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Microbiology: An evolving science Chapter 24
Terms in this set (60)
Our immune system's memory relies on special lymphocytes in the circulation called memory B cells and T cells
These form during an infection and "remember" it for years.
Armed with this "knowledge," the lymphocytes circulate throughout the body like tiny sentries, ready to detect and quickly respond to a second attack.
From birth, the immune system is able to adapt and recognize billions of possible foreign antigens
Adaptive immunity develops as the need arises.
Two types are recognized:
Mediated by B cells (B lymphocytes)
Produce antibodies that directly target antigens of invaders
Involves T cells (T lymphocytes)
Control antibody production and can directly kill infected host cells
The immune system does not recognize the whole microbe, but innumerable tiny pieces of it.
Each small segment of an antigen that elicits an immune response is called an epitope or an antigenic determinant.
measures the effectiveness by which an antigen elicits an immune response.
Proteins are the most effective antigens.
Form more diverse chemical forms and maintain their tertiary structures
Degrade pathogen proteins
Attach antigens to MHC for presentation
Link between innate immune response and adaptive immune response
Immunological specificity means that antibody made to one epitope will not bind to other epitopes.
However, cross-binding to similar epitopes can happen.
For example, the antibody to the cowpox virus will bind to a similar epitope on the smallpox virus.
This technique has been used in vaccination.
Antigens that can elicit antibody production by themselves
Small molecules that are not immunogenic on their own
Must first be covalently attached to a large carrier protein or other molecule
An antibody (immunoglobulin) is a Y-shaped structure made up of four polypeptides:
Two large heavy chains and two smaller light chains, connected by disulfide bonds
An antibody has:
highly conserved amino acids
Denoted CH and CL, for heavy and light chains
highly different amino acids
Denoted VH and VL, for heavy and light chains, respectively
Form the antigen-binding site
Heavy chains and light chains come in different types, which differ in their own constant regions:
There are five different types of heavy chains:
Alpha (a), mu (m), gamma (g), delta (d), and espilon (e)
But only two types of light chains:
Kappa (k) and lambda (l)
There are five classes of antibodies, which are defined by the type of heavy chain they possess:
IgG, IgM, IgA, IgD, and IgE
Differences in the constant regions may lead to:
shared by all members of a particular species
shared by some, but not all members
changes within the same antibody class, in the same individual
All antibody isotypes have the same basic structure.
However, each has a unique "super" structure.
a monomer with four subclasses
Most abundant antibody in blood and tissue fluids
most commonly found as a dimer
Secreted across mucosa
Abundant in secretions such as tears and breast milk
monomer (on B cells) or pentamer
First antibody detected during an immune response
Found abundantly on B cells; rare in blood
monomer found on mast cells and basophils
Mediates inflammatory response
Primary antibody response
Via disease or vaccination
Antibodies appear in serum after several days
-B cells that bind antigen make antibodies
-IgM, then IgG (isotype switching)
-Some B cells become memory cells
Secondary antibody response
Via a second exposure to pathogen or booster dose
Antibodies appear in blood within hours
A much bigger response, with mostly IgG
B Cells Differentiate into Plasma Cells by Clonal Selection
Each B cell circulating throughout the body or hidden in a lymphoid organ is programmed to synthesize antibody that reacts with a single epitope.
In a process called clonal selection, an invading antigen will inadvertently select which B-cell clone will proliferate to large numbers.
When a B cell contacts its related antigen, it is stimulated to proliferate and differentiate into plasma cells (which secrete antibodies) and memory cells.
serve as a link between humoral immunity and cell-mediated immunity.
They develop in the thymus and contain surface antigens different from those of B cells.
T-cells can be divided into two broad groups:
Helper T cells (TH cells)
Cytotoxic T cells (TC cells)
Helper T cells (TH cells)
Display the surface antigen CD4
Assist activation of B cells and other T cells
Cytotoxic T cells (TC cells)
Display the surface antigen CD8
Destroy bacteria and infected host cells
Helper T cells come in three types:
TH0: is the precursor that can differentiate into the other two types
responds to antigens from infected cells
Virus proteins replicating in cell
Cancer cells expressing mutated proteins
Activates cytotoxic T cells to destroy infected cells
responds to antigens in bloodstream
Bacterial, viral proteins in bloodstream
Activates B cells to produce antibodies
Aids in having invaders engulfed by phagocytes
Consists of membrane proteins with variable regions that can bind antigens
MHC proteins differ between species and among individuals within a species
They help determine whether a given antigen is recognized as self or nonself (foreign)
Two classes of MHC proteins are found on cell surfaces
Class I MHC
Class II MHC
Class I MHC
found on all nucleated cells
Class II MHC
found only on antigen-presenting cells
MHC class I presents INTRACELLULAR antigens:
Microbial proteins made in the host cytoplasm are degraded.
Peptides are imported into the ER and loaded onto MHC class I molecules.
MHC class II presents EXTRACELLULAR antigens:
Microbial proteins made outside the cell are endocytosed in an endosome.
They are then degraded and placed on MHC class II molecules.
T-cell receptors (TCRs)
are the antigen-binding molecules present on the surfaces of T cells
TCRs associate with CD3 proteins on cell surface
Bind antigens only if attached to MHC
Complex transduces signal into cell, triggering T-cell proliferation
TH0 binds MHC II on antigen-presenting cell:
recognition of antigen presenting cell (APC)
CD4 binds MHC II: recognition of presenting molecule
If T-cell receptor binds antigen:
Activation to become TH1 or TH2 cell
Depends on what cytokines are present
Clonal selection: proliferation of TH cells
Activate nearby cytotoxic T (TC) cells
Cell-Mediated Immune Response
Activated TC binds MHC I present on all cells.
CD8 binds MHC I: recognition of presenting molecule
If T-cell receptor binds presented antigen:
Cell is infected, must be destroyed
TC secretes perforin
Forms pore in target cell membrane
TC secretes toxic "granzymes" (indudes apoptosis)
Enter target cell through perforin-formed pores
Humoral Immune Response
TH0 binds MHC II on antigen-presenting cell
If T-cell receptor binds antigen
Activation to become TH2 cell
Proliferation of TH2 cells
B cells bind antigen with antibodies:
Endocytoses and presents antigen on MHC II
TH2 CD154 binds CD40
Recognition of B cell
CD4 binds MHC II
If T-cell receptor binds to antigen
Activated TH2 activates B cell:
Stimulates B-cell proliferation, differentiation
B cells undergo class switching:
Form plasma cells
Secreted antibodies coat extracellular antigens
Prevent virus binding to target cells
Form memory B cells
Reactivate if binds antigen again
Fast response—no need for TH2 binding
Sometimes the immune system may overreact to certain foreign antigens, or react against itself.
These immune miscues are called allergic hypersensitivity reactions, and the antigen causing the reaction is called an allergen.
Autoimmunity is the inability to distinguish between self-antigens and foreign antigens
Immune system reacts to host cells.
Autoreactive B cells escape clonal deletion.
TC cells can kill host cells that make self-protein closely resembling a foreign antigen.
Autoimmune diseases include:
Systemic lupus erythematosus
Type 1 diabetes
THIS SET IS OFTEN IN FOLDERS WITH...
Microbiology: an evolving science Chapter 23
Microbiology: An evolving science Chapter 25
Microbiology: an evolving science Chapter 9
Microbiology: An evolving science Chapter 12
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