Transcript Antigens and antibidies

Microbiology, virology, immunology
department
ANTIGENS AND
ANTIBODIES.
STRUCTURE OF IMMUNE
SYSTEM
By as. O.B. Kuchmak
The name antigens (Gk. anti against,
genos genus) is given to organic
substances of a colloid structure
(proteins and different protein
complexes in combination with lipids
or polysaccharides) which upon
injection into the body are capable of
causing the production of antibodies
and reacting specifically with them.
Antigenic substances must have certain
properties: a colloid structure,
and solubility in the body fluids.
Antigenic properties are pertinent to
 toxins of a plant origin (ricin, robin, abrin, cortin,
etc.),
 toxins of an animal origin (toxins of snakes,
spiders, scorpions, phalangia, karakurts, bees),
 enzymes,
 native foreign proteins,
 various cellular elements of tissues and organs,
 bacteria and their toxins, rickettsiae and
viruses.
Antigens, consequently, are
characterized by the following
main properties:
1. the ability to cause the
production of antibodies
(antigenicity),
2. the ability to enter into an
interaction with the
corresponding antibodies
(antigenic specificity).
The features of molecules that
determine antigenicity and
immunogenicity are as follows:
1.Foreignness.
2.Molecular Size.
3.Chemical-Structural Complexity
4.Antigenic Determinants
(Epitopes)
Foreignness: In general,
molecules recognized as "self”
are not immunogenic; ie, we are
tolerant to those self-molecules.
To be immunogenic, molecules
must be recognized as "nonself,"
ie, foreign.
Molecular Size: The most potent
immunogens are proteins with high
molecular weights, ie, above 100,000.
Generally, molecules with molecujar
weight below 10,000 are weakly
immunogenic, and very small ones, eg,
an amino acid, are nonimmunogenic.
Certain small molecules, eg, haptens,
become immunogenic only when linked
to a carrier protein.
Chemical nature:
Protein and polysaccharides are most
antigenic. Lipids and nucleic acids are
less antigenic. Their antigenicity is
enhanced by combination with proteins.
A certain amount of chemical complexity
is required, for example, amino acid
homopolymers are less immunogenic than
heteropoymers containing two or three
different amino acids.
Antigenic Determinants
(Epitopes):
Epitopes are –
small chemical groups on the
antigen molecule that can elicit and
react with antibody.
An antigen can have one or more
determinants. Most antigens have
many determinants; ie, they are
multivalent.
Antigen epitopes
epitopes
Antigen
Hapten (incomplete antibody)
is a molecule that cannot induce
an immune response by itself
but can react with specific
antibody.
Nevertheless, haptens can induce
a response if combined with larger
molecules (normally proteins) which
serve as carrier.
Haptens are usually small. Many
drugs, eg, penicillins, are haptens,
and the catechol in the plant oil that
causes poison oak and poison ivy is a
hapten.
hapten
Immunogenic antigen
epitope
protein
No antibidies
produced
Antibody against
epitope on antigen
Immunogenic antigen as carier for hapten
epitope
hapten
Antibody against
epitope on hapten
Antibody against
epitope on antigen
It is well known that the properties of
chemical,
structural
and
functional
specificity are inherent in all natural
proteins. Proteins of different species of
animals, plants, bacteria, rickettsiae and
viruses
can
be
differentiated
by
immunological reactions. The antigenic
function of bacteria, rickettsiae and
viruses is characterized not only by
species, but also by type specificity.
A typical antigen:antibody reaction: gramnegative bacterial pathogen may have several
antigens, or immunogens (flagella, pili and cell
wall)
Antigenic structure of a bacterium
Antigenic structure of a virus
H
N
RNA
Protein М2
Protein М1
Lipid membrane
Antigenic properties of bacteria,
toxins, rickettsiae and viruses,
used in the practice of reproducing
artificial
immunity
against
infectious diseases, are of most
practical importance.
When the antigenic structures of
the host are similar to those of the
causative agent, the macroorganism
is incapable of producing immunity,
as the result of which the disease
follows a graver course. It is
possible that in individual cases the
carrier state and inefficacy of
vaccination are due to the common
character of the microbial antigens
and the antigens of the person's
cells.
It has been established that human
erythrocytes have antigens in
common with staphylococci,
streptococci, the organisms of
plague, E. coli. Salmonella paratyphi,
Shigella organisms, smallpox and
influenza viruses, and other
causative agents of infectious
diseases. Such a condition is called
antigenic mimicry.
Isoantigens are those substances
which have antigenic properties and are
contained in some individuals of a given
species.
