Question

B cells can attempt to increase the strength of antibody binding to antigen. In the answer box below, answer the following questions (in order). Where does this occur? What cells provide additional sources of antigen? What is this process called? Does this involve DNA rearrangement/recombination? What enzyme is required for this process(full name)?

Answer 1

Ans) B-cells are activated by the binding of antigen to receptors on its cell surface which causes the cell to divide and proliferate. Some stimulated B-cells become plasma cells, which secrete antibodies. Others become long-lived memory B-cells which can be stimulated at a later time to differentiate into plasma cells.

- The antigen-recognition molecules of B cells are the immunoglobulins, or Ig. The secretion of antibodies, which bind pathogens or their toxic products in the extracellular spaces of the body, is the main effector function of B cells in adaptive immunity.

- Immunoglobulins (Igs) are antigen-binding proteins produced by B cells. Igs contain two identical heavy (H) chains and two identical light (L) chains. Each H chain associates with an L chain to form an antigen-binding site so that each Ig is bivalent. Each L chain has a variable (V) domain and an invariant constant (C) domain. Each H chain has a V domain and three to four C domains. The V domains form the Ig antigen-binding site, whereas the C domains define the Ig isotype and its associated functions. Membrane-bound Ig (mIg) is part of the B cell receptor (BCR), soluble Ig (sIg) is secreted by plasma cells as antibody, and secretory antibodies (SIg) are polymeric sIg forms found in secretions. The Ig genes encoded in the Igh, Igl and Igk loci contain large numbers of V (variable), D (diversity) and J (joining) gene segments upstream of one or more C (constant) exons. In a developing B cell, Ig gene rearrangement randomly assembles V, D and J gene segments to create a complete V exon in the Igh locus and in either the Igk or Igl locus. Antibody diversity arises from combinatorial joining of gene segments, junctional diversity and random H–L pairing. Antigen-antibody binding affinity is defined by multiple non-covalent binding forces at a single binding site, whereas avidity is the overall strength of binding between multivalent antibody and multivalent antigen.