Antigens and Antibodies Flashcards
Define immunogen.
macromolecules capable of triggering an adaptive immune response by inducing formation of antibodies or sensitized cells in an immunocompetent host
What is the action of an antigen?
reacts with antibody and/or sensitized cells but may not be able to invoke an immune response in the first place
What is the rule regarding the relationship between immunogens and antigens?
all immunogens are antigens, but not all antigens are immunogens
What is the ability of an antigen to stimulate a host response (immunogenicity) dependent on?
macromolecular size (molecular weight), chemical composition, molecular complexity, foreignness
What is the general rule regarding molecular weight and reactivity, and what is the minimum weight an antigen must be to be recognized?
the greater the molecular weight, the more potent the molecule is; 10,000 Daltons or more
What are considered the “best” materials for antigens to be made of?
proteins and polysaccharides
What makes carbohydrates less immunogenic than proteins?
the number of sugar units in carbohydrates are more limited than the number of amino acids in proteins
Can nucleic acids and lipids be immunogenic? If so, under what conditions?
yes, if they occur in conjunction with other substances
What makes proteins the ideal antigen material?
the variety of amino acids can arrange themselves into an enormous variety of 3D molecules
What makes synthetic polymers such as nylon and Teflon a good material for artificial medical appliances, such as heart valves?
they are made of up a few simple repeating units with no bending or folding within the molecule; therefore, they are non-immunogenic
What is the most common form of antigens composed of carbohydrates?
glycolipids or glycoproteins
Which blood group antigens are glycolipids?
A, B, H
Which blood group antigens are glycoproteins?
Rh and Lewis
What are epitopes?
small, molecular shapes or configurations on antigens that are actually recognized by antibodies or T-cells
What is another name for epitope?
determinant site
What is the relationship between molecular size and epitope number?
larger molecules may have more epitopes than smaller molecules; generally speaking, there is approximately one epitope for every increase of 10,000 Daltons in molecular weight
What is the dominant epitope?
the most exposed region of a molecule; the immune response is almost always directed against this particular area’s surface protein components
Are all of the epitopes on a single molecule the same?
they can be; also capable of having differing specificities
What is a linear epitope?
amino acids following one another on a single chain
What is a conformational epitope?
results from the folding of one or multiple chains, bringing certain amino acids from different segments into close proximity so they can be recognized together
What is the difference between B-cell (antibody) and T-cell recognition of antigens?
B-cells are capable of recognizing antigens in their original state; for T-cells, the antigen-presenting cell must degrade the immunogen into small peptides before T-cells can recognize it
What are haptens?
non-immunogenic materials that, when combined with a carrier, create new antigenic determinants; they are too small to be recognized on their own
Once antibody formation has been initiated, can a hapten react with that antibody even if it is not complexed?
yes, but it will not produce an agglutination or precipitation reaction; because a hapten has only one determinant site, it cannot form the cross-links with more than one antibody that are required to produce such a reaction
What are autoantigens?
antigens that belong to the host that do not produce an immune response under normal circumstances
What are alloantigens?
antigens from other member’s of the host’s species that are capable of eliciting immune responses; these play an important role in tissue transplants and blood transfusions
What are heteroantigens?
antigens from other species, such as animals, plants, or microorganisms
What are heterophile antigens?
heteroantigens that exist in unrelated plants or animals but are either identical or closely related in structure, so that antibody to one will react with the other (cross-reactivity)
What are adjuvants and how do they work?
Substances administered with an antigen that increase the immune response, such as metal salts made from aluminum or calcium. These complex with the antigen and increase its size to prevent a rapid escape from the tissues, which prolongs its existence in the area and increases the number of macrophages involved in the antigen processing
What are human leukocyte antigens (HLA)?
antigens found on all nucleated cells in the body that play a key role in the immune response
What is the major histocompatibility complex (MHC)?
the genes controlling the expression of human leukocyte antigens by coding for proteins that regulate the immune system
What is the main function of the MHC, and what specific conditions does it play a role in?
brings antigen to the cell surface for T-cell recognition; plays a role in graft rejection, transfusion reactions, and autoimmune diseases
Where are the MHC genes found and how are they categorized?
found on chromosome 6; Classes I, II, and III
Describe Class I MHC genes, including where the molecules are found.
coded for at three different loci termed A, B, and C; expressed on all nucleated cells
Describe Class II MHC genes, including where the molecules are found.
coded for in the D region with at least three different loci known as DR, DQ, and DP; found primarily on B-cells, activated T-cells, monocytes, macrophages, dendritic cells, and endothelium
Describe Class III MHC genes.
code for complement proteins
What kind of cells do Class I molecules present antigen to?
CD8 (cytotoxic) T-cells
What kind of cells involve Class II molecules in antigen recognition?
