Immunology CH 9: B Cell Activation

where do B2 cells sample antigens?

in the B cell areas of secondary lymphoid tissues called lymphoid follicles

Where are follicular dendritic cells located?

in B cell areas of secondary lymphoid tissue

What are follicular dendritic cells function?

To present antigen to B cells

What is a naive B cell?

a B cell that has not yet become activated

What is a plasma cell?

An activated B cell that secretes antibodies

What is signal 1 needed for B cell activation?

The antigen has to bind to the B cell receptor

What is signal 2 for B cell activation?

C3d has to bind to the B cell co-receptor

What is signal 3 for B cell activation?

2 signals from conjugate Tfh cell

What is signal 1 of 2 that conjugate Tfh cells give to B cells in the process of being activated?

the CD40L from the T cell binds to the CD40 receptor of the B cell

What is signal 2 of 2 that conjugate Tfh cells give to B cells in the process of being activated?

T cell cytokines bind to the B cell’s receptors for those cytokines

What are the T cell cytokines required for in terms of B cells?

B cell activation, proliferation, somatic hypermutation, and class switching.

What type of T cells provide activating signals for B cells?

T follicular helper cells (Tfh)

What is linked recognition?

The idea that B cells and Tfh cells recognize different antigens from the same pathogen

What type of antigens do B cells recognize and what do they do with them ? (think about this in terms of linked recognition)

They recognize surface antigens

They ingest and break down those surface antigens into small peptide fragments and then put them on MHC class ll molecules

What type of antigens to Tfh cells recognize and what do they do with them? (think about this in terms of linked recognition)

They recognize small peptide fragments of the surface antigen that was presented to them on MHC class ll molecules.

What are self reactive T cells?

T cells that mistakenly recognize and respond to the body’s own proteins (self antigens) instead of foreign invaders. If they aren’t eliminated, they can cause autoimmune disorders.

How do B cells needing a Tfh cell to help them become activated serve as a form of protection/negative selcetion?

Some self reactive B cells escape the first round of negative selection and enter the blood stream. If the self reactive B cell encounters a self antigen it will look to the Tfh cell to help it become activated to it. The self reactive Tfh cells have been removed by this point though so the Tfh cell won’t give the B cell the activation signals it needs because it won’t be able to recognize the self antigen.

Where does a B cell that has received the first 2 activating signals move to?

The boundary region between the B cell area and the T cell area of the lymph nodes

What is a B cell that has received the first 2 activating signals looking for when it moves to the boundary region?

It is looking for its conjugate Tfh cell that is going to give it the rest of the signals it needs to become activated.

When does the primary focus form?

It forms after B cell activation.

Where does the primary focus form?

It forms in the medullary cords

What happens to B cells in the primary focus?

They undergo rapid proliferation and some of them turn into plasma cells that secrete low affinity IgM antibodies for a quick but temporary protection. The rest of the B cells will leave and go somewhere else.

When does the germinal center form?

It forms after B cell/Tfh cell proliferation in the primary focus.

Where is the germinal center formed?

It is formed in the lymphoid follicles

What enzyme is located in the germinal center?

AID enzyme

What happens in the germinal center because of the AID enzyme?

The B cell/conjugate Tfh cell undergo somatic hypermutation and class switching

What are the zones where affinity maturation takes place called in the germinal center?

The dark zone and light zone

What happens in the dark zone of the germinal center?

Centroblasts divide rapidly and their receptors undergo somatic hypermutation and class switching.

What is a centroblast?

B cells in the dark zone that will undergo somatic hypermutation and class switching.

What happens in the light zone of the germinal center

Centrocytes are displaying their receptor on the surface to see if it can bind antigens with it.

What is a centrocyte?

a B cell in the light zone that has undergone somatic hypermutation and class switching.

What happens to centrocytes with high affinity receptors?

The centrocytes will successfully bind antigen and present it to Tfh cells and then receive survival signals.

What happens to centrocytes that successfully bind antigen?

The centrocytes will undergo more class switching/selection and eventually differentiate into plasma cells or memory B cells

What determines if B cell turns into a plasma cell or a memory B cell?

The cytokines given from the Tfh signals.

What happens to centrocytes with low affinity receptors?

The centrocytes will not be able to bind antigen to present to Tfh cells, meaning they won’t get their survival signals and they will eventually die by apoptosis.

What is a plasma cell?

A fully developed B cell that secretes antibodies.

T/F: Plasma cells express the B cell receptor

False, naive B cells express the B cell receptor

T/F: Plasma cells display MHC class ll molecules

False, naive B cells display MHC class ll molecules

T/F: Plasma cells divide, go through affinity maturation, and class switching.

False, Naive B cells do

What is a thymus dependent response?

a B cell requires Tfh cells for activation

What is a thymus independent response?

a B cell does not require Tfh cells for activation

What is the primary antibody class secreted during a thymus independent response?

IgM

What are the two types of thymus independent responses?

TI-1 and TI-2

What type of antigens are TI-1 responses activated by?

TI-1 antigens

Ex: common bacterial PAMPs like lipopolysaccharide

What is the advantage of B cells responding to TI-1 antigens without T cell help?

The response to TI-1 antigens results in a rapid first line of defense against infections, especially when the adaptive immune system is still developing (early life) or if a pathogen is spreading rapdily.

What type of antigens are TI-2 responses activated by?

TI-2 antigens

Ex: highly repetitive carbohydrate structures that cross link many BCR and send an activation signal

What is the advantage of B cells responding to TI-2 antigens without T cell help?

The response to TI-2 antigens is critical for protection against encapsulated bacteria, which bypass phagocytosis.

The TI-2 antigens fight the bacteria off and tag the cells so more help can come.

What type of B cells do TI-2 antigens activate?

They primarily activate B-1 cells