Microbiology – Chapter 15
The Immune system: Specific defenses –
Specific immunity
The immune system is a network of cells
and organs that extends throughout the
body and functions as the third line of
defense against invaders. It is a specific
response and it generates specific
chemicals to counteract invaders.
1. Foreign substances are antigens
2. Proteins made by the body in response
are called antibodies
Microbiology – Chapter 15
Two types of immunity:
1. Active: the individual own immune system
produces the immunity
A. Naturally acquired: by disease
B. Artificially acquired: vaccination
2. Passive: either have antibodies passed from
mother to child or having immune globulin
administered medically
Microbiology – Chapter 15
Antigens: The body can recognize materials as non-self
or foreign material. These materials are called
antigens.
1. Generally protein or large polysaccharides, nucleic
acids or lipids are antigenic only if combined with
protein or polysaccharides
2. Any cell, part of a cell, or chemical that induces
an
immune response bythe B-cells or Tcells
(lymphocytes), is called antigenic.
3. Usually large molecules 10,000 mw, in many cases the
antigen is some particular part of a cell – like a cell wall
polysaccharide, capsule material,
flagella, or fimbriae
Microbiology – Chapter 15
Antigens: The body can recognize materials as non-self
or foreign material.
4. Viral protein, pollen, other protein (egg or milk protein)
can cause an immune response and are antigenic.
5. Antibodies tend to react with specific parts of an antigen
– called and antigenic determinant or epitope. Size
and shape; lock-key just like in enzyme substrate
interactions.
6. Small molecules that are too small to cause an immune
response are called haptens. Penicillin is an
example. By itself, too small to be antigenic, but
it combines with serum proteins and then can
become antigenic (penicillin allergy )
Fig. 15.8
Microbiology – Chapter 15
Antibodies are produced by Lymphocytes
1. T or B cell lymphocytes recognize foreign material as
antigens.
2. They have receptor sites on their cell surface that bind
antigens in order to eliminate them as foreigners.
3. The antibody molecules are large proteins that are
specific in size and shape to interact chemically with
their particular antigen.
Antibody structure: several kinds of antibodies (chart
on pg ____)
Microbiology – Chapter 15
G M A D E
1. IgG – Monomer, simple antibody, Y shaped, composed of two heavy
chains and two light chains,
a. The “y” ends have a variable region, amino acid sequence can
vary,
thus allowing specific interaction with their specific antigen – they
have two antigen binding sites
b. Constant region, on the molecule’s stem, this c region is called
constant, it can be different (actually have 5 different c region types –
giving 5 different types of antibodies)
c. IgG- most prevalent ab, found in blood and it is called monomer for
its simple shape
d. When acting on antigen, enhances phagocytosis, neutralizes toxins
e. Since it is small, it passes the placenta and provides passive
immunity to infants
Microbiology – Chapter 15
G M A D E
2. IgM – Pentamer, composed of 5 monomers (5 Y
monomer units)
a. Large and stays in blood stream or attaches to
blood cells
b. The first kind of ab to appear after an antigenic
challenge
c. Involved in clumping (agglutination) reactions,
works with complement and clumps antigens and
cells, so the can be easily phagocytized
d. Kind of reaction seen with ABO blood grouping
Microbiology – Chapter 15
G M A D E
3. IgA – Secretory antibody, secreted along epithelium
linings
a. Found in respiratory tract, GI tract, mother’s milk
b. Localized protection
4. IgD – function not well known, found on b cell surfaces,
may function in initiation of immune response (B
cell activation)
5. IgE – bound antibodies, found on surfaces of mast cells,
stimulates inflammatory response, may be a trigger
for allergic response
Microbiology – Chapter 15
The immune response: B cell immunity (Humoral
immunity) **** Know for test ****
The B cells are lymphocytes that develop from stem cells
located in the red bone marrow.
1. In embryonic development, stem cells differentiate
into b cells
2. Some move to thymus gland and become T cells
3. Both B and T cells later migrate to other lymph
tissue (lymph nodes, spleen)
4. When B cells are exposed to antigens, they are
activated, they start to divide and become a clone
of many effector cells called plasma cells
Microbiology – Chapter 15
The B cells are lymphocytes that develop from stem cells located in the
red bone marrow.
