“Crowd Surfing” Particulate Antigens Are Shuttled to B Cells in Lymph Node Follicles

The source of the antigen that drains from peripheral tissues can vary widely, but it typically includes pathogens, foreign particles, or cell debris. These antigens can originate from various sources such as invading bacteria, viruses, or other pathogens, as well as from damaged or dying cells in the tissue.

When we say an antigen is “particulate,” it means that it exists as discrete, solid particles rather than being dissolved or freely floating in a fluid. Particulate antigens can include things like bacteria, viruses, cell fragments, or other insoluble particles.

Once inside the lymph node, several types of cells participate in transferring the antigen to the B cells:

1. Dendritic cells: Dendritic cells are specialized antigen-presenting cells (APCs) that capture antigens in the peripheral tissues and migrate to the lymph nodes. In the lymph node, dendritic cells present the antigen to B cells and T cells, initiating an adaptive immune response.
2. Follicular dendritic cells (FDCs): FDCs are stromal cells found within the B cell follicles of lymph nodes. They trap antigens, including particulate antigens, on their surfaces and present them to B cells. FDCs play a crucial role in the germinal center reaction, where B cells undergo proliferation, somatic hypermutation, and antibody class switching.
3. Macrophages: Macrophages are phagocytic cells that engulf and digest foreign particles, including antigens. They can also present antigen fragments to B cells and T cells, contributing to the activation of the adaptive immune response.

When particulate antigen reaches B cells in lymph nodes, the B cell receptors (BCRs) play a crucial role in recognizing the antigen. BCRs are membrane-bound immunoglobulin molecules present on the surface of B cells. Each B cell expresses a unique BCR with specificity for a particular antigen.

Here’s how BCRs are involved in recognizing and processing the antigen:

1. Recognition: BCRs recognize the antigen directly. If the antigen matches the specificity of the BCR, it binds to the BCR on the surface of the B cell. This binding triggers a series of signaling events within the B cell, leading to B cell activation.
2. Internalization and processing: Once the antigen is bound to the BCR, the B cell internalizes the antigen-BCR complex through a process called receptor-mediated endocytosis. Inside the B cell, the antigen is processed into smaller peptide fragments by proteolytic enzymes. These peptide fragments are then presented on the B cell surface bound to major histocompatibility complex (MHC) class II molecules.

Next Topic: “Meet Ups” between T Cells and Professional Antigen Presenting Cells in Lymph Nodes

Source: ChatGPT response prompted and edited by Joel Graff.

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