipids, carbohydrates, and nucleic acids

Two important characteristics of antigens 

Reactivity: The ability of the antigen to react specifically (form complex) with the produced antibodies or cells

Immunogenicity: Ability to provoke an immune response.

Stimulates production of specific antibodies or proliferation of specific T cells, or both (B & T lymphocytes)

Antigenic determinants

Specific cell surface molecules on the surface of the antigen.

Most antigens have several determinate sites (epitopes) to which free antibodies or activated B and/or T lymphocytes can bind

A large antigenic protein may have hundreds of chemically different antigenic determinate sites

Thus many different antibodies and T-lymphocytes are activated against it

Lymphocytes have tens of thousands of the same unique receptor on their individual cell surface 

Haptens

Partial or incomplete antigens that give reactivity without immunogenicity.

Ordinarily do not initiate immune response

However if hapten is bound to a larger carrier protein molecule, such as a serum protein, it can become immunogenic

The hapten becomes the antigenic determinate site on the body protein

e.g. small lipid toxin in poison ivy triggers an immune response only after binding with a body protein

Hapten stimulated responses are responsible for hypersensitivity reactions

Immunocompetence

Refers to the cells ability to recognize a specific antigen by binding to it.

The immune system is competent to face any immunological threat

Before a particular antigen enters the body there are lymphocytes with surface proteins that can recognize the intruder and respond to it, ready and waiting.

There are millions of different lymphocyte populations distributed throughout the body

Each population consists of several thousand cells with receptors on their membranes that differ from those of other lymphocytes populations

These are highly specific surface receptor proteins on the cell membrane

They number approximately 100,000 per cell and are highly specific for a single antigen

Each group of lymphocytes will respond to the presence of a different antigen

Once activated they undergo proliferation to produce clones with identical specificity

Where a given lymphocyte develops into a B or T-cell determines its immunocompetence

T-cells in thymus and B-cells in bone marrow.

The B and T-cells acquire distinctive surface proteins capable of recognizing a specific antigen

Once they become immunocompetent they disperse to lymph tissue to await antigen encounters

Note: The initial stage of lymphocyte development does not require the presence of an antigen

Once the cells express mature TCR and BCR receptors further differentiation is antigen dependent 

B-cell development

Occurs in the bone marrow

Cytokine IL-7 stimulates the stem cell to differentiate into the B cell

T cell development

Occurs in the thymus

Immature T cells migrate continuously to the thymus from red bone marrow

In the thymus a selection process occurs that allows T cells that are not autoreactive to enter into systemic circulation 

Surviving T cells must be able to recognize

MHC molecules without being autoreactive

Peptides derived from foreign antigens

More than 95% of T cells die in the thymus because either:

They were autoreactive

They didn’t recognize MHC molecules

Positive and Negative selection

Positive selection - Allows T-cells that demonstrate an interaction with MHC complex and an unresponsiveness toward self-antigens to survive

Negative selection - Process by which T-cells capable of producing immune reactions against self-antigens are destroyed

Early in thymic education immature T cells express both CD₈ and CD₄ surface molecules

As T cells mature in the thymus expression of one of the CD molecules is lost and they will display either a CD₈+ surface protein or a CD₄+ surface protein

CD8+ cells may become Killer T cells and will interact with MHC-I molecules

CD4+ cells may become Helper T cells and will interact with MHC-II molecules

Helper T cells aid both the CMI and AMI responses

In 2 – 4 days newly immunocompetent naïve T cells leave the thymus

Antibody Mediated Immunity (Humoral Immunity)

Involves the B lymphocytes

Co-stimulated by type II (TH2) helper T cells which secrete IL - 4, IL - 5, and IL - 6

Effective against extracellular pathogens

Antibody Mediated Immunity

Refers to antibodies produced by the plasma cells and carried in the blood and lymph

B-cells do not destroy pathogens directly but instead produce plasma cells which secrete specific antibody proteins

These antibodies bind to bacteria, bacterial toxins and viruses in body fluids inactivating them and marking them for destruction

Via phagocytosis or complement activation which leads to cell lysis 

B-Cells

Each population of B cell produces a distinct antibody molecule (immunoglobulin) displayed on cell membrane

Millions of populations of B cells

Each population consists of thousands of B cells

All BCR on a given B cell have the same specificity

B - cells do not destroy pathogens directly, instead they produce plasma cells

Plasma cells produce and secrete specific antibodies

B-cell activation typically occurs in the germinal center of the lymph node

Sequence of events for B cell activation

1. Immunocompetent (naïve) B lymphocyte encounters invading antigen in lymphoid tissue

2. Antigen is presented to, or binds to, cell surface receptor on B-cell

Antigen binding is essential but insufficient to produce an effective response

This is the first signal needed for proliferation

A given B-cell will exhibit antigen-receptor molecules with a specificity for a particular antigen

