B Cells and Antibodies of Adaptive Immunity Flashcards
What are the two main weapons of the adaptive immune system?
Antibodies and T cells
What is the main defense against extracellular bacteria?
Antibodies
Antibodies can also help protect against infection by viruses
What cells produce antibodies?
B cells
Where are B cells derived?
Like T cells, B cells are derived from common lymphoid progenitors in the bone marrow.
Where do B cells develop and mature?
Unlike T cells, B cells remain in the bone marrow throughout their development and maturation process.
What is the BCR?
The B cell receptor is a surface-bound immunoglobulin (mIg) with an associated Igα/Igβ component; antibodies are secreted immunoglobulin (sIg).
The BCR is responsible for antigen binding and determines the antigen specificity of the B cell.
What do B cells develop during maturation?
B cells begin to express the B cell receptor, BCR (also called membrane-bound immunoglobulin, mIg) as well as the Igα/Igβ co-stimulatory proteins.
What is the Igα/Igβ subunit responsible for?
Sending a signal to the inside of the cell once an antigen has been bound
(this is analogous to the CD3 complex on T cells)
What is clonal deletion?
Process of eliminating self-reactive B cells
During the developmental process, if the BCR of an immature B cells binds an antigen (usually a self-antigen) while still in the bone marrow, the Igα/Igβ co-stimulatory proteins sends a signal to the inside of the cell causing the cell to die.
(This is analogous to the negative selection that thymocytes undergo in the thymus.)
Discuss positive selection in regards to B cells.
As BCRs do not need to interact with processed proteins and MHC like the TCR of T cells, there is no similar process of positive selection.
What happens to B cells that complete the maturation process?
B cells that are not deleted develop into mature B cells that leave the bone marrow, circulate in the blood and migrate to the spleen and lymph nodes.
What is the the BCR of a mature, naïve B cell?
The BCR of a mature, naïve B cell is either membrane-bound immunoglobulin M (mIgM) or membrane-bound immunoglobulin D (mIgD).
Describe the structure of a BCR molecule, including the formation of its antigen binding pocket.
Each BCR molecule consists of 4 polypeptide chains: 2 identical heavy (H) chains and 2 identical light (L) chains.
Each of the immunoglobulin H and L chains has one V region (V = variable) and one C region (C = constant).
The VH (variable region of the Heavy chain) and VL (variable region of the Light chain) regions combine to form an antigen-binding pocket.
There are two antigen-binding sites per BCR (or immunoglobulin molecule).
How many antigen-binding sites are there per BCR molecule?
There are two antigen-binding sites per BCR (or immunoglobulin molecule).
What happens first in the expression of BCR?
The expression of the BCR first involves the rearrangement of immunoglobulin “gene segments” during development to form complete genes for the H chain and the L chain.
This process is covered in MICB 302.
How many BCRs will a single mature B cell have, and where?
A single mature B cell will have approximately 100,000 BCRs in its membrane.
How many epitopes will a single B cell bind?
All of the BCRs (i.e., both mIgM and mIgD) on a single B cell. Because the mIgM and mIgD have the same VH and VL regions, all of the BCRs will have the same antigen specificity.
Thus, a single B cell will bind only one or a few similar epitopes (antigenic determinants).
How do different B cells have different antigen specificity?
Different B cells will have different VH and VL regions in the BCRs (because they used different gene segments when constructing the gene for the H and L chains).
Describe antibody synthesis.
Antibodies are secreted proteins made by B cells.
The heavy (H) and light (L) chain mRNAs are translated into proteins by membrane-bound ribosomes.
The H and L chain polypeptides are translocated into the endoplasmic reticulum (ER) as they are synthesized.
In the ER, two H and two L chains are joined by di-sulfide bonds and assembled into a complete antibody molecule.
What is the main difference between membrane-bound immunoglobulin (mlgs) and secreted antibodies (slgs)?
Amino acid sequences at the carboxyl-terminus of the H chain.
For mIg, the amino acids are hydrophobic so that the polypeptide is anchored in the membrane when the vesicle fuses with the plasma membrane.
For sIg, the amino acids tend to be hydrophilic so that when the vesicles with the plasma membrane, the antibodies are released into the extracellular fluids.
