B cells Flashcards
Antigen (Antibody generator): compound that elicits an
adaptive immune response
– Can be bound by?
A specific BCR (antibody) or TCR
Nature of BCR antigens is greatly varied
– Proteins and polysaccharides are typical antigens
– Lipids and nucleic acids are rarely antigens
– Size is important for antigenicity
>10 kDa more likely antigens
What is an Immunogen?
A molecule that causes adaptive immune response if injected alone
An Epitope is?
Antigenic determinant; part of antigen where antibody (or TCR binds)
Basic unit of the antibody structure is the?
Y-shaped monomer
– 4 chains held by disulfide bonds
2 “heavy” chains
2 “light” chains
Antibodies are?
proteins made up of 4 amino acid chains
Each chain of an antibody has a constant region
– This region of the heavy chain contains the Fc region
– Heavy chain determines Fc region and isotype
Each chain of an antibody has a variable region
– Variable region is unique
– 1 heavy chain variable region and 1 light chain variable region together forms the antigen binding site
2 Ag binding sites/antibody
Blocks toxin or virus from interacting with cell
Neutralization
Binds cellular structures to interfere with function
Immobilization & prevention of adherence
Clumping of bacterial cells by specific antibody. Bacteria more easily phagocytized
Agglutination & precipitation
Multiple antibodies bind a target cell making it a target for NK cells
Antibody-dependent cellular cytotoxicity
Isotype determines functional relationships of antibody
– Interactions with cells
– Interactions with cells
Fc receptor binding
– Tissue distribution
– Monomeric, dimeric, or pentameric structure
IgG and IgM are the primary?
plasma isotypes
IgG and monomeric IgA are the?
major extracellular fluid isotypes
Dimeric IgA is in?
secretions across epithelia, including breast milk
IgG is the only?
transplacental isotype
IgE is typically associated with?
mast cells (especially in the respiratory tract, GI tract, & skin)
The brain is normally devoid of?
immunoglobulin
During lymphocyte development, B and T cells
acquire ability to?
recognize distinct epitopes
– Once they make a TCR or BCR, cells “checked out”
to ensure proper function
– B cells undergo developmental stages in bone marrow
– T cells go through process in thymus
Antigen receptor diversity: Recombination of DNA allows ?
generation of novel variable regions of antigen receptors
– Applies to BCR and TCR
Antigen receptor diversity: Junctional diversity
nucleotide additions occur at the junction sites
– Applies to BCR and TCR
Two chains form antigen binding site
– Applies to BCR and TCR:
BCR = light and heavy chain
TCR = α and β chains form TCR
Somatic hypermutation for?
affinity maturation
– Applies to BCR/antibody only
B Lymphocyte characteristics include?
– Naïve
* Have antigen receptor but have not encountered antigen
– Activated
* Able to proliferate
* Have bound antigen
– Effectors:
* Descendants of activated lymphocytes
* Able to produce specific cytokines
* Plasma cells
– Memory lymphocytes
* Long-lived descendants of activated lymphocytes
* Memory cells responsible for seed and effectiveness of secondary response
– Remembers antigen on subsequent exposure
When antigen enters body, only specific antibody binds
– Initiates multiplication of antigen specific B cells. Process called clonal selection
Repeated cycles of cell division generates population of copied antibodies
– Termed clonal expansion
- Without sustained stimulation, cells undergo apoptosis
Humoral Immunity
Mediated by B lymphocytes a.k.a B cells
– Develops in bone marrow
– B cells may be triggered to proliferate into plasma cells
Plasma cells produce antibodies
– Antibodies produce when antigen bonds B cell receptor
– Some B cells differentiate into memory cells
B Lymphocyte Activation 7 steps
- Ag binds to B cell receptor
– Poises B cell to respond; still needs help from CD4 T cells - Ag is phagocytized and processed by B cells
- B cell presents a peptide cut from the antigen on MHCII
- TH cell with specific TCR recognizes the peptide: MHCII
complex and co-stimulates the B cell - B cell is activated & proliferates
- Optimizes BCR/antibody
- Plasma & memory cells
B Lymphocyte and Antibody Response:
Characteristic of primary response
Lag period of 10 to 12 days occurs before antibody
detection in blood
* Activated B cells proliferate and differentiate into increasing numbers of plasma cells as long as antigen is present
– Net result is slow steady increase in antibody titer
Over time, some B cells undergo changes enhancing immune response including?
Affinity maturation
Isotype (Class) switching
Affinity maturation by somatic hypermutation
– Demonstration of evolution: natural selection
* Occurs among activated proliferating B cells in 2° lymphoid tissue
– Point mutants in the variable region of the BCR are formed
* Many cells have unproductive mutations (nonsense) and the cell will die
* Many cells will have less effective BCRs and will die
* Rarely, a mutation will make the antibody bind better and that cell will be further stimulated and optimized
* Because this is a rare event LOTS of B cells need to be produced to generate the best antibody
– Fine tunes the quality of antibody binding to antigen
Isotype (Class) switching
– B cells initially programmed to produce IgM
– Helper T cells produce cytokines upon co-stimulation
Can induce B cells to switch isotypes
– Differentiate to plasma cells that secrete other classes of
antibodies
Commonly IgG
Can be IgA or IgE too
Formation of memory B cells
B cells that have undergone class switching
IgG is primary antibody of memory
IgG antibody can circulate in body for years allowing
protection against specific antigens
Characteristics of secondary response for B cells
Memory cells responsible for swift effective reaction
of secondary response
* Often eliminate invaders before noticeable harm is done
Vaccine exploits phenomenon of immunologic memory
– Some memory B cells will differentiate into plasma cells
Results in?
rapid production of antibodies
T-independent antigens
– Can stimulate antibody response
* Activate B cells without helper T cells
* Few antigens are T-independent
– B cell receptors bind antigen simultaneously
* Clusters BCRs and leads to B cell activation
– Some polysaccharides and lipopolysaccharides act as T-independent antigens
– No T cells, so only IgM produced and no memory
TH cells in B cell activation
If TH cell encounters B cell bearing peptide: MHCII complex
– TH cell responds by producing cytokines
– B cell is activated in response to cytokine stimulation
* B cell proliferates and undergoes class switching
Also drives formation of B memory cells
Natural killer cells descend from lymphoid stem cells. They?
They lack antigen specificity
- No antigen receptors
- Recognize antigens by means of Fc portion of IgG antibodies
– Fc receptors allow NK cells to attach to antibody-coated cells
NK Actions assist the adaptive immune response
Important in process of antibody dependent cellular toxicity
- Enable killing of host cells with foreign protein in membrane
Natural killer cells recognize damaged host cells with no MHC class I surface molecules, Important in?
viral infection