Approximately 100,000 Ig on the cell surface membrane

3. Clustering or cross-linking of BCR with epitopes initiates an enzymatic chain reaction inside the cell that initiates genetic activation of the B cell

This activation is enhanced if complement receptors on the B cell surface simultaneously bind complement fragments on opsinized antigen

4. Antigen is taken into the B-cell

Receptor mediated endocytosis of antigen receptor complex

B cell receptor (BCR) IgM

5. Antigen is broken down into peptide fragments

Vesicle containing antigen fuses with lysosome

Antigen is broken down into fragments

Peptide fragments join with class II histocompatibility (MHC-II) molecule

6. Antigen is presented on the B-cell surface to Helper-T cells

The B – cell serves the APC function

Antigenic fragments are displayed on MHC – II markers

7. Helper-T (TH2) locks on to antigenic fragment presented on MHC-II

CD₄+ docks onto MHC-II

T-cell receptor (TCR) “recognizes” peptide (Ag) displayed on MHC-II

Coupling of the MHC-II with the TCR and the binding of CD40 ligand on the Th₂ cell with the CD40 on the B cell is the second signal needed for proliferation (The 1^st signal was clustering)

8. If the MHC-II; Ag; TCR is a match from the same source and there is complimentary CD40 and CD40L binding and up regulated B7 proteins on the B cell dock with CD 28 proteins on the Th2 cell than….

TH2 cell secrete cytokines:

IL – 4 ( B – cell differentiation factor )

IL – 5 ( B – cell stimulating factor )

IL – 6 ( B – cell growth factor )

9. Proliferation and differentiation

Proliferation: B-cells are stimulated to grow rapidly and divide into cloned cells identical to the original stimulated B-cell. Results in a population of B-cells with a high affinity and antigen specificity for the immunizing antigen

Differentiation: B-cells differentiate into plasma cells and memory B-cells 

Plasma cells

The effector cells of the humoral response

In about four days there are approx 500 plasma cells for each B-cell stimulated

Secrete antibodies which have the same structure (variable region) as the initially stimulated surface receptor

Antibodies bind with free antigen and mark them for destruction

Plasma cells are viable for only 6 -7 days

Memory B-cells

Activated B-cells cloned from the original B-cell

They do not differentiate into plasma cells

Can initiate an almost immediate humoral response if and when they encounter the same antigen

Upon 2^nd encounter they will divide and differentiate into plasma cells that secrete antibodies in massive amounts

Primary Response

Occurs when the body is first exposed to a particular antigen

From the time of antigen challenge to the production of antibodies by that cell is about 4 days

time required for the B-cell specific to the particular antigen to proliferate and differentiate

The body reaches peak antibody levels in about 10 days

On average serum titers peak in 7 – 14 days and then decline thereafter if antigen is no longer present 

Secondary Responses

Occurs on any subsequent exposure to the same antigen

Memory cells which are dispersed throughout the body provoke a faster, more prolonged response

Able to produce greater levels of antibodies with a greater affinity for the antigen

The cells have been primed and express surface proteins able to respond to cytokines released during early exposure to an antigen

They may not require the second signal needed for naïve B cell activation and thus respond quickly to epitopes

From the time of secondary antigen encounter to peak antibody levels is about 2 days and can stay raised for weeks

Active and Passive Humoral Immunity 

Active: Occurs after an exposure to an antigen; Stimulates own B-cells to produce antibodies

Natural: infection

Artificial: vaccination of dead or almost dead (attenuated) pathogens

Delayed effect but long duration; Can last for years

Passive: Produced by transfer of antibodies from an external source

Antibodies are given to person rather than being produced by plasma cells

Natural: placenta or mothers milk

Artificial: injection of gamma globulins (antibodies)

Immediate effect but short duration

ANTIBODIES

Immunoglobulins, or Igs.

Belong to a group of glycoproteins called gamma globins

These proteins are secreted by activated B cells or by the plasma cell offspring in response to an antigen

Igs combine specifically with the antigenic determinant that triggered their production

Function to neutralize bacterial toxins and viruses and to opsonize bacteria  

Antibody structure 

 Igs classification is based of the constant region of the heavy chain component

Variable regions: F_ab (antigen binding fragment)

The antigen binding sites of the antibody

Different for each specific antigenic determinate

Constant regions: F_c (crystallizable fragmnet)

Are nearly the same in all antibodies of the same class, but differs from one class of antibodies to another

