Adaptive Immunity 1 Flashcards

Question

_Comparison of B cell and T cell development_ * B cells are produced throughout life in the bone marrow (-50million new B cells every day) - 5 days life expectancy * T cells are produced in the thymus which involutes at the end of ... - but thymus in adults does have some residiual ... tissue and new T cells are also generated in extrathymic sites (liver and intestine) - also very long lived - 4x10(^11) circulating T cells * Both have diverse repertoires of antigen receptors - generated by genetic rearrangements

Answer 1

* B cells are produced throughout life in the bone marrow (-50million new B cells every day) * T cells are produced in the thymus which involutes at the end of **puberty** - but thymus in adults does have some residiual **corticomedullary** tissue and new T cells are also generated in extrathymic sites (liver and intestine) - also very long lived - 4x10(^11) circulating T cells * Both have diverse repertoires of antigen receptors - generated by genetic rearrangements

Answer 2

* B cells are produced throughout life in the bone marrow (-50million new B cells every day) * T cells are produced in the thymus which involutes at the end of puberty - but thymus in adults does have some residiual corticomedullary tissue and new T cells are also generated in extrathymic sites **(liver and intestine)** - also very long lived - 4x10(^11) circulating T cells * Both have diverse repertoires of antigen receptors - generated by genetic rearrangements

Answer 3

* B cells are produced throughout life in the bone marrow (-50million new B cells every day) * T cells are produced in the thymus which involutes at the end of puberty - but thymus in adults does have some residiual corticomedullary tissue and new T cells are also generated in extrathymic sites (liver and intestine) - also very long lived - 4x10(^11) circulating T cells * Both have diverse repertoires of **antigen receptors** - generated by genetic rearrangements

Answer 4

* These cells, just like T cells, originate from a common lymphoid progenitor in the bone marrow. The first evidence for commitment to becoming a B cell is the emergence of a **pro-B cell,** which has undergone genetic rearrangement of its **diversity** (D) and **joining** (J) segments. * There then follows differentiation to pre-B cells (large then small) and the first expression of a pre B cell receptor on the cell surface. Immature B cells then undergo a process of positive and negative selection – only those B cells that are positively selected and avoid negative selection are finally released into the peripheral blood. * Failure at either of these steps leads to apoptosis or programmed cell death.

Answer 5

* These cells, just like T cells, originate from a common lymphoid progenitor in the bone marrow. The first evidence for commitment to becoming a B cell is the emergence of a pro-B cell, which has undergone genetic rearrangement of its diversity (D) and joining (J) segments. * There then follows differentiation to **pre**-B cells (large then small) and the first expression of a **pre** B cell receptor on the cell surface. Immature B cells then undergo a process of positive and negative selection – only those B cells that are positively selected and avoid negative selection are finally released into the peripheral blood. * Failure at either of these steps leads to apoptosis or programmed cell death.

Answer 6

* These cells, just like T cells, originate from a common lymphoid progenitor in the bone marrow. The first evidence for commitment to becoming a B cell is the emergence of a pro-B cell, which has undergone genetic rearrangement of its diversity (D) and joining (J) segments. * There then follows differentiation to pre-B cells (large then small) and the first expression of a pre B cell receptor on the cell surface. Immature B cells then undergo a process of **positive** and **negative** selection – only those B cells that are **positively** selected and avoid **negative** selection are finally released into the peripheral blood. * Failure at either of these steps leads to apoptosis or programmed cell death.

Answer 7

* These cells, just like T cells, originate from a common lymphoid progenitor in the bone marrow. The first evidence for commitment to becoming a B cell is the emergence of a pro-B cell, which has undergone genetic rearrangement of its diversity (D) and joining (J) segments. * There then follows differentiation to pre-B cells (large then small) and the first expression of a pre B cell receptor on the cell surface. Immature B cells then undergo a process of positive and negative selection – only those B cells that are positively selected and avoid negative selection are finally released into the peripheral blood. * Failure at either of these steps leads to **apoptosis** or **programmed** cell death.

Answer 8

* For T cells, the T cell precursors migrate to the thymus where they undergo four different stages of **DN** differentiation. * **DN** stands for **double negative** and this signifies that these cells express neither CD4 nor CD8. DN4 cells express a pre T cell receptor and then transiently go through an immature single positive phase before becoming double positive for CD4 and CD8. * These cells, like immature B cells, undergo both positive and negative selection before being released into the peripheral blood as either CD4 or CD8 T cells.

