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13 Cards in this Set
- Front
- Back
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Humoral immunity
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Humoral immunity is mediated by antibodies and is the arm of the adaptive immune response that functions to neutralize and eliminate extracellular microbes and microbial toxins.
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Why is humoral immunity more important than cellular immunity?
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Humoral immunity is more important than cellular immunity in defending against microbes with capsules rich in poly-saccharides and lipids, and against polysaccharide and lipid toxins.
The reason for this is that B cells respond to, and produce antibodies specific for, many types of molecules, but T cells, the mediators of cellular immunity, recognize and respond only to protein antigens. |
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Clonal expansion
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The activation of B lymphocytes results in the proliferation of antigen-specific cells, called clonal expansion, and in their differentiation into effector cells, called plasma cells, that actively secrete antibodies.The secreted antibodies have the same specificity as that of the naive B cell membrane receptors that recognized antigen to initiate the response.
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Heavy chain isotope, or class switching
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During their differentiation, some B cells may begin to produce antibodies of different heavy chain isotypes (or classes), which mediate different effector functions and are specialized to combat different types of microbes.
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Affinity maturation
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Repeated exposure to a protein antigen results in the production of antibodies with increasing affinity for the antigen.
Leads to the production of antibodies with improved capacity to bind to and neutralize microbes and their toxins. |
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T-dependent response
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Protein antigens are processed in antigen-presenting cells (APCs) and recognized by helper T lymphocytes, which play an important role in B cell activation and induce heavy chain isotype switching and affinity maturation.
In the absence of T cell help, protein antigens elicit weak or no antibody responses. Therefore, protein antigens, and the antibody responses to these antigens, are called "T-dependent." |
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T-independent response
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Polysaccharides, lipids, and other nonprotein antigens stimulate antibody production without the involvement of helper T cells. Therefore, these nonprotein antigens, and the antibody responses to them, are called "T-independent."
The antibodies produced in response to T-independent antigens show relatively little heavy chain isotype switching and affinity maturation. |
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Follicular B cells
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A majority of B cells are called follicular B cells because they reside in the follicles of lymphoid organs.
These follicular B cells make the bulk of T-dependent, class-switched, and high-affinity antibody responses to protein antigens and give rise to long-lived plasma cells. |
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Marginal zone B cells
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Marginal zone B cells, which are located in the marginal zones of the splenic white pulp, respond to blood-borne polysaccharide antigens.
Express antigen receptors of limited diversity and make predominantly IgM responses, which lack many of the features of T-dependent antibody responses to protein antigens |
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B-1 B cells
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B-1 B cells respond to nonprotein antigens in the mucosal tissues and peritoneum.
Express antigen receptors of limited diversity and make predominantly IgM responses, which lack many of the features of T-dependent antibody responses to protein antigens. |
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Antigen induced signaling in B cells
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In B cells, Ig receptor-mediated signal transduction requires the bringing together, or cross-linking, of two or more receptor molecules.
Membrane IgM and IgD of naive B cells recognize antigens but do not transduce signals. The receptors are noncovalently attached to 2 proteins Igα and Igβ, to form the B cell receptor (BCR) complex. Phosphorylation causes the recruitment of a number of signaling molecules. The net result of receptor-induced signaling in B cells is the activation of transcription factors that turn on genes whose protein products are involved in B cell proliferation and differentiation. |
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Igα and Igβ
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When attached to IgM and IgD of naive B cells, called the BCR complex.
The cytoplasmic domains of both Igα and Igβ contain conserved immunoreceptor tyrosine-based activation motifs, or ITAM's. When 2 or more antigen receptors of a B cell are clustered, the tyrosine's in the ITAM's of Igα and Igβ are phosphorylated. These phosphotyrosines become docking sites for adapter proteins that themselves get phosphorylated and then recruit other signaling molecules. |
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The complement system
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B lymphocytes express a receptor for a protein of the complement system that provides signals for the activation of the cells.
The complement system is a collection of plasma proteins that are activated by microbes and by antibodies attached to microbes. Complement proteins provide second signals for B cell activation, functioning in concert with antigen (which is "signal 1") to initiate B cell proliferation and differentiation. |