Isoantigens of leukocytes, blood
platelets, lymphocytes, granulocytes,
blood sera, liver, and kidneys and interorgan (cell nuclei, mitochondria,
ribosomes, etc.) and pathological
(cancerous, bum, radiation) isoantigens
have been revealed.
Isoantigens have been found in the
erythrocytes of animals and man. At first
it was established that in human
erythrocytes there are two antigens (A and
B), and in the sera — beta- and alphaantibodies. Only heterogenic antigens and
antibodies (agglutinins) can be found in
human blood.
On the basis of antigenic structure the
erythrocytes of all people can be
subdivided into 4 groups (A, B, AB and 0).
These data are taken into account
during blood transfusion.
Autoantigens are substances
capable of immunizing the body from
which they are obtained.
Self antigens
 are ordinarily non-antigenic,
 not normally found free in circulation
or tissue fluids,
 are not recognized as self antigens.
Thus, they become modified and are
capable of bearing an antigenic function.
Autoantigens
These substances include
 the eye lens,
 spermatozoids,
 homogenates of the seminal gland,
 skin,
 emulsions of kidneys, liver, lungs
and other tissues.
Autoantigens
Under ordinary conditions they do
not come in contact with the
immunizing systems of the body,
therefore antibodies are not produced
against such cells and tissues. However,
if these tissues are injured, then
autoantigens may be absorbed, and may
cause the production of antibodies
which have a toxic effect on the
corresponding cells.
The origination of autoantigens is
possible under the influence of
 cooling, radiation,
 drugs (amidopyrine, sulphonamides,
preparations of gold, etc.),
 virus infections "(virus pneumonias and
mononucleosis),
 acterial proteins and toxins of
streptococci, staphylococci, tubercle
bacilli, paraproteins,
 aseptic autolysis of brain tissue,
 and other factors.
Histocompatibility antigens
There are the antigens present on the
cells of each individual of species . These
proteins are alloantigens; ie, they differ
among members of the same species.
These antigens are encoded by genes
known as histocompatibility genes which
collectively constitute major
histocompatibility complex (MHC), These
are located on the short arm of
chromosome 6.
Major Histocompatibility Complex
MHC products present on the
surface of leucocytes are known as
human leukocyte antigens (HLA).
The success of tissue and organ
transplants depends on the donor's
and recipient's human leukocyte
antigens (HLA). If the HLA proteins
on the donor's cells differ from
those on the recipient's cells, an
immune response occurs in the
recipient.
Major Histocompatibility Complex
Three of these genes (HLA-A, HLA-B,
and HLA-C) code for the class I MHC
proteins. Several HLA-D loci determine the
class II MHC proteins, ie, DP, DQ, and DR.
There are many alleles of the class I and
class II genes. For example, there are at
least 47 HLA-A genes, 88 HLA-B genes, 29
HLA-C genes, and more than 300 HLA-D
genes, but any individual inherits only a
single allele at each locus from each parent
and thus can make no more than two class I
and II proteins at each gene locus.
Expression of these genes is codominant,
ie, the proteins encoded by both the
paternal and maternal genes are produced.
Each person can make as many as 12 HLA
proteins: 3 at class I loci and 3 at class II
loci, from both chromosomes. Between the
class I and class II gene loci is a third locus,
sometimes called class III. This locus
contains several immunologically important
genes, encoding two cytokines (tumor
necrosis factor and lymphotoxin) and-two
complement components (C2 and C4).
Class I MHC Proteins. These are
glycoproteins found on the surface of
virtually all nude ated cells. There are
approximately 20 different proteins
encoded by the allelic genes at the A
locus, 40 at the B locus, and 8 at the C
locus.
Class II MHC Proteins. These are
glycoproteins found on the surface of
certain cells, including macrophages, B
cells, dendritic cells of the spleen, and
Langerhans cells of the skin.
BIOLOGIC IMPORTANCE OF MHC
The ability of T cells to recognize
antigen is dependent on association of
the antigen with either class I or class
II proteins. For example, cytotoxic T
cells respond to antigen in association
with class I MHC proteins. Thus, a
cytotoxic Tcell that kills a virusinfected cell will not kill a cell infected
with the same virus if the cell does not
also express the appropriate class I
proteins.
BIOLOGIC IMPORTANCE OF MHC
MHC genes and proteins are also
important in two other medical
contexts. One is that many autoimmune
diseases occur in people who carry
certain MHC genes, and the other is
that the success of organ transplants
is, in large part, determined by the
compatibility of the MHC genes of the
donor and recipient.
