Adaptive Immunity

Define adaptive immunity

The response of antigen-specific lymphocytes (B & T cells) to antigen, including the development of immunological memory

What are characteristics of adaptive immunity?

1. Acquired (not innate)

2. Highly specific - efficient

3. Increase recognition of pathogens by the innate immune system

4. Gradual response (generated over 3-4 days)

What are three advantages of the adaptive immune system?

1. Precise

2. Memory

3. Adapts

What are the 2 branches of the adaptive immune system?

Humoral (HI) and Cell Mediated (CMI)

What does the Humoral immune system target? What is it mediated by? Is it transferable?

Targets extracellular pathogens (bacteria), mediated by antibodies, transferable

What does the cell mediated immune system target? What is it mediated by? Is it transferable?

Targets intracellular pathogens (virus), mediated by cytotoxic T cells, not transferable

What cell is like a “keystone” that connect the innate and adaptive immune response?

CD4T cell

Define antigen. What is the most common type?

Any molecule, macromolecule, virus, particle, or cell that contains a structure recognized and bound by an Ig or TCR. Majority are proteins.

Define antigenic determinant (Epitope)

Portion of antigen recognized by antibody

How do B & T cells each recognize antigens? And do they bind specifically?

• B cells recognize by BCR or immunoglobulin (soluble form = Ab)

• T cells recognize by TCR (no soluble form)

• Both bind specifically (lock & key)

Describe the structure of immunoglobulins and BCR

• 2 heavy and 2 light chains

• Constant and variable region on each chain

• Some soluble, some with transmembrane region on B cells

• Antigen bind to variable region

Describe the structure of T-cell receptors

• Alpha and beta chain

• Constant and variable region on each chain

• Transmembrane region

• Antigen bind to variable region

Are antibodies specific?

Very

Define antibody repertoire

Total number of antibody specificities to an individual

Increased diversity of antibody specificities is due to what processes?

• Gene rearrangement (Somatic recombination)

• Junctional diversification

Define gene rearrangement (somatic recombination)

Enzyme catalyzed process of recombination

Define junctional diversification and the types of junctions in each chain

Junctional diversification: Gene arrangement with nucleotide insertions at the joint to produce a functional gene

1. Heavy & beta chain: V, D, & J

2. Light & alpha chain: V & J

What are the 4 simplified postulates of clonal selection?

1. Each cells has 1 receptor for 1 antigen (specificity)

2. Cell binds tightly to antigen = activation

3. Differentiated effector cells are exact copies (clonal expansion)

4. Cells that bind tightly to “self” are destroyed early on (clonal deletion)

What cells link innate and adaptive immunity? Why?

Dendritic cells carry antigen to lymphoid tissue and activate T-cells

Describe the steps of antigenic processing and presentation

1. Dendritic cell takes up pathogen for degradation

2. Pathogen is taken apart in dendritic cell

3. Pathogen proteins are unfolded and cut into small pieces

4. Peptides bind to MHC molecules and the complexes go to the cell surface

5. T-cell receptors bind to peptide:MHC complexes go to the cell surface

What does MHC stand for?

Major histocompatibility complex

What the two classes of MHC? What type of antigen do they present? On what cell types are they present?

1. MHC Class I

   1. Presents peptides from intracellular pathogen

   2. Present on all nucleated cells

2. MHC Class II

   1. Presents peptides from extracellular pathogens

   2. Present only on professional Antigen Presenting Cells (pAPCs)

T cells antigen recognition is assisted by internal and external co-receptors. What type of cell does each type of receptor activate? And what class of MHC receptor presents antigen to it?

• Intracellular infection: TCR on CD8 (Cytotoxic T cell) binds to APC MHC I

• Extracellular infection: TCR on CD4 (Helper T cell) binds to APC MHC II

Describe the differences in B and T cell antigen recognition

1. B-cell recognition

   1. Free antigen (Native)

   2. BCR - membrane bound antibody

   3. No antigen processing required

2. TCR recognition

   1. Processed antigen (Peptide fragment)

   2. TCR - T cell receptor

   3. Must be presented on MHC molecule

      1. CD8 → MHC I (Intracellular)

      2. CD4 → MHC II (Extracellular)

Name the 3 pAPCs

1. Macrophages

2. Dendritic cells

3. B-cells

Describe the steps of basic activation of B cells

1. B cells bind bacteria, cells engulf and degrade them, producing peptides

2. Peptides bound by MHC II in endocytic vesicle

3. Bound peptide MHC II complex is transported to cell surface

4. Helper T cell recognizes complex and activates B cell

What are the 2 major functions of antibodies?

• Neutralization: Prevents pathogen growth, replication or interaction with host cells

• Opsonization: Aid in engulfment and destruction by phagocytosis

What are the isotypes of antibodies?

1. IgA

2. IgD

3. IgE

4. IgG

5. IgM

Describe the benefits of somatic recombination, junctional diversification, isotype switching, and somatic hyper mutation

• Somatic recombination: increases variety of antibodies

• Junctional diversification: increases variety of antibodies

• Isotype switching: improves specialization of function and ability to recruit effectors

• Somatic hypermutation: improves antigen binding by the antibody