The Innate Immune System's Response to Viruses

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Immunology

Summary

The innate immune system is the first line of defense against viral infection. When a cell is infected by a virus, the virus hijacks the cell’s machinery and begins to replicate itself. When this happens, the host cell releases interferon alpha and interferon beta– both type I interferons– to prevent the virus from further replicating within itself, and to also warn neighboring cells of the virus. Two other immune cells are involved in the innate immune response against viruses. The first is the plasmacytoid dendritic cell, which has toll-like receptors on its surface that can bind to viral antigens, causing them to produce large amounts of type I interferon. The second cell is the natural killer cell, which produces interferon gamma, a type 2 interferon. The NKC destroys virus-infected cells by determining whether they have decreased or absent MHC 1 molecules, and they are able to detect this via the NKC’s inhibitory and activating receptors. Once the NKC has targeted the virus-infected cell for destruction, it has two primary methods to induce apoptosis: it uses perforin to form pores in the cell membrane so they can insert granzyme B, and the Fas ligand on the NKC can also bind to the death receptor Fas on the infected cell, both of which trigger signaling cascades that lead to cell death.

Lesson Outline

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FAQs

What is the difference between T-cell dependent and T-cell independent B-cell activation?

T-cell dependent B-cell activation requires interaction with Helper T-cells for B-cell maturation and differentiation into antibody-secreting plasma cells. In this process, antigens are first captured by the B-cells through receptor-mediated endocytosis and then presented on MHC Class II molecules to Helper T-cells, which provides co-stimulatory signals necessary for full activation. On the other hand, T-cell independent B-cell activation doesn't require T-cell help and is typically triggered by multivalent antigens that can cross-link B-cell receptors, leading to direct B-cell activation and differentiation.

What roles do secondary lymphoid organs play in B-cell activation?

Secondary lymphoid organs, such as lymph nodes, spleen, and mucosa-associated lymphoid tissues, are the major sites where B-cell activation takes place. These locations facilitate interaction between B-cells and antigens or antigen-presenting cells. Additionally, they provide a supportive environment where B-cells can encounter Helper T-cells for the co-stimulatory signals in T-cell dependent B-cell activation.

What happens during B-cell maturation?

B-cell maturation is a multi-stage process starting from the production of B-cell precursors in the bone marrow. It involves antigen-independent gene rearrangement of B-cell receptors and expression of different surface markers. After leaving the bone marrow, the mature B-cells migrate to secondary lymphoid organs, where they undergo further maturation upon encountering antigens by either T-cell dependent or T-cell independent activation pathways. This process leads to the production of memory B-cells and antibody-secreting plasma cells.

How does class switching occur during B-Cell Activation?

Class switching is a process that takes place during B-cell activation, specifically in T-cell dependent activation. It allows a B-cell to produce a different class of antibody other than IgM or IgD by changing the constant region of the heavy chain of the antibody. This process is driven by cytokines provided by the helper T-cells and occurs in the germinal centers of the secondary lymphoid organs.

What is the role of Toll-like receptors in B-cell activation?

Toll-like receptors (TLRs) are a type of pattern recognition receptor present on B-cells. They recognize certain microbial patterns and can lead to activation of B-cell immune responses. They play a role particularly in T-cell independent B-cell activation, where they sense and bind to specific microbial components, aiding in the immediate activation and differentiation of B-cells into antibody-secreting plasma cells.

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