Answer 9

* For T cells, the T cell precursors migrate to the thymus where they undergo four different stages of DN differentiation. * DN stands for double negative and this signifies that these cells express neither **CD4** nor **CD8**. DN4 cells express a pre T cell receptor and then transiently go through an immature single positive phase before becoming double positive for **CD4** and **CD8**. * These cells, like immature B cells, undergo both positive and negative selection before being released into the peripheral blood as either **CD4** or **CD8** T cells.

Answer 10

* For T cells, the T cell precursors migrate to the thymus where they undergo four different stages of DN differentiation. * DN stands for double negative and this signifies that these cells express neither CD4 nor CD8. **DN4** cells express a pre T cell receptor and then transiently go through an immature single positive phase before becoming double positive for CD4 and CD8. * These cells, like immature B cells, undergo both positive and negative selection before being released into the peripheral blood as either CD4 or CD8 T cells.

Answer 11

* For T cells, the T cell precursors migrate to the thymus where they undergo four different stages of DN differentiation. * DN stands for double negative and this signifies that these cells express neither CD4 nor CD8. DN4 cells express a pre T cell receptor and then transiently go through an immature single positive phase before becoming double positive for CD4 and CD8. * These cells, like immature B cells, undergo both **positive** and **negative** selection before being released into the peripheral blood as either CD4 or CD8 T cells.

Answer 12

* We can summarise the common stages of B and T cell development into three separate phases: * 1st phase is the generation of a unique antigen receptor through genetic rearrangement of the V (**D**) and **J** segments. * The 2nd phase involves the **refinement** of the antigen receptor repertoire. The receptor is first challenged with self antigen and only those cells that recognize “self” are selected for further development. (Positive selection). Those receptors that bind too strongly to self antigen are then deleted (negative selection). So, how much self recognition is too much? You can imagine that release of a cell that recognizes self a little too much may be the basis of autoimmune problems. * The 3rd phase involves encounter with foreign antigen. This phase takes place in the secondary lymphoid tissues (lymph nodes or spleen) and results in clonal selection and expansion – ultimately leading to the development of effector and memory lymphocytes.

Answer 13

* We can summarise the common stages of B and T cell development into three separate phases: * 1st phase is the generation of a unique antigen receptor through genetic rearrangement of the V (D) and J segments. * The 2nd phase involves the refinement of the antigen receptor repertoire. The receptor is first challenged with self antigen and only those cells that recognize “self” are selected for further development. (**Positive** selection). Those receptors that bind too strongly to self antigen are then deleted (**negative** selection). So, how much self recognition is too much? You can imagine that release of a cell that recognizes self a little too much may be the basis of autoimmune problems. * The 3rd phase involves encounter with foreign antigen. This phase takes place in the **secondary** lymphoid tissues (lymph nodes or spleen) and results in clonal selection and expansion – ultimately leading to the development of effector and memory lymphocytes.

Answer 14

* 1st phase: generation of an **antigen** receptor * V(**D**) **J** gene rearrangement – producing a new **antigen** receptor

Answer 15

* 1st phase: generation of an antigen receptor * V(D) J gene **rearrangement** – producing a new antigen receptor

Answer 16

* 2nd phase: **refinement** of the antigen receptor repertoire * Antigen receptor is tested for antigen **recognition** (self antigens) * Only those receptors that recognise self antigen are selected * POSITIVE SELECTION * Those receptors which bind strongly to self antigen are deleted * NEGATIVE SELECTION

Answer 17

* 2nd phase: refinement of the antigen receptor repertoire * Antigen receptor is tested for antigen recognition (self antigens) * Only those receptors that recognise self antigen are selected * **POSITIVE** SELECTION * Those receptors which bind strongly to self antigen are deleted * **NEGATIVE** SELECTION

Answer 18

* 3rd phase: Stimulation by **foreign** **antigen** * Clonal selection of lymphocytes * Generation of effector and memory lymphocytes

Answer 19

* 3rd phase: Stimulation by foreign antigen * **Clonal** selection of lymphocytes * Generation of **effector** and **memory** lymphocytes