Disease
HLA
Addison’s disease
DR5
Behtyer’s
disease
of a
presence
at healthy
persons
Antigene on which
the immune answer
develops
70
20
Adrenal cortex
B27
89
9
Unknown
Hashimoto’s
thyreoiditis
DR3, DR5
51
24
Thyroglobulin
Juvenile
diabetes
DR3, DR4
72
24
Insulin receptor
Rheumatoid
arthritis
DR7,
59
21
Colagen, Fc
fragment of IgG
Frequency of
an antigene
at ill
persons
DR21
Narcolepsy
DR2
100
34
Unknown
Goodpasture’s
syndrom
DR2
88
29
Basement
membrane of a
kidney and lung
ANTIBODIES (IMMUNOGLOBUL1NS)
are globulin proteins (immunoglobulins) that
react specifically with the antigen that
stimulated their production. They make up
about 20% of the protein in blood plasma.
Blood contains three types of globulins:
alpha, beta, and gamma based on their
electrophoretic migration rate. Antibodies
are gamma globulins.
IMMUNOGLOBUL1N STRUCTURE
Immunoglobulins are glycoproteins made
up of light (L) and heavy (H) polypeptide
chains. The terms "light" and heavy" refer to
molecular weight; light chains have a
molecular weight of about 25,000, whereas
heavy chains have a molecular weight of
50,000-70,000. The simplest antibody
molecule has a Y shape and consists of four
polypeptide chains: two H chains and two L
chains. The four chains are linked by
disulfide bonds. An individual antibody
molecule always consists of identical H chains
and identical L chains.
IMMUNOGLOBUL1N STRUCTURE
If an antibody molecule is treated with a
proteolytic enzyme such as papain, peptide
bonds in the "hinge" region are broken,
producing two identical Fab fragments, and
one Fc fragment.
The variable regions are responsible for
antigen-binding,
whereas the constant
regions are responsible for various biologic
functions, eg, complement activation and
binding to cell surface receptors, placental
transfer.
Structure of IgG
H-chain
L-chain
disulfide bond
Variable
Варіабельна
ділянка
Region
(Fab)
"hinge"
region
Constant
region (Fc)
L and H chains are subdivided into
variable and constant regions. The regions
are composed of three-dimensionally
folded, repeating segments called domains.
An L chain consists of one variable (VL)
and one constant (CL) domain. Most H
chains consist of one variable (VH) and
three constant (CH) domains.
The variable regions of both L and H
chains have three extremely variable
("hypervariable") amino acid sequences at
the amino-terminal end that form the
antigen-binding site.
There are five classes of antibodies:
Ig G, Ig M, Ig A, Ig D, and Ig E.
L chains belong to one of two types,
k (kappa) or λ (lambda), on the basis of
amino acid differences in their constant
regions. Both types occur in all classes of
immunoglobulins.
H chains are distinct for each of the
five immunoglobulin classes and are
designated γ (IgG), μ (IgM) , α ( IgA),
ε ( IgE), and δ ( IgD).
IMMUNOGLOBULIN CLASSES
Ig G. Each IgG molecule consists of two L
chains and two H chains linked by disulfide bonds
(molecular formula H2L2). Because it has two
identical antigen-binding sites, it is said to be
divalent.
IgG is the predominant antibody in the
secondary-response and constitutes an important
defense against bacteria and viruses. IgG is the
only antibody to cross the placenta. Only its Fc
portion binds to receptors on the surface of
placental cells. It is therefore the most abundant
immunoglobulin in newborn. IgG is one of the two
immunoglobulins that can activate complement;
IgM is the other. IgG is the immunoglobulin that
opsonizes.
Ig A is the main immunoglobulin in
secretions such as colostrum, saliva, tears,
and respiratory, intestinal, and genital tract
secretions. It prevents attachment of
microorganisms, eg, bacteria and viruses, to
mucous membranes.
Each secretory IgA molecule consists of
two H2L2 units plus one molecule each of J
(joining) chain and secretory component. The
secretory component is a polypeptide synthesized by
epithelial cells that provides for IgA passage to the
mucosal surface. It also protects IgA from being
degraded in the intestinal tract. In serum, some
IgA exists as monomeric H2L2.
Ig M.
IgM is the main immunoglobulin
produced early in the primary response. It is
present as a monomer on the surface of virtually
all B cells, where it functions as an antigenbinding receptor. In serum, it is a pentamer
composed of 5 H2L2 units plus one molecule of J
(joining) chain. Because the pentamer has 10
antigen-binding sites, it is the most efficient
immunoglobulin in agglutination, complement
fixation (activation), and other antibody
reactions and is important in defense against
bacteria and viruses. It can be produced by the
fetus in certain infections. It has the highest
avidity of the immunoglobulins; its interaction
with antigen can involve all 10 of its binding sites.