Fc acts the ligand for the FcR on the phagocytic cells

Fc also allows for complement fixation 

Antibody Classes

Five major classes each playing a different role in the immune response

IgG: Most abundant: about 75% of all antibodies in the body

Monomer

Triggers the complement system

IgG₃ is most active in complement fixation

Only antibody to cross placenta

Produced by memory cells during secondary responses

23 day half-life

IgA: About 15% of all antibodies

Monomer and dimers

Found on external surfaces in tears, saliva, mucus, G.I. secretions, and colostrum

Provides localized protection on mucus membranes

Respiratory, enteric, and genitourinary

Levels decrease during stress, which decreases resistance to infection

6 day half-life  

IgM: 5-10% of all antibodies

Pentamers

BCR - Found on surface of naive B cells as antigen receptors; Involved in the activation of B cells

First antibodies to be secreted by the plasma cells after any initial exposure to an antigen (primary response)

Thus an increase in IgM in plasma indicates infection

Triggers the complement system

Can serve as B-cell receptor to activate B cells

ABO plasma agglutinins are IgM antibodies

5 day half- life 

IgD

IgD: Less than 1%

monomers

BCR: Found on surface of naive B cells as antigen receptors; Involved in the activation of B cells

3 day half-life

IgE: Less than 0.1%

monomers

F_c on mast cells and basophils

Involved in allergic reactions

2 day half-life 

Classes of antibodies

Different classes of antibodies from the same plasma cell all have the same antigen-binding site

Same variable region

Primary humoral response mainly involves IgD (BCR) & IgM (BCR and 1st released)

Secondary humoral response mainly involves IgG (most abundant), IgA & IgE

Antibody Function

o form antigen-antibody complexes.

Do not directly destroy invading microbes but rather tag them, inactivating them and marking them for destruction

a. Neutralization: Antibodies cover up surface proteins on pathogen so that it cannot bind and react with body tissue cells to cause pathology

b. Complement fixation: Antibody binding triggers complement fixation and activation along with subsequent cell lysis

c. Agglutination: Cell clumping of foreign cells

More than one microbe at a time can be neutralized by free binding sites on certain antibodies (IgM)

This results in cross linking and cell clumping.

d. Precipitation: Molecules instead of cells are cross-linked 

Major Histocompatibility Complex Antigen 

Also called Human Leukocyte Antigens (HLAs)

Everyone has unique MHC antigens (self antigens) which mark the surface of all body cells (Except RBC's)

There about one hundred thousand which mark each cell

Each MHC posses a distinct 3 dimensional shape with a central groove for antigen fragments

Their primary function appears to help T cells recognize foreign invaders

These self antigens are manufactured in the rough E.R. and then processed in the golgi apparatus before being secreted on the cell surface via exocytosis of their membrane bound vesicle

Proteins in body cells are continually being broken down via lysosomal activity and their amino acids recycled.

Some of these broken down products will join with the vesicles of newly synthesized MHC self antigens and these intracellular fragments will be presented on the cell membrane
 

Two classes: 

Class I MHC: glycoprotein on the plasma membrane of all body cells (except RBC's)

Present endogenous peptide fragments produced inside the cell to naïve CD8+ cells

Endogenous self peptides and foreign peptides, such as viruses, that are synthesized in body cells are cleaved by an enzyme called proteasome. These cytosolic proteins are degraded into smaller peptides

These small peptides enter the ER through TAP 1 and 2 channels

Transporter associated with antigen processing (TAP)

Peptides attach to the class 1 MHC groove

MHC-1 molecule and associated peptide are transported to plasma membrane

MHC I molecules display peptide fragments 8-9 amino acids long

Class II MHC: Glycoproteins on antigen presenting cells such as macrophages; B-cells; Dendtritic cells

Class II MHC only appear when the APC is processing exogenous antigens for presentation

Process: Phagolysome joins with vesicle containing MHC-II

Peptide fragments of 14 – 20 a.a. long bind to MHC-II groove after removal of invariant chain

Invariant chain prevents self peptides from binding with MHC-II in ER by blocking groove

Vesicle with MHC-II molecule travels to APC plasma membrane for presentation

Cellular immunity

Also called cell-mediated

T-cells act directly against the foreign cell by lyses, or indirectly by promoting the inflammatory response

via released chemicals

May also activate other lymphocytes and macrophages

Dendritic cells

Major function is to pinocytize foreign material and act as antigen presenters and to activate T-cells

Major function of macrophages is to phagocytize antigens and act as antigen presenters and to activate T-cells

Typically present to both T and B cells in interstitium

Activated B cells can also act as antigen presenting cells

Dendritic cells such as the Langerhan’s cells continually monitor the immediate environment via pinocytosis.