Answer 20

Only those receptors that recognise self antigen are selected

Answer 21

Those receptors which bind strongly to self antigen are deleted

Answer 22

* B cells and T cells are the lymphocyte components of the adaptive immune system * Their critical function is to produce immunological **memory** * B cells and T cells begin life in the bone marrow – T cells complete their development in the **thymus** * All lymphocyte development is guided by interactions with **stromal** cells * All undergo both **positive** and **negative** selection * There are many subsets of T cells but the most important are CD4+ **Thelper** and CD8+ **Tcytotoxic**

Answer 23

how to identify B cells and T cells using immunophenotyping; **B** cells express CD19, CD20, **T** cells express CD3 and either CD4 (T helper) or CD8 (cytotoxic T cell)

Answer 24

* Both cell types undergo positive and negative selection; **positive** selection identifies cells that can bind antigen (in the case of T cells, MHC-bound peptide) and signal through their receptor (B cell receptor or T cell receptor) – the signalling promotes their survival. **Negative** selection involves the binding of self-antigen to the B cell receptor or T cell receptor, which results in deletion of the cell by apoptosis·

Answer 25

* Both cell types undergo positive and negative selection; positive selection identifies cells that can bind antigen (in the case of T cells, MHC-bound peptide) and signal through their receptor (B cell receptor or T cell receptor) – the signalling promotes their survival. Negative selection involves the binding of self-antigen to the B cell receptor or T cell receptor, which results in deletion of the cell by apoptosis·

Answer 26

* B cells, unlike T cells, can recognize native antigen and in some instances can produce a full immunological response against the pathogen without the need for T cell help. However, for the majority of antigens, B cells need to interact with T cells in order to become fully activated. * The two different classes of antigen therefore are described as ’**thymus** independent’ and ‘**thymus** dependent’. * **T**-independent antigens are classically polysaccharides, lipids or nucleic acids and are characterised by repeating motifs, which allow for B cell receptor cross-linking. * The majority of antigens are proteins and are **T**-dependent antigens as they do not contain repeating epitopes.

Answer 27

* For the majority of antigens, **B** cells need to interact with **T** cells in order to become fully activated. * The two different classes of antigen therefore are described as ’thymus independent’ and ‘thymus dependent

Answer 28

T-**independent** antigens are classically polysaccharides, lipids or nucleic acids and are characterised by repeating motifs, which allow for B cell receptor cross-linking.

Answer 29

T-independent antigens are classically **polysaccharides**, lipids or nucleic acids and are characterised by repeating motifs, which allow for B cell receptor cross-linking.

Answer 30

The majority of antigens are proteins and are T-**dependent** antigens as they do not contain repeating epitopes.

Answer 31

Thymus-independent antigen is an immunogen that can stimulate B cells to synthesize antibodies without participation by T cells.

Answer 32

An antigen that requires the presence of T cell help to stimulate the B cell to secrete antibody

Answer 33

* Simple, repetitive antigens * Mostly **IgM** (no class switching) * Modest **affinity** * No memory (plasma cells are short lived) * B cells activated by direct BCR crosslinking * B cells still require a second activation signal – often this is via Toll-like receptor (TLR) engagement

Answer 34

* Simple, repetitive antigens * Mostly IgM (no class switching) * Modest affinity * No **memory** (plasma cells are short lived) * B cells activated by direct BCR **crosslinking** * B cells still require a second activation signal – often this is via Toll-like receptor (TLR) engagement

Answer 35

* Simple, repetitive antigens * Mostly IgM (no class switching) * Modest affinity * No memory (plasma cells are short lived) * B cells activated by direct BCR crosslinking * B cells still require a **second** activation signal – often this is via **Toll-like receptor** (TLR) engagement

Answer 36

* Simple, repetitive antigens * Mostly IgM (no class switching) * Modest affinity * No memory (plasma cells are short lived) * B cells activated by direct BCR crosslinking * B cells still require a second activation signal – often this is via Toll-like receptor (TLR) engagement

Answer 37

Naïve B cells express both **IgM** and **IgD** on their surface - *Following activation with T-independent antigens, these B cell most commonly secrete IgM antibodies. They are not capable of class switching to produce different immunoglobulin isotypes as this requires T cell help.*

Answer 38

Naïve B cells express both IgM and IgD on their surface. Following activation with T-independent antigens, these B cell most commonly secrete **IgM** antibodies. They are not capable of **class** switching to produce different immunoglobulin isotypes as this requires T cell help.

