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B 细胞激活和分化
适应性免疫反应是一种复杂的防御机制，依赖于 B 淋巴细胞或 B 细胞的激活和分化。这些过程使我们的身体能够针对特定病原体（如细菌、游离病毒颗粒、毒素和寄生虫）产生特定的反应。
当未成熟的 B 细胞遇到可与其表面的 B 细胞受体 (BCR) 结合的特定抗原时，它们会发生敏化以对抗原的存在作出反应。敏化…
当初始B细胞遇到与其相应的抗原时，抗原会结合细胞表面的B细胞受体（BCR），从而使B细胞致敏。
结合后，B细胞通过受体介导的内吞作用吞噬抗原，将抗原-BCR复合物内化。
在B细胞内，抗原被加工成较小的肽片段，并与MHC II分子结合。
抗原-MHC II 复合物被转运至B细胞表面进行展示，从而完成致敏过程。
接着，致敏的B细胞结合辅助性T细胞上的T细胞受体（TCR），辅助性T细胞释放细胞因子作为共刺激信号，从而激活B细胞。
活化后，B细胞会经历克隆扩增，迅速增殖产生能够与相同抗原结合的 identical B细胞。
一些活化的细胞会分化为效应性浆细胞，产生能够中和抗原或标记抗原以使其被清除的抗体。
与此同时，其他活化的B细胞会分化为长寿命的记忆B细胞，从而在将来再次遇到相同抗原时提供免疫记忆
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Q1: How does a naive B cell become sensitized to an antigen?
Sensitization begins when a naive B cell encounters an antigen that binds to its B cell receptor (BCR) on the surface. The B cell then engulfs the antigen through receptor-mediated endocytosis, internalizing the antigen-BCR complex. Inside the cell, the antigen is processed into smaller peptide fragments and combined with MHC II molecules, which are transported to the B cell surface for display.
Q2: What role do Helper T cells play in B cell activation?
After sensitization, the B cell presents the antigen-MHC II complex to Helper T cells. When the Helper T cell's TCR binds to this complex, the Helper T cell releases cytokines as co-stimulatory signals that activate the B cell. This interaction is essential for triggering the B cell's proliferation and differentiation into effector and memory cells.
Q3: What happens during clonal expansion of activated B cells?
Upon activation by Helper T cell signals, the B cell undergoes clonal expansion, rapidly proliferating to generate many identical B cells capable of binding to the same antigen. This expansion amplifies the immune response by creating a large population of cells with identical antigen specificity, enabling a more effective defense against the pathogen.
Q4: How do plasma cells and memory B cells differ in function?
Some activated B cells differentiate into effector plasma cells, which produce antibodies that neutralize antigens or mark them for destruction by phagocytes. Other activated B cells become long-lived memory B cells that provide immunological memory, enabling rapid recognition and response upon future encounters with the same antigen.
Q5: What pathogens can B cells help defend against?
B cells mount tailored responses against specific pathogens including bacteria, free virus particles, toxins, and parasites. Through antibody production and antigen presentation, B cells enable the adaptive immune system to recognize and eliminate these diverse threats with precision and specificity.
Q6: Why is immunological memory important after B cell differentiation?
Memory B cells generated during initial antigen exposure provide immunological memory that enables faster and stronger responses to subsequent encounters with the same antigen. This long-lived population of cells allows the immune system to recognize previously encountered pathogens more quickly, providing enhanced protection against reinfection.
Q7: How does antigen processing contribute to B cell sensitization?
During sensitization, the internalized antigen is broken down into smaller peptide fragments and combined with MHC II molecules to form an antigen-MHC II complex. This processed complex is transported to the B cell surface where it is displayed, allowing the B cell to function as an antigen-presenting cell for Helper T cells and enabling proper immune activation.