Final answer:
Allele exclusion in T and B cells ensures that each lymphocyte expresses only one type of antigen receptor, either from the paternal or the maternal allele, which is crucial for maintaining the specificity of the immune response.
Step-by-step explanation:
The mechanism of allele exclusion in T and B cells ensures that each cell expresses a receptor with a single specificity for an antigen. During the development of T and B cells, VDJ recombination allows the assembly of gene segments to produce a unique T-cell receptor (TCR) or antibody. In T cells, TCR diversity is achieved via DNA recombinase randomly excising and recombining V, D, and J segments. A similar process occurs for B cells, but concerning their immunoglobulin (Ig) genes for light and heavy chains.
Upon successful rearrangement and expression of one of the alleles (either paternal or maternal), the cell undergoes allelic exclusion, preventing the rearrangement of the other allele. This ensures that each T or B cell produces a TCR or antibody with a unique specificity, avoiding the possibility of dual specificity that could confuse the immune response.
Moreover, mechanisms like peripheral tolerance ensure that if B cells bind self-antigens without receiving assistance from helper T cells, they undergo apoptosis instead of becoming active, thereby preventing autoimmunity.