B cell antigen processing and presentation

Definition

Target type: biologicalprocess

The process in which a B cell expresses antigen (peptide or lipid) on its cell surface in association with an MHC protein complex. [GOC:add, ISBN:0781735149, PMID:15771591]

B cells are lymphocytes responsible for humoral immunity, the antibody-mediated component of the adaptive immune system. They play a crucial role in recognizing and responding to specific antigens present on pathogens. B cell antigen processing and presentation involves a series of steps that enable the B cell to effectively present the antigen to T cells, initiating a coordinated immune response.

1. Antigen Capture: B cells possess a unique surface receptor called the B cell receptor (BCR), which is a transmembrane immunoglobulin molecule. The BCR specifically binds to intact antigens, such as proteins, carbohydrates, or lipids, present on pathogens or foreign substances. This binding event triggers the internalization of the antigen-BCR complex.

2. Antigen Processing: Once internalized, the antigen-BCR complex is delivered to intracellular compartments called endosomes. Within these compartments, the antigen is subjected to enzymatic degradation by proteases, breaking it down into smaller peptides. This process is known as antigen processing.

3. MHC Class II Association: Concurrently, major histocompatibility complex (MHC) class II molecules are synthesized within the endoplasmic reticulum (ER) of the B cell. These MHC class II molecules are composed of two polypeptide chains, α and β, which form a groove that can bind to peptides. Once synthesized, MHC class II molecules are transported to the endosomes where they encounter the processed antigen peptides.

4. Peptide Loading: The processed antigen peptides bind to the MHC class II groove within the endosomes. This binding is facilitated by specialized chaperone proteins, such as invariant chain (Ii). Ii initially occupies the MHC class II groove to prevent premature peptide binding in the ER. Once the MHC class II molecule reaches the endosome, Ii is degraded, allowing for peptide loading.

5. Surface Expression: The MHC class II molecule loaded with the antigen peptide is then transported to the B cell surface. This process is mediated by the exocytic pathway, which involves the fusion of endosomes with the plasma membrane. The surface expression of the MHC class II-peptide complex allows the B cell to present the antigen to other immune cells.

6. T Cell Activation: When a T helper cell encounters the B cell displaying the antigen-MHC class II complex, it recognizes the antigen through its T cell receptor (TCR). The TCR binds specifically to the peptide presented in the MHC class II groove. This interaction triggers the activation of the T helper cell, leading to the production of cytokines and other signaling molecules.

7. B Cell Activation and Differentiation: The cytokines released by the activated T helper cell act on the B cell, providing signals that promote B cell activation and differentiation. Activated B cells undergo clonal expansion, producing a large number of daughter cells specific for the same antigen. These activated B cells differentiate into antibody-secreting plasma cells or memory B cells.

8. Antibody Production: Plasma cells are specialized B cells that secrete large amounts of antibodies. Antibodies are Y-shaped proteins that recognize and bind to specific epitopes on the antigen. By binding to antigens, antibodies can neutralize pathogens, opsonize them for phagocytosis, or activate complement, a part of the innate immune system.

9. Memory B Cell Formation: Memory B cells are long-lived B cells that retain the memory of the encountered antigen. Upon subsequent exposure to the same antigen, memory B cells can rapidly differentiate into plasma cells, providing a faster and more robust immune response.'
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Proteins (1)

Compounds (3)