5. The plasma cells produce the antibodies that counteract the
specific antigen that activated the original B cells
6. Theory of antibody production – Clonal selection
a. During development the B cells undergo tremendous genetic
recombination that results in literally millions of different
receptor sites on their surfaces. These receptor sites are able
to bind with the specific shape of specific antigens.
b. Because of the tremendous number of potential genetic
combinations on the gene regions that code for these antigen
recognition sites – millions of possibilities – result – millions of
genetically different B cells
Microbiology – Chapter 15
The B cells are lymphocytes that develop from
stem cells located in the red bone marrow
c. Recognition – an antigen enters the host, only one or a
few b cells have a site on its surface that fits that
antigen (better the fit, the betterthe immune
response) – antigenic selection – antigen selects
its B cell
d. The specific matching b cell is now activated and
undergoes cell division into many cells (a clone) –
*****Clonal selection*****
e. See pg ___ -know this for the test – be able to
diagram andexplain
Microbiology – Chapter 15
i. Recognition
ii. Activation
iii. Proliferation
iv. Differentiation (plasma cells, memory cells)
v. Production of antibodies (secreted into plasma)
vi. Memory cells – long lived cells, survive and can
respond very quickly if encounter antigen again
(immunological memory)
Microbiology – Chapter 15
The first reaction – recognition - activation –proliferation - etc. Takes
time - This is the primary response – 1 to 2 weeks (pg____)
Secondary response is very quick – memory B cells can respond
quickly to produce more b cells and antibodies, just a few days
How does the body know the difference between self and non-self
material??
Still a mystery – self tolerance, the forbidden clone?? Maybe
the B and T cells that are exposed to self antigens are
destroyed in fetal development when they pass through the
thymus gland (clonal deletion)
Microbiology – Chapter 15
T cell response
T cells – T lymphocytes, cell mediated immunity – just
touch on it a little
1. T cell activation is different than that of B cells
2. The antigen that is presented to the T cell is first
processed by antigen presenting cells – usually
a macrophage
a. Macrophage encounters antigen – ingests it,
digests it and then seperates it into antgenic
determinants
b. Antigenic determinants migrate to the phagocyte
cell surface
c. Antigens are held on surface of the phagocyte
Microbiology – Chapter 15
3. The correct T cell encounters the antigen on the phagocyte and
is activated
4. T cells then can divide and differentiate into different types of T cells
(be able to list and give a function)
a. T-helper cells (Th) – most prevalent, secrete lymphokines
(interleukins – chemical messengers between cells of
the immune system) see pg ____
b. Cytotoxic T cells (Tc) – destroy target cell when in contact
(virus infected cell killed before virus can replicate)
c. T suppressor cells (Ts) – may regulate the immune
response, turns it off when not needed
d. Td – delayed hypersensitive cells – cause inflammation
reaction associated with hypersensitivity, and some of
the problems with tissue rejection
Microbiology – Chapter 15
Diagnostic Immunlogy
Many diagnostic tests in microbiology are based on immunology
1. Ag – Ab reactions can b used to determine presence of infection or
an exposure to an antigen
2. Hiv test, Hep C are used to determine exposure to these viruses
3. Clinical immunology (serology) is the branch of immunology involved
with identifying cause of diseases based on the presence of
antigens or antibodies in serum of patients
4. a good diagnostic immunological test should meet two criteria
a. Specificity – test will indicate the presence of only one
particular antibody or antigen , won’t react with even
closely related Ag or Ab
b. Sensitivity – can detect even tiny amounts of antibodies or
antigens in serum
Microbiology – Chapter 15
c. Titer – the serum, with antibodies, is diluted and the dilutions are
tested for antigens. The highest dilution that still tests positive
(a visible reaction) with the antigen is the titer
5. Agglutination test: easy to see and read, type of test for blood
types.
a. Whole cells are tested for presence of antigens on their
surfaces
b. Antiserum is added, if positive (specific ag –ab reaction),
clumping or aggregation of cells occurs ( blood typing
pg – ___)
c. A,B,O blood groups, rh factor testing ( Photo atlas, in lab,
page 108-109)
i. A – a ag, antib ab
ii. B- b ag, anti a ab
iii. AB – both a and b ag and no anti a or b ab
iv. O – neither a or a antigen, both anti a and anti b ab
Microbiology – Chapter 15
6. Elisa test (EIA test) – enzyme linked immunosorbent assay (pg 532)
a. Widely used in clinical labs, adaptable for direct assay to
detect presence of antigen or indirect assay by testing
for the presence of antibody
b. These tests can be easily automated and results determined
by a computer
Direct ELISA (photo atlas pg 110 –111) handout
1. Ab for a specific Ag attached to the plastic microtiter well
2. Test Ag is added, if it binds, then add enzme-linked ab
specific for the test ag
3. Then a substrate that is specific for that enzyme is added.
This will produce a specific color change
4. The enzyme and substrate are selected so that a specific
color is produced that can be detected in the computer –
using optical density reader
Microbiology – Chapter 15
ii. Indirect – like the HIV test.
Known antigen is attached to the well. Test
antiserum is added. If it is complementary to the
antigen, it binds. p.532
Other tests:
Complement fixation test – pg 529
Fluorescent AB pg 530,
Precipitin,
Western blot test (confirmatory for HIV after ELISA)