Medical Immunology (Exam III Material) Lecture 9
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What does Ig stand for and where can it be found/created?
1. Ig = immunoglobulin
2. Can be found bound to the cell surface B-cells as a B-cell receptor, or secreted by B-cells as an antibody (IgA, IgG, IgD, IgE, IgM)
What are the characteristics of antibodies (Ab's) in an immune response?
1. Can opsonize antigens; binds to the antigen tagging it for phagocytosis or complement binding in the classical pathway.
2. Can neutralize antigen; interferes with the antigens function by blocking the pathogen's cell surface proteins that are necessary for growth, replication, and infection. (Ex. snake venom toxin)
What is the typical shape of an Ig molecule?
2. Contains two heavy chains and two light chains that are composed of Ig domains
3. Each arm of the Y shaped molecule contains a Fab region (Fragment antigen binding)
4. Each Ig molecule contains a stem that consists only of Heavy chains, called the Fc region (Fragment crystallizable); responsible for the effector function of Ab's.
B-cell activation requires______of surface Ig (B-cell receptor).
What are the common themes of receptor signaling during B-cell activation that requires cross-linking of surface Ig?
1. On the surface of a given cell, there are many copies of the same (transmembrane) receptor.
2. Due to transmembrane proteins diffusing laterally in the cell membrane, many copies of the same receptor can be distributed all over the cell surface; polyvalent Ag can induce diffusion of several copies to the same region of the cell membrane (called cross-linking or clustering).
3. Leads to phosphorylation of cytoplasmic tails of Igα and Igβ, which initiates intracellular signaling cascade that leads to changes in gene expression in the nucleus.
B-cell activation requires signals from B-cell_____.
What do B-cell co-recpetors recognize?
C3d, which is a breakdown product of C3b complement fragment, deposited on a pathogen.
How does pathogen binding affect the B-cell receptor and co-receptor in regards to its region on the B lymphocyte cell membrane?
What does this cause in regards to kinases?
1.Pathogen binding clusters B-cell receptors and co-receptors in the same region.
2. Kinases associated with Igα and Igβ can now phosphorylate cytoplasmic tails of B-cell co-receptors leading to the intracellular signaling cascade.
How would a small soluble Ag affect B-cell activation?
B-cell and co-receptors recognize epitopes and Ag-attached complement fragments on the same molecule of AG.
Why is the thymus needed to have activated B-cells?
The thymus is needed to develop competent cytotoxic and helper T-cells.
Helper T-cells (CD4) are responsible for interacting with B-cells in accordance to antigens, to activate them.
What is the rare exception causing an induced B-cell response without the aid of Helper T-Cells?
thymus independent antigen
How do thymus independent antigens cause extensive cross-linking of B-cell receptors?
They are highly polyvalent
(ex. bacterial cell wall polysaccharides coat the entire surface of the bacterium)
How are B-cells typically activated?
- a. Ag needs to get to lymph node i. In its native (intact) form à for recognition by B-cell receptors
- ii. In its processed, loaded form as part of a peptide:MHC complex
b. T-cell needs to get to lymph node
c. B-cell needs to get to lymph node
d. B and T cell need to interact
What complement is responsible for opsonizing Ag?
What is this complement degraded to once it opsonizes Ag?
2. C3b is broken down to C3d
What does it mean for Ag (antigen) to be in its native form?
When Ag is in its native form, what part of the immune system is it recognized by? What is it recognized by in its processed form?
1. Native Ag means that it is in its intact form (not broken-down/processed)
2. Ag is recognized by B-cells in its native form; Ag is recognized by peptide:MHC complex in its processed form.
Lymphatics can directly drain Ag to the lymph nodes.
Follicular Dendritic cells are from hemopoietic cells.
Follicular Dendritic cells reside in the lymph nodes have _____ receptors that bind _____ on opsonized Ag.
What do the FDC's serve as for Ag?
1. CR2; C3d
2. Follicular Dendritic cells serve as repositories (storage) of Ag
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