*Development of the Adaptive Immune

What are the 3 main types of lymphocytes + function

B-cells — produces antibodies

T-cells — activates B-cells (helper T-cells) or kills infected cells (cytotoxic T-cells)

NK cells — part of innate immunity

What percentage of WBC are lymphocytes?

20-40% of WBC population

Where to lymphocytes perform immune surveillance?

Lymph nodes and other secondary immune organs (full of lymphocytes)

What do TCRs do?

They bind to antigens presented by MHC.

What is the structure of a TCR?

2 chains: α-chain + β-chain

Each chain has a variable region (for antigen specificity) and a constant region (to anchor into plasma membrane)

What does CD3 do in the TCR complex?

Transmits the activation signal (for immune response) into the T-cell when TCR recognises and binds to an antigen.

→ present on ALL T-cells

How do BCRs differ from TCRs?

BCRs bind to free-floating antigens that are not already processed. T-cells can ONLY bind to antigens presented by MHC.

Consists of a light chain and heavy chain. 

What is unique about each lymphocytes receptor?

Each lymphocyte receptor will only recognise one specific antigen/epitope.

What co-receptor do helper T-cells use and what do they bind to?

Helper T-cells with CD-4 bind to MHC-II.

What co-receptor do cytotoxic T-cells use and what does it bind to?

Cytotoxic T-cells with CD8 bind to MHC-I.

How does a T-cell know its binding to an APC?

Co-receptor binds to MHC first, verifying cell type, before TCR tests specific epitiope.

List the 5 steps of antigen presentation and T-cell activation

1. APC with MHC-II presents the antigen

2. Helper T-cell uses CD4 to bind to MHC-II, interacting with the correct cell

3. TCR tests epitope

4. If epitope is correct, adaptive immune response initiated

5. T-helper differentiates based on cytokines secreted by APC

Where does antigen presentation and T-cell activation primarily occur?

Secondary lymphoid organs (primarily) and blood

What is an immunological synapse?

When TCR recognises and bind the antigen, it forms a synapse.

A sustained physical interaction between APC and T-cell.

→ cytokines released by the APC drive T-cell differentiation

What are lymphokines?

Cytokines secreted by lymphocytes.

→ lymphocyte activation, differentiation, function

What is the function of IL-2?

Secreted by T-helper cells that stimulates itself and other T-cells to proliferate (divide and multiply)

What do IL-4, IL-6, IL-10, TNF-β all do?

Cytokines that collectively coordinate and regulate the activation of B-cells.

What does CD stand for and what are CD proteins typically involved in?

“Clusters of differentiation”

Cell surface receptors typically involved in:

• leukocyte adhesion

• cytokine response

• chemotaxis

What does CD3 do?

Part of the T-cell receptor complex that allows the cell to recognize and epitope and transmits the activation signal into the T-cell.

→ present on ALL T-cells

What does CD4 do?

• Defines T-helper lymphocytes.

• Binds to MHC-II

• Required to initiate antibody response

• Helps coordinate adaptive immunity

What does CD8 do?

• Defines cytotoxic T-lymphocytes

• Binds to MHC-I

• Performs surveillance of nucleated cells and kills infected cells

How can CD proteins be used to differentiate T-cell subtypes?

By labelling CD proteins with monoclonal antibodies to identify lymphocyte subpopulations. (Using labels)

What is CD34, why is it clinically relevant?

Hematopoietic stem cell marker. Used to identify and isolate stem cells.

What are CD19 and CD20, whats their clinical relevance?

Early lymphocyte markers (B-cell markers)

Relevant for leukemia therapy as treatments target cells with these markers.

What is CD2? What is it used for in the Erythrocyte Rosette Assay?

CD2 receptor on T-cells binds to LFA-3 (mimics MHC/adhesion proteins)

In the assay, SRBCs (with LFA-3) bind to T-cells, forming rosette structures for T-cell isolation.

Where do B and T lymphocytes originate?

Both develope in the bone marrow from hematopoietic stem cells.

What unique process occurs in lymphoblasts to generate receptor diversity?

• Genetic recombination (VDJ recombination)

• Unique to lymphocytes and is stochastic (random)

• Each T/B cell develops a unique receptor to a specific unique epitope.

Where are autoreactive lymphocytes eliminated?

B-cells — eliminated in BM stroma

T-cells — eliminated in thymus

What are colony stimulating factors (CSFs)

A specific type of hematopoietic GF that stimulate the proliferation of specific leukocytes.

Give examples of CSFs and what they stimulate

GM-CSF — stimulates the production of granulocytes and macrophages

G-CSF — promotes production of neutrophils

What is the clinical significance of CSFs?

1. Can be tested when diagnosing immunodeficiencies

2. Can be used as drugs to repopulate WBCs of cancer patients following radiation and chemotherapy

What is the ‘immune repertoire’?

The comination of all epitopes T and B cells can recognize.

Diversity is built in early during development of T and B cells.

how is immune repertoire diversity generated?

Lymphoid progenitors use DNA recombination to give rise to a huge assortment of lymphocytes with specific receptors.

What does V(D)J recombination consist of?

Up to 3 gene segments:

V → variable

D → diversity

J → joining

Each segment is copied many times with different sequences.

How does VDJ recombination work?

One copy of each segment is selected by each cell during B-cell and T-cell development. Remaining copies are deleted - once selection is done, no turning back.

How many possible epitopes can be detected through VDJ recombination?

300 billion possible epitopes through mixing and matching gene segments.

Where does T-cell receptor recombination take place?

thymus

What are thymocytes?

Lymphoid progenitors that migrate to the thymus to become mature T-cells

Name 3 thymic peptide hormones involved in T-cell differentiation

Thymulin, thymosin, thymopoietin

What is thymic education?

The process by which thymocutes differentiate into mature T-cells in the thymus under the influence of thymic peptide hormones. T-cells that fail thymic education are killed.

What % of thymocytes survive thymic education?

Vast majority DO NOT survive — only ~2% make it.

What are double negative thymocytes?

Thymocytes taht express CD3 but no CD4 or CD8

What are double positive thymocytes?

Thymocytes that express CD3, CD4, AND CD8 and they move through the thymus.

What are the 4 steps of thymic education?

1. Thymocytes arrive via circulation

2. Migrate to edge and are successively tested

3. Tested by cortival epithelial cells (+ selection) → commits to lineage

4. Tested by dendritic nurse cells for autoreactivity (- selection)

What is positive selection in the thymus?

Thymocytes must bind to either MHC-I or MHC-II of cortical epithelial cells.

No binding = apoptosis

Leads to lineage commitment (supresses either CD4 or CD8)

What is negative selection in the thymus?

Dendritic nurse cells test for autoreactivity to self-antigen.

Autoreactive T-cells are phagocytosed and eliminated to prevent autoimmunity.

What determines whether a double positive thymocyte becomes a helper or cytotoxic?

Which MHC they bind to during positive selection.

What are the CD markers for a mature helper T-cell?

CD3⁺, CD4⁺, CD8^-

What are the CD markers for a mature cytotoxic T-cell?

CD3⁺, CD4^-, CD8⁺

What are the CD markers for a double negative thymocyte?

CD3⁺, CD4^-, CD8^-

What are the CD markers for a double positive thymocyte?

CD3⁺, CD4⁺, CD8⁺

How does the StemSep technology work for T-cell selection?

Magnetically labeled T-cells are retained by a magnet while other cells flow through.

If selecting for T-cells in general, which receptor would antibody bind to?

CD3 — present in ALL T-cells