B Cell Mediated Immunity

B cell activation requires how many signals?

2

What signals are required for B-cell activation in thymus-dependent antigens?

Cross linking of BCR by antigen

Helper T cells recognize peptide fragments presented on B cell surface

How do helper T cells activate the B cell?

Secreting cytokines

Presenting CD40/CD40L

T-B cell interaction process

1. B cell processes and present peptide/MHC II

2. Activated CD4 T cell binds via TCR & CD4

3. T cell activates B cell via CD40/CD40L & cytokines

Cognate interaction

B cells and helper T cells must recognize epitopes of the same molecular complex in order to interact

Where in the lymph node does B cell activation occur?

T cell area (paracortex)

What occurs in the germinal center of the lymph node (outcomes of germinal center formation)?

• Proliferation of B cells

• Somatic mutation and selection of high affinity B cells

• Isotype switching

• Memory cell induction

The selection of high affinity B cells is called…

affinity maturation

Enlargement of lymph node

Lymphadenomegaly

Lymph node pathology (unwanted changes); can also refer to enlargement of lymph node

Lymphadenopathy

An enlarged lymph node is referred to as…

reactive node (reactive hyperplasia)

Inflammation of the lymph node (can also refer to enlarged lymph node)

Lymphadenitis

Outcomes of activated B cell undergoing somatic hypermutation

1. Creates B cell with low-affinity - doesn’t cross link or present antigen to T cell - dies by apoptosis

2. Creates B cell with high-affinity - cross links with antigen - proliferation and maturation - Creates memory B cell or plasma cells

B cell activation by CD40L & cytokines released by T cell leads to…

B cell proliferation and differentiation to antibody-secreting plasma cells

What is the initial class of antibody before isotype switching?

IgM

What classes can the antibody become after isotype switching?

IgG, IgA, IgE

What occurs during isotype switching?

B cells recombine heavy chain constant domain coding gene segments and switch to other classes

What remains constant during isotype switching?

Variable domain coding part of the gene

What is the function of plasma cells?

High-rate antibody secretion only

Mitogenic

Induce B cell activation and proliferation (requires LPS)

T-independent antigens

Mitogenic multivalent antigen with identical epitopes (polysaccharides)

What signals are involved in B cell activation for T-independent antigens?

1. B cell receptors are activated by crosslinking with antigens

2. B-cell receptor and co-receptor cooperate in B cell activation

Immune response to T-independent antigens

• Weaker response (lower levels of antibodies)

• No isotype switching (only IgM)

• No memory B cell development

Vaccination purpose

Induction of long-lived plasma cells and memory cells

Development of memory B cells can only occur with..

T cell help

Secondary immune response is the result of…

• Increased antigen-specific T & B cells

• Lower threshold for activation due to memory cells

Conjugate vaccine

Strategy to induce memory cells to polysaccharide antigens

Effector functions of antibodies

• Neutralization of virus and toxins

• Blocking of bacteria

• Phagocytosis of opsonized bacteria

• Antibody-dependent cell-mediated cytotoxicity

• Mast cell/basophil degranulation

• Complement activation

What antibody functions require binding of Fc region with Fc receptors?

• Phagocytosis of opsonized bacteria

• Antibody-dependent cell-mediated cytotoxicity

• Mast cell/basophil degranulation

What must occur for antibodies to activate complement?

Binding of Fc region with complement component