Answer 39

Its important to note that although T-**independent** antigens can drive a B cell response. This response is short lived and does not produce immunological **memory**.

Answer 40

* B and T cells generate a diverse array of antigen receptors through the process of **V(D)J** recombination * The B cell receptor is made up of two heavy chains and two light chains making the classic “Y shaped” antigen receptor expressed on their surface. This B cell receptor can also be secreted as **antibody**. * The T cell receptor is made up of an alpha chain and a beta chain heterodimer. * Antigen engagement of both the BCR and TCR results in cell signaling, which is regulated by the B cell co-receptor complex and the CD3 complex respectively. * B cells can recognise native antigens, but T cells can only bind processed peptide antigens presented to them in the context of MHC molecules.

Answer 41

* B and T cells generate a diverse array of antigen receptors through the process of V(D)J recombination * The B cell receptor is made up of two heavy chains and two light chains making the classic “Y shaped” antigen receptor expressed on their surface. This B cell receptor can also be secreted as antibody. * The T cell receptor is made up of an alpha chain and a beta chain heterodimer. * Antigen engagement of both the BCR and TCR results in cell signaling, which is regulated by the B cell co-receptor complex and the **CD3** complex respectively. * B cells can recognise native antigens, but T cells can only bind processed peptide antigens presented to them in the context of **MHC** molecules.

Answer 42

* B cells can bind intact **protein** antigen in solution * T cells bind **peptides** displayed on the surface of another cell - an “antigen presenting cell” (dendritic cell, macrophage, or B cell) - in the context of **MHC** I or **MHC** II molecules

Answer 43

B and T cells generate a diverse array of antigen receptors through the process of V(D)J **recombination.**

Answer 44

The **B** cell receptor is made up of two heavy chains and two light chains making the classic “Y shaped” antigen receptor expressed on their surface

Answer 45

The **T** cell receptor is made up of an alpha chain and a beta chain heterodimer.

Answer 46

* Both B and T cells signal by associating with signaling complex in membrane: * Ig-a and Ig-b for B cells; **CD3** complex for T cells

Answer 47

* Both B and T cells signal by associating with signaling complex in membrane: * **Ig-a and Ig-b** for B cells; CD3 complex for T cells

Answer 48

* T-independent B cell activation follows a **2-signal** activation model. * T-dependent B cell activation also requires multiple signals – typically this would involve **three** cognate ligand/receptor interactions.

Answer 49

* T-independent B cell activation follows a 2-signal activation model. * T-dependent B cell activation also requires multiple signals – typically this would involve three cognate ligand/receptor interactions.

Answer 50

* For the vast majority of antigens, B cells need T cell help in order to produce an effective immune response. * B cells and T cells have evolved an array of reciprocal ligand/receptor expression which allows them to interact with each other. * Some of the most important are **CD40** and **CD40ligand**; CD80/CD86 and CD28.

Answer 51

antibody response to complex antigens - proteins/peptides (requires direct, physical B-T interaction)

Answer 52

Both B and T cells recognise antigen but not usually the same **epitope -** *B cells can recognise native antigen whereas T cells can only “see” processed antigen, which is presented to them in the context of MHC*

Answer 53

Both B and T cells need signal 1 (through antigen **receptor**) and signal 2 (co-**stimulation**)

Answer 54

* T-dependent B cell responses requires a three-signal process: * Sequence of events: * Antigen binding to **BCR** provides “Signal 1” to B cell * Antigen is internalised, processed and antigenic peptides are displayed on MHC for T cell recognition * T**H** (**helper** T-cell) recognises antigen-MHC complex via the T cell antigen receptor (TCR): provides “Signal 1” to T cell * CD80/CD86 on B cell binding to CD28 on T cell provides “Signal 2” to T cell * T cell activation leads to up-regulation of CD40L which bind to CD40 providing “Signal 2” to B cell * Cytokine production by activated T cell also help to activate B cell…“Signal 3” * B cell proliferates and differentiates into antibody secreting B cell (plasma cell) or becomes a memory B cell

