B Cell Activation

Question 1

clonal selection

Answer 1

fundamental tenet of the immune system stating that every individual possess numerous clonally derived lymphocytes, each clone having risen from a single precursor (w/o influence of antigen), expressing one antigen receptor, and capable of recognizing and responding to distinct antigenic determinant. When encounters antigen in the periphery, specific preexisting clone is ‘selected’ and activated

Question 2

Describe the B cell subsets.

Answer 2

B-1: developed from fetal liver-derived stem cells; responde to non-protein Ag in the mucosa (CD5+)

B-2: develop from BM progenitors after birth
follicular B cells - recirculating B cells (majority)
marginal B cells - reside in spleen and recognize blood-borne polysaccharide Ags

Question 3

antigen dependent phase

Answer 3

response initiated by recognition of antigen (epitope) by B cell specific for that antigen; antigen bind to mTg on naive cells and activates cell (can be T-dependent or T-independent)

Question 4

Describe the two signals needed to activate a B cell.

Answer 4

1. signal 1: antigen recognition either through
   - cross-linking of 2 or more BCR: Ig beta, alpha
   - interaction with Ag+C3d attached (more immunogenic): Ig alpha, beta, CR2, CD19
2. signal 2:
   - T independent: produce IgM
   - T dependent: interaction w/ T cause activation by CD40L (T cell) w/ CD40 (B cell) leading to isotype switching and somatic hypermutation

Question 5

class switching

Answer 5

gene recombination of heavy chain dictating the isotype class of antibody produced and effector function; determined by cytokine released by activated T cell and CD40:CD40L interaction

Question 6

somatic hypermutation

Answer 6

introduction of point mutations in the V region of Ig genes at random. when combined with survival selection by FDC and T(fh) cells in germinal center, gives rise to B cell that produces antibodies with high affinity for antigen.

Question 7

activation-induced deaminase (AID)

Answer 7

enzyme activated by CD40 signals which is responsible for initiating class switching and performs point mutations of Cs to Us during somatic hypermutation

Question 8

plasma cells

Answer 8

terminally differentiated Ab secreting B cells that secrete at rates ranging from 100s to 1000s of antibodies per second

Question 9

memory cells

Answer 9

differentiated B cells which survive for long period of time w/o additional Ag stimulation and are capable of mounting a rapid immune response to subsequent exposure (secondary immune response)