#5 Antibody-Mediated Immune Response

Question 1

Negative selection of B cells

Answer 1

If a B cell recognizes self-Ags, then it will undergo RECEPTOR EDITING (Ig gene recombination to make new light chain)

Question 2

The activation of B cells by Ag in the lymph node does the following:

Answer 2

↑ B cell proliferation

↑expression of MHC class 2, B7(CD80/CD86), receptors for cytokines made by Th cells

secrete low level of IgM

Question 3

Help from Complement

Answer 3

CR2 is important in beginning of infection b/c there isn’t much Ag

CR2 binds C3d (a fragment of C3b that’s on the bacteria)→makes B cell very sensitive to Ags

*B cell recognizes Ag labeled by innate immunity

Question 4

Where do B cells go after activation?

Answer 4

Lymphoid follicle → T cell-rich zones of lymph node

Question 5

B cells are professional APC

Answer 5

True

They can engulf and present via MHC class II. This is required in order to have T helper cells activate B cells

(Many different CD4+ cells can activate a single B cell)

Question 6

How T cells activate B cells

Answer 6

Stimulatory signals

CD40L (T helper cell) + CD40 (B cell) → signal sent

If BCRs have been cross-linked, then B cell is activated

Question 7

B cell Ig Class Switching

Answer 7

First activated, B cell makes mostly IgM →can switch to IgG, IgA, IgE

• Ab class is determined by Fc heavy chain region
• B cell cuts off IgM constant region DNA and puts on a different one

Question 8

IgE

Answer 8

IgE: Immunity vs helminths, mast cell degranulation

IgA: Mucosal Immunity

IgM: Complement activation

Question 9

What are the 2 cells that a B cell can become?

Answer 9

Plasma cell (Ab factory) or memory cell

Question 10

Where do plasma cells travel?

Answer 10

To spleen or back to bone marrow

Question 11

How long does plasma cell live for?

Answer 11

5 days

Question 12

B cell → Memory cell

Answer 12

CD40-CD40L interaction is critical

*T cell needs to activate B cell in order to make plasma or memory cells

Question 13

B cells via Thymus-dependent and Independent

Answer 13

DEPENDENT: Proteins need help of Th2 cells to stimulate B cells to make Ab and memory cells → Thymus DEPENDENT. Thymus dependent antigens require B cells to have direct contact with T helper cells in order to form antibodies. This is how a typical humoral immune response is carried out in the case of most antigens. An APC presents antigen to a T helper cell, the T helper cells can then present antigen to the B cell and it is activated to produce antibodies.

INDEPENDENT: Chemical nature: polymeric antigens (polysaccharides, glycolipids, nucleic acids) (2nd signal via Toll-like receptors)type 1) or B cell receptor clustering(type 2))

Thymus independent antigens activate B cells by other mechanisms. They can be bacterial cell wall components, like lipopolysaccharides. These are called TI-1 Antigens. Or they can be highly repetitious molecules, like the proteins that make up bacterial flagella. These are TI2-Antigens.

Question 14

What are the 2 types of TI Ags? (thymus independent)

Answer 14

TI-1 Ags are polyclonal activators of B cells

TI-2 Ags (not polyclonal) are Ags with repeating epitopes for cross-linking BCRs

Question 15

TI-2 Ags

Answer 15

-Generated against repetitive Ags, direct BCR cross-linking, mostly IgM, low affinity, no memory

Question 16

Clinical Significance of TI Ags

Answer 16

• Many bacterial cell wall polysaccharides (pneumococcus, meningococcus, Haemophilus) come from TI Ags
• Ab-mediated immunity against encapsulated bacteria (B cells can be activated right away and not have to wait for T cells)
• Patients with congenital or acquired deficiencies of Ab-mediated response → susceptible to infections with encapsulated bacteria