Answer 55

* T-dependent B cell responses requires a three-signal process: * Sequence of events: * Antigen binding to BCR provides “Signal 1” to B cell * Antigen is internalised, processed and antigenic peptides are displayed on MHC for T cell recognition * TH (helper T-cell) recognises antigen-MHC complex via the T cell antigen receptor (TCR): provides “Signal 1” to T cell * **CD80/CD86** on B cell binding to CD28 on T cell provides “Signal 2” to T cell * T cell activation leads to up-regulation of CD40L which bind to CD40 providing “Signal 2” to B cell * **Cytokine** production by activated T cell also help to activate B cell…“Signal 3” * B cell proliferates and differentiates into antibody secreting B cell (plasma cell) or becomes a memory B cell

Answer 56

* T-dependent B cell responses requires a three-signal process: * Sequence of events: * Antigen binding to BCR provides “Signal 1” to B cell * Antigen is internalised, processed and antigenic peptides are displayed on MHC for T cell recognition * TH (helper T-cell) recognises antigen-MHC complex via the T cell antigen receptor (TCR): provides “Signal 1” to T cell * CD80/CD86 on B cell binding to CD28 on T cell provides “Signal 2” to T cell * T cell activation leads to up-regulation of **CD40L** which bind to **CD40** providing “Signal 2” to B cell * Cytokine production by activated T cell also help to activate B cell…“Signal 3” * B cell **proliferates** and differentiates into antibody secreting B cell (plasma cell) or becomes a memory B cell

Answer 57

* T-dependent B cell responses requires a three-signal process: * Sequence of events: * Antigen binding to BCR provides “Signal 1” to B cell * Antigen is internalised, processed and antigenic peptides are displayed on MHC for T cell recognition * TH (helper T-cell) recognises antigen-MHC complex via the T cell antigen receptor (TCR): provides “Signal 1” to T cell * CD80/CD86 on B cell binding to **CD28** on T cell provides “Signal 2” to T cell * T cell activation leads to up-regulation of CD40L which bind to CD40 providing “Signal 2” to B cell * Cytokine production by activated T cell also help to activate B cell…“Signal 3” * B cell proliferates and **differentiates** into antibody secreting B cell (plasma cell) or becomes a **memory** B cell

Answer 58

After signal **3** (of T-dependent B cell activation), the B cell will then proliferate and undergo a process called somatic hypermutation to “improve” the affinity of the B cell receptor for the antigen. Ultimately this leads to the production of immunoglobulin-secreting plasma cells and memory B cells.

Answer 59

After signal 3 (of T-dependent B cell activation), the B cell will then proliferate and undergo a process called somatic **hypermutation** to “improve” the affinity of the B cell receptor for the antigen. Ultimately this leads to the production of immunoglobulin-secreting plasma cells and memory B cells.

Answer 60

T-dependent responses require antigen presentation in the context of **MHC** molecules and T cell receptor recognition. The activation is reinforced by a second signal following the binding of CD40L (on T cells) with CD40 (on B cells); the binding of co-stimulatory molecules (CD80 and CD86 on B cells; the mouse equivalent is B7.1 and B7.2) and CD28 (on T cells) provides a second activation signal to the T cell. T cell secreted cytokines (e.g. IL-4) provide the third activation signal to the B cells.

Answer 61

T-dependent responses require antigen presentation in the context of MHC molecules and T cell receptor recognition. The activation is reinforced by a second signal following the binding of **CD40L** (on T cells) with **CD40** (on B cells); the binding of co-stimulatory molecules (CD80 and CD86 on B cells; the mouse equivalent is B7.1 and B7.2) and CD28 (on T cells) provides a second activation signal to the T cell. T cell secreted cytokines (e.g. IL-4) provide the third activation signal to the B cells.

Answer 62

T-dependent responses require antigen presentation in the context of MHC molecules and T cell receptor recognition. The activation is reinforced by a second signal following the binding of CD40L (on T cells) with CD40 (on B cells); the binding of co-stimulatory molecules (**CD80 and CD86** on B cells; the mouse equivalent is B7.1 and B7.2) and CD28 (on T cells) provides a second activation signal to the T cell. T cell secreted cytokines (e.g. IL-4) provide the third activation signal to the B cells.

Answer 63

T-dependent responses require antigen presentation in the context of MHC molecules and T cell receptor recognition. The activation is reinforced by a second signal following the binding of CD40L (on T cells) with CD40 (on B cells); the binding of co-stimulatory molecules (CD80 and CD86 on B cells; the mouse equivalent is B7.1 and B7.2) and **CD28** (on T cells) provides a second activation signal to the T cell. T cell secreted cytokines (e.g. IL-4) provide the third activation signal to the B cells.

