Final answer:
The antibody response to thymus-independent antigens does not require T cell help because repetitive epitopes on the antigen lead to the crosslinking of BCRs, which activates B cells without the need for T cell involvement. The correct choice is (a), highlighting the importance of BCR crosslinking in T cell-independent activation.
Step-by-step explanation:
The antibody response to thymus-independent antigens does not require T cell help because some highly repetitive epitopes cause extensive crosslinking of the BCRs and their co-receptors, which can activate mature naive B cells. This process is known as T cell-independent activation of B cells, and it typically occurs with antigens that have repetitive epitope units within their structure, such as polysaccharide capsules or lipopolysaccharide found on bacteria. These repetitive units allow for the cross-linkage of multiple BCRs, providing the signal necessary for B cell activation without the need for T cell involvement.
The correct answer to the student's question is (a), as T-independent antigens enable the cross-linkage of BCRs on the B cell surface due to their repeated nature, thereby activating the B cell in the absence of help from T cell cytokines. This crosslinking is sufficient to activate B cells, leading to the production of IgM antibodies by plasma cells.