CD4 (helper) T-cells
What is the main role of Class I molecules?
bind peptides that have been synthesized inside the cells by viruses, tumors, and intracellular pathogens and transport them to the plasma membrane where they can be recognized by the T-cells
What is the main role of Class II molecules?
bind exogenous proteins that have been taken into the cells from the outside and degraded (ex., bacteria attacking cells from the outside)
What is a haplotype?
the genes inherited at a particular chromosomal region
How likely is it that two individuals will express the same human leukocyte antigens?
very low; due to the number of genes in the haplotype and the way in which they are inherited, an individual’s MHC type is about as unique as a fingerprint; this makes organ donation matching extremely difficult
How would one go about resolving a discrepancy caused by cross-reactivity?
perform an adsorption on the serum to remove all but the antigen intended for testing
What are immunoglobulins?
substances produced in response to an antigenic stimulation that are capable of specific interaction with the provoking antigen
What property distinguishes and antibody from an immunoglobulin?
antibodies have specificity for a particular antigen
Which heavy chain type is found in the IgG class?
gamma
Which heavy chain type is found in the IgA class?
alpha
Which heavy chain type is found in the IgM class?
mu
Which heavy chain type is found in the IgD class?
delta
Which heavy chain type is found in the IgE class?
epsilon
What is the constant region of an immunoglobulin molecule associated with?
the biological properties that differ between each immunoglobulin class
What is the variable region of an immunoglobulin molecule associated with?
provides the specificity for antigen binding
What chemical property forms the connecting links within an immunoglobulin molecule?
disulfide bonds
What is the function of the internal disulfide links found in the heavy and light chains of immunoglobulins?
they form loops in the peptide chains that compactly fold to form globular domains; these domains can in turn activate complement, bind antigen, and adhere to the monocyte surface, among other things
Which immunoglobulin is found in the highest concentration in normal adults?
IgG
What is the main function of IgG?
antibody capable of destroying bacteria, neutralizing viruses, and precipitation reactions; also make up som hemagglutinins and hemolysins
Which immunoglobulin can cross the placenta?
IgG
Which subclasses of IgG fix complement?
1-3; 4 does not
Is IgG capable of opsonization?
yes
Which immunoglobulin is the largest?
IgM; made up of five basic structural units in a circular arrangement (pentameter)
How can the bonds holding an IgM molecule together be broken?
using 2-mercaptoethanol
What is the name of the chains that hold the IgM pentameter together?
J (joining) chains
Which immunoglobulin is associated with primary response?
IgM
Which immunoglobulin is associated with secondary (anamnestic) response?
IgG
What class of immunoglobulin is first produced by infants during development?
IgM
What is the relationship between IgM and IgG?
synthesis of IgM decreases as concentration of IgG increases
Which class of antibody is most often formed in response to Gram-negative bacteria, and what makes it so effective?
IgM; multiple binding sites
What four components are characteristically IgM in nature?
heterophile antibody, rheumatoid factor, cold agglutinins, and hemagglutinins
Can IgM activate complement?
yes, better than IgG
What is the main immunoglobulin found in various body secretions, such as saliva, tears, sweat, and breast milk?
IgA, also known as secretory IgA
What is the nature of the secretory component of IgA?
thought to protect the IgA dimer from enzyme destruction, and aid in transport across mucus membranes into secretions
What is the main function of IgA?
binds antigen at portals of entry, and confers immunity from mother to baby through breast milk
Which subclass is predominant?
IgA2
Can IgA fix complement?
no
Which immunoglobulin is found in the lowest concentration in the body?
IgE
What makes IgE unique?
has an extra domain that is capable of binding with receptors on mast cells and basophils
What effect does the attachment of IgE to mast cells and basophils have on the body in regards to allergic reactions?
mast cells and basophils release vasoactive amines/pharmacologic agents (histamine and heparin) which are important chemical mediators of allergic reactions; the interaction of IgE and active mast cells or basophils stimulates the development of an immediate physiologic response from life-threatening (anaphylaxis) to milder discomforts associated with allergies
What role is IgE thought to have in the defense of parasitic infections?
recruits eosinophils to areas of infection
Where are the major tissues that form IgE located?
respiratory and GI tract
Can IgE fix complement?
no
What is the synonym for IgE and what is important to remember about it?
reagin; this is NOT the same reagin as that found in syphilis
Does IgE participate in agglutination or opsonization?
no
Which immunoglobulin class was discovered in 1965 in a patient with multiple myeloma?
IgD
Where is most IgD found?
on the surface of immunocompetent but unstimulated B-cells
What is thought to be the role of IgD?
it is the second immunoglobulin to appear (and therefore may play a role in) B-cell activation, maturation, and differentiation
Can IgD fix complement?
no
What is the Fab fragment?
the site of antigen binding on an immunoglobulin (Fab = fragment antigen binding)
Is the Fab fragment capable of causing agglutination or precipitation reactions?
no
What is the Fc fragment?
the site of complement attachment/activation on an immunoglobulin (Fc = fragment crystallizable)
Which proteolytic enzyme can be used to break apart IgG molecules into three equal parts?
papain
Which proteolytic enzyme can be used to break apart IgG at the carboxy-terminal side of the interchain disulfide bonds?
pepsin
What does a treatment of IgG with pepsin yield?
a single fragment weighing approximately 100,000 Daltons with all of the antigen-binding ability (known as Fab2), and an additional fragment known as Fc3