Ig E. Ig E is medically important for two
reasons: (1) it mediates immediate (anaphylactic)
hypersensitivity, and (2) it participates in host
defenses against certain parasites, eg, helminths
(worms). Although Ig E is present in trace amounts
in normal serum (approximately 0.004%), persons
with allergic reactivity have greatly increased
amounts, and Ig E may appear in external
secretions. Ig E does not fix complement and does
not cross the placenta.
Ig D. This immunoglobulin has no known antibody
function but may function as an antigen receptor;
it is present on the surface of many B
lymphocytes. It is present in small amounts in
serum.
The Structures of the Different Classes of Antibodies
Ig
Major Functions
Ig G
Main antibody in the secondary response. Opsonizes
bacteria, making them easier to phagocytize. Fixes
complement, which enhances bacterial killing. Neutralizes
bacterial toxins and viruses. Crosses the placenta.
Ig A
Secretory IgA prevents attachment of bacteria and viruses
to mucous membranes Does not fix complement.
Ig M
Produced in the primary response to an antigen. Fixes
complement. Does not cross the placenta. Antigen receptor
on the surface of B cells.
Ig D
Uncertain. Found on the surface of many B cells as well as
in serum.
Ig E
Mediates immediate hypersensitivity by causing release of
mediators from mast cells and basophils upon exposure to
antigen (allergen). Defends against worm infections by
causing release of enzymes from eosinophils. Does not fix
complement. Main host defense against
helminth
infections.
Immune responce
Primary response. After first injection of the
antigen there is a long lag phase of several days
before antibody appears.
Secondary immune response. If the same
host is subsequently exposed to the same
antigen, then the secondary immune response
is usually mote pronouced and occur more
rapidly. Because of the availability of specific
memory cells, an increased number of effector
cells are produced.
The antibody formed in primary response is
predominantly IgM and in secondary response
IgG.
Primary and Secondary Responses to an Antigen
Primary and Secondary Responses to an Antigen
Structure of the immune
system
Lymphoid organs can be classified
into
primary (central) lymphoid organs
secondary (peripheral) lymphoid
organs
Structure of the immune system
Primary (central) lymphoid organs:
 thymus
 avian bursa of Fabricius (the
equivalent of it in mammals is bone
marrow)
 foetal liver (during embrionic
development)
Structure of the immune system
Secondary (peripheral) lymphoid organs:
 lymph nodes
 spleen
 mucosa-associated lymphoid tissues
ORIGIN OF IMMUNE CELLS
The capability of responding to
immunologic stimuli rests mainly with
lymphoid
cells.
During
embryonic
development,
blood
cell
precursors
originate mainly in the fetal liver and yolk
sac; in postnatal life, the stem cells reside
in the bone marrow. Stem cells
differentiate into cells of the erythroid,
myeloid, or lymphoid series. The latter
evolve
into
two
main
lymphocyte
populations: T cells and B cells. The ratio
of T cells to B cells is approximately 3:1.
Differentiation of stem cells into B cells and T cells.
This occurs in the bone marrow and thymus.
T cells
T cell precursors differentiate into
immunocompetent T cells within the thymus. Stem
cells lack antigen receptors and CD3, CD4, and CD8
molecules on their surface, but during passage
through the thymus they differentiate into T cells
that can express these glycoproteins. The stem
cells, which initially express neither CD4 nor CD8
(double-negatives), first differentiate to express
both CD4 and CD8 (double-positives) and then
proceed to express either CD4 or CD8. A doublepositive cell will differentiate into a CD4-positive
cell if it contacts a cell bearing class II MHC
proteins but will differentiate into a CD8-positive
cell if it contacts a cell bearing class I MHC
proteins.
Lymphoid progenitors migrate from the bone marrow
to
the thymus where they develop into mature T cells
Thymocytes are intimately associated with epithelial cells as they
develop in the thymus
B cells perform two important functions: (1) They
differentiate into plasma cells and produce antibodies,
and (2) they are antigen-presenting cells (APCs).
Origin. During embryogenesis, B cell precursors are
recognized first in the fetal liver. From there they
migrate to the bone marrow, which is their main
location during adult life. Unlike T cells, they do not
require the thymus for maturation. Pre-B cells lack
surface immunoglobulins and light chains but do have μ
heavy chains in the cytoplasm. The maturation of B
cells has two phases: the antigen-independent phase
consists of stem cells, pre-B cells, and B.cells, whereas
the antigen-dependent phase consists of the cells that
arise subsequent to the interaction of antigen with the
B cells, eg, activated B cells and plasma cells. B cells
display surface IgM, which serves as a receptor for
antigens.