Answer 64

Antigenic challenge of the adaptive immune system results in immunological **memory**. This is important as it allows faster and more effective responses to repeat exposure to antigen.

Answer 65

CD4 T cells recognise **MHC II** and CD8 T cells recognise **MHC I.**

Answer 66

Most antigens recognised by B cells (and other antigen presenting cells) require T cell help in order to generate an immune response. Some antigens (e.g. bacterial **polysaccharides**) can trigger B cell receptor **cross** linking and the production of antibodies (usually non-class switched IgM) without T cell help (T-independent).

Answer 67

Most antigens recognised by B cells (and other antigen presenting cells) require T cell help in order to generate an immune response. Some antigens (e.g. bacterial polysaccharides) can trigger B cell receptor cross linking and the production of antibodies (usually non-class switched **IgM**) without T cell help (T-independent).

Answer 68

B cells take up antigen through the **specific binding** to their B cell receptor. Other antigen presenting cells are capable of recognising a large range of pathogens via their **pattern recognition** receptors (e.g. TLRs, CLRs, NLRs, RLRs). In all cases, the bound receptor is internalised through the process of endocytosis and the endosomes (phagosomes) then bind to lysosomes, which create a highly acidic environment resulting in proteolytic degradation of the pathogen. The peptides are subsequently complexed with MHC II molecules and trafficked to the cell surface where they are presented to CD4 T cells. The recognition of the MHC II-bound peptide by a CD4 T cell results in a cascade of events leading to full activation of both the presenting B cell and the T cell.

Answer 69

B cells take up antigen through the specific binding to their B cell receptor. Other antigen presenting cells are capable of recognising a large range of pathogens via their pattern recognition receptors (e.g. TLRs, CLRs, NLRs, RLRs). In all cases, the bound receptor is internalised through the process of **endocytosis** and the endosomes (phagosomes) then bind to lysosomes, which create a highly acidic environment resulting in proteolytic degradation of the pathogen. The peptides are subsequently complexed with MHC II molecules and trafficked to the cell surface where they are presented to **CD4** T cells. The recognition of the MHC II-bound peptide by a **CD4** T cell results in a cascade of events leading to full activation of both the presenting B cell and the T cell.

Answer 70

The development of immunological memory in a population leads to the concept of **herd immunity** and this slows or even prevents the spread of infectious agents. However, for **herd immunity** to be effective \>60% of the population needs to have been infected and recovered from the pathogen.

Answer 71

For herd immunity to be effective \>**60**% of the population needs to have been infected and recovered from the pathogen.

Answer 72

* Some B cells are capable of responding to antigen without T cell help – most are not * T-independent antigens usually contain repeating epitopes classically polysaccharides, glycolipids and nucleic acids * Cross-linking of the B cell receptors is the first signal for activation, but a second signal is required e.g. from toll-like receptor engagement * T-dependent antigens are usually proteins/peptides * This type of T-dependent activation requires 3 signals – B cell receptor engagement (signal 1), Binding of CD40L to CD40 (signal 2) and cytokine stimulation (signal 3)

Answer 73

* Some B cells are capable of responding to antigen without T cell help – most are not * T-independent antigens usually contain repeating **epitopes** classically polysaccharides, glycolipids and nucleic acids * **Cross**-**linking** of the B cell receptors is the first signal for activation, but a second signal is required e.g. from **toll**-like receptor engagement * T-dependent antigens are usually **proteins**/**peptides** * This type of T-dependent activation requires 3 signals – B cell receptor engagement (signal 1), Binding of **CD40L** to CD40 (signal 2) and **cytokine** stimulation (signal 3)

Answer 74

* Some B cells are capable of responding to antigen without T cell help – most are not * T-independent antigens usually contain repeating **epitopes** classically polysaccharides, glycolipids and nucleic acids * **Cross**-**linking** of the B cell receptors is the first signal for activation, but a second signal is required e.g. from **toll**-like receptor engagement * T-dependent antigens are usually **proteins**/**peptides** * This type of T-dependent activation requires 3 signals – B cell receptor engagement (signal 1), Binding of **CD40L** to CD40 (signal 2) and **cytokine** stimulation (signal 3)