When activated by the processing of antigens and through the secretion of TNF by macrophages, or the LPS of gram negative bacteria they will up regulate the expression of MHC-II molecules and B7 proteins (CD80/86) on their cell surface. They will also begin migration into the surrounding lymphatic capillaries to present to the T and B cells in the lymph nodes

Cell Mediated Immunity

Involves the CD8+

Co- stimulated by helper T cells

Involves type I (TH₁) helper T cells which secrete IL-II

Effective against intracellular pathogens

CELL-MEDIATED IMMUNE RESPONSE

Begins with recognition of a particular antigen by a small number of T cells.

Once antigen is recognized and co stimulation occurs T cells undergo:

Proliferation: Divides several times into clone cells

Differentiation: Forms more highly specialized cells

In 7 days a single activated T_c cell can produce a few thousand Killer T cells

T-cells have a unique receptor (TCR) capable of binding with a specific antigen-MHC combination

T cells recognize fragments of antigenic proteins which have been processed and presented in association with MHC self-antigens

Cytotoxic T cells

Develop from the T-cells that display the CD8+ surface protein.

Directly attack infected host cells

Able to recognize foreign antigens bound with MHC-1 molecules on the surface of body cells infected by viruses, tumor cells or tissue transplants

To become a killer T-cell they need costimulation by IL-2 or other cytokines produced by the helper T-cells

thus maximal activation of killer T-cells requires presentation of antigen associated with both MHC-I and MHC-II molecules

After antigen recognition by the CD8 cell, IL-2 is the major cytokine responsible for proliferation of the antigen activated Tc cells

The primary source of IL-2 is activated T_H1 cells

Not only does IL-2 co-stimulate the antigen primed CD8 cells it also stimulates activation of the NK cells

Memory cells do not differentiate but if an antigen appears a second time they will immediately differentiate into cytotoxic T cells; Evoking a rapid and effective response

Cytotoxic T cell Function

Release cytotoxins and cytokines

Cytotoxins: Non-specific toxins that produce apoptosis in the target cell

These are released at the interface between the affect cell membrane and the killer T cell membrane

Two main types of cytokines released by the killer T

1. Perforin: Cytotoxic T-cell binds to target cell and releases granules of perforin via exocytosis.

Perforin fuses with the plasma membrane and perforates it allowing ions to flow in along their electrical chemical gradients

Results in cell lysis

2. Granzymes: enter through newly formed pores and induce apoptosis

Activates damaging agents in target cell causing DNA to fragment and the cell to die.

Cytokines

Molecules including interleukins and lymphokines that modulate the activities of other immune cells

Target cells must posses specific recetors to bind cytokines

Two important cytokines released by the activated killer T cell are:

TNF

IL-2: Acts as an autocrine to allow the proliferation of other activated CD8+ cells

Helper T cells

Naïve helper T lymphocytes are referred to as CD4+ or Th0

Develop from the T-cells displaying CD4 surface proteins

CD4+ are inactive T cells until first signal is given.

Activation of the Th cell requires two signals

The first signal is antigen recognition of peptides associated with MHC-II molecules on APC cells

This occurs as TCR binds to MHC –II along with CD4 coupling

The second signal is provided by a co-stimulatory binding of surface molecules

B7 (CD80/86) molecules on APC ligate the CD28 receptor on the Th0 (CD4+)

Note: The molecular interaction and activation of the Th cell results in up regulation of CD40L on the Th cell surface

These will be needed for B cell activation

The CD40L binding with the CD40 protein on the APC surface up-regulates the APCs expression of MHC-II molecules so that when the Th and APC disengage the APC can continue to present within the lymph node

Once co-stimulation signals are received the Th cell undergoes proliferation and differentiation

into helper, cytotoxic and memory T-cells

Once disengaged from the APC the activated Th will proliferate and differentiate into helper T and memory cells

The Th1 will secrete large amounts of IL-II which assist activation of the naïve Tc cell

Two types of T_H cells arise form the naïve CD4 (T_H0) cells

T_H1: Induced primarily by secretion of IL-12 released by the macrophage in response to bacteria

T_H1 secrete IL-2 and activate cell mediated immune responses

T_H2: Induced primarily by secretion of IL-4 released by the NK cell

T_H2 secrete IL-4,5,6 and activate antibody mediated immune responses

Helper T cell function 

Produce lymphokines which enhance humoral and cell-mediated responses and stimulate inflammatory response.

T_H1 cell secrete cytokine IL – 2 (CD8+ cell differentiation factor)

T_H2 cell secrete cytokines

IL – 4 ( B – cell differentiation factor )

Also stimulates the development of Th2 cells from CD4 cells

Enhances eosinophil activity and stimulates B-cells to produce IgE

IL – 5 ( B – cell stimulating factor )

IL – 6 ( B – cell growth factor )

Deactivation of Killer T cells

Killer T-cells undergo apoptosis

They express surface proteins that signal their own destruction

Suppressor T cells inhibit both the cell mediated and the humeral immune responses