Pre-B cells are found in the bone
marrow, whereas B cells circulate in the
blood stream. B cells constitute about 30%
of the recirculating pool of small
lymphocytes, and their life span is short,
ie. days or weeks. Within lymph nodes,
they are located in germinal centers;
within the spleen, they are found in the
white pulp. They are also found in the gutassociated lymphoid tissue, eg, Peyer's
patches.
T Cells
perform several important functions, which can be
divided into two main categories, namely, regulatory and effector.
The regulatory functions are mediated primarily by helper (CD4positive) T cells, which produce interleukins. For example, helper T
cells make (1) interleukin-4 (IL-4) and IL-5, which help B cells produce
antibodies; (2) IL-2, which activates CD4 and CD8 cells; and (3)
gamma interferon, which activates macrophages,
CD4 and CD8 Types of T Cells. Within the thymus, perhaps
within the outer cortical epithelial cells (nurse cells), T cell
progenitors differentiate under the influence of thymic hormones
(thymosins and thymopoietins) into T cell subpopulations. These cells
are characterized by certain surface glycoproteins, eg, CD3, CD4, and
CD8. All T cells have CD3 proteins on their surface in association
with antigen receptors (T cell receptor [see below]). The CD3 complex
of five transmembrane proteins is involved with transmitting, from
the outside of the cell to the inside, the information that the antigen
receptor is occupied. CD4 is a single transmembrane polypeptide
whereas CD8 consists of two transmembrane polypeptides.
T
Cells
perform several important
functions, which can be divided into two main
categories, namely, regulatory and effector.
The regulatory functions are mediated
primarily by helper (CD4-positive) T cells,
which produce interleukins. For example,
helper T cells make
(1) interleukin-4 (IL-4) and IL-5, which help B
cells produce antibodies;
(2) IL-2, which activates CD4 and CD8 cells;
and
(3) Gamma-interferon,
which
activates
macrophages, effect delayed hypersensitivity
(eg, limit infection by M tuberculosis).
These
functions
are
performed
by
2
subpopulations of CD4 cells: Th-1 cells help
activate cytotoxic T cells by producing IL-2 and
help initiate the delayed hypersensitivity response
by producing primarily IL-2 and gamma interferon,
whereas Th-2 cells perform the B cell helper
function by producing primarily IL-4 and IL-5. One
important regulator of the balance between Th-l
cells and Th-2 cells is interleukin-12 (IL-12), which
is produced by macrophages. IL-12 increases the
number of Th-1 cells, thereby enhancing host
defenses against organisms that are controlled by a
delayed
hypersensitivity
response.
Another
important regulator is gamma interferon which
inhibits the production of Th-2 cells. CD4 cells make
up about 65% of peripheral T cells and predominate
in the thymic medulla, tonsils, and blood.
CD 8 lymphocytes perform cytotoxic
functions; that is, they kill virus
infected, tumor, and allograft cells.
They kill by either of two mechanisms,
namely, the release of performs, which
destroy cell membranes, or the
induction of programmed cell death
(apoptosis).
CD 8 cells predominate in human
bone marrow and gut lymphoid tissue
and constitute about 35% of peripheral
T cells.
Types of T cells: After T cells are challenged by
antigens, the cells differentiate into one of several types
of functioning T cells
T Cell Receptor
(TCR) for antigen consists of
two polypeptides, alpha and beta ,which are
associated with CD3 proteins. TCR proteins are
similar to immunoglobulin heavy chains.
Note that each T cell has a unique T cell
receptor on its surface, which means that hundreds
of millions of different T cells exist in each person.
Activated T cells, like activated B cells, clonally
expand to yield large numbers of cells specific for
that antigen.
Although TCRs and immunoglobulins are
analogous in that they both interact with antigen in
a highly specific manner, the T cell receptor is
different in two important ways: (1) it has two
chains rather than four, and (2) it recognizes
antigen only, in conjunction with MHC proteins,
whereas immunoglobulins recognize free antigen.
Antibodies produced
by humoral immune
responses eliminate
foreign agents in three
ways:
1. Neutralization
2. Opsonization
3. Immune complexes
Summary of Humoral Immunity
The Reactions in Cell-Mediated Immunity
Colorized SEM of a small T lymphocyte attacking
two large tumor cells