62 確定每一個成熟B細胞只能產生辨識一種抗原決定位抗體的機制為何？

對偶基因排除（allelic exclusion） 親和力成熟（affinity maturation）

RNA多樣裁接（alternative splicing） 類型轉換（class switch）

answer A

allelic exclusion:

Allelic exclusion is a process by which only one allele of a gene is expressed while the other allele is silenced.

For most genes, the individual inherits one copy of each gene from each parent. Each copy of the gene is called an allele.

Allelic exclusion has been observed most often in genes for cell surface receptors and has been extensively studied in immune cells such as B lymphocytes.^[1]In B lymphocytes, successful heavy chain gene rearrangement on one chromosome results in the shutting down of rearrangement on the second chromosome. If no successful rearrangement occurs, rearrangement takes place on the second chromosome. If no successful rearrangement occurs on either chromosome, the cell dies. As a result of allelic exclusion, all the antigen receptors on an individual lymphocyte will have the same amino acid sequence in the variable domain of the heavy chain protein. As the specificity of the antigen receptor is modulated by the variable domain of the light chain encoded by one of the immunoglobulin light chain loci, the specificities of B cells containing the same heavy chain recombination event can differ according to their light chain recombination event.

affinity maturation

affinity maturation is the process by which B cells produce antibodies with increased affinity for antigen during the course of an immune response. With repeated exposures to the same antigen, a host will produce antibodies of successively greater affinities. A secondary response can elicit antibodies with several logfold greater affinity than in a primary response. The main principles of the in vivo affinity maturation namely mutation and selection are utilised for the biotechnological approach of the in vitro affinity maturation.

alternative splicing

Alternative splicing (or differential splicing) is a process by which the exons of the RNA produced by transcriptionof a gene (a primary gene transcript or pre-mRNA) are reconnected in multiple ways during RNA splicing. The resulting different mRNAs may be translated into different protein isoforms; thus, a single gene may code for multiple protein

class switch

Immunoglobulin class switching (or isotype switching or isotypic commutation orclass switch recombination(CSR)) is a biological mechanism that changes a B cell's production of antibody from one class to another, for example, from an isotype called IgMto an isotype called IgG. During this process, the constant region portion of the antibodyheavy chain is changed, but the variable region of the heavy chain stays the same (the terms "constant" and "variable" refer to changes or lack thereof between antibodies that target different epitopes). Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.