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Samenvatting How the Immune System Works - Introduction to Human Immunology (CBI20803)

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Wageningen University (WUR)

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How the Immune System Works

Samenvatting van alle relevante hoofdstukken voor het tentamen van het vak Introduction to Human Immunology.

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immunology
antibodies
receptors
cytokines
t cells
b cells
th cells
nk cells
macrophages
dendritic cells

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SAMENVATTING BOEK
INTRODUCTION TO HUMAN IMMUNOLOGY

Chapter 1 – An overview
First line of defence against invaders consists of physical barriers, and to cause trouble,
viruses, bacteria, parasites and fungi must penetrate these shields.

INNATE IMMUNE SYSTEM
The Innate Immune System is our 2nd line of defence. It is a naturally obtained system,
what all animals have. One of the defender cells that is stationed in your tissues is the
most famous innate immune system: the macrophage.

Macrophages have antennae (receptors) on their surface which are
tuned to recognize danger molecules characteristic of common
microbial invaders. When it encounters a bacterium, a macrophage
first engulfs it in a pouch (vesicle) called a phagosome. The vesicle
containing the bacterium is then taken inside the macrophage, where
it fuses with another vesicle termed a lysosome.  these contain
powerful chemicals and enzymes which can destroy bacteria.

 This whole process is called phagocytosis

Macrophages and all other blood cells in your body are descendants of
self-renewing blood stem cells.

When the cells that can mature into macrophages first exit the bone marrow and enter
the blood stream, they are called monocytes. Monocytes remain in the blood for an
average of 3 days.

During the battle between the bacteria and the chemicals, macrophages produce and
give off (secrete) proteins called cytokines  hormone like messengers which facilitate
communication between cells of the immune system. Some of these cytokines alert
monocytes and other immune system cells traveling in nearby capillaries to encourage
these cells to exit the blood to help fighting.

ADAPTIVE IMMUNE SYSTEM
The 3rd level of defence is the adaptive immune system. This system can actually
adapt to protect us against almost any invader. It adapts to defend against specific
invaders.

Immunity is conferred by special proteins that circulated in the blood
of immunized individuals: antibodies. The agent that caused the
antibodies to be made was called an antigen. Prototype antibody;
immunoglobulin G (IgG). An IgG antibody molecule is made up of two
pairs of two different proteins, the heavy chain (Hc) and the light
chain (Lc). Each molecule has two identical “hands” (Fab regions) that
can bind to antigens. Next to IgG you also have: IgA, IgD, IgE, and
IgM.

Each kind of antibody is produced by B cells – white blood cells that are born in the bone
marrow, and which can mature to become antibody factories called plasma B cells.

An antibody mole also has a constant region (Fc) “tail” which can bind to receptors (Fc
receptors) on the surface of cells such as macrophages. It is the structure of an antibody

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,that determines its class, which immune system cells it will bind to, and how it will
function.

In every B cells, on the chromosomes that encode the antibody heavy chain there are
multiple copies of four types of DNA molecules (gene segments) called V, D, J, and C.
The DNA that encodes the light chain of the antibody molecule is also assembled by
picking gene segments and pasting them together.

Proliferation: B cell is triggered to double in size and divide into two daughter cells.

Antibodies identify invaders and let other players do the
dirty work. Antibodies can opsonize these invaders. When
antibodies opsonize bacteria or viruses, they do so by
binding to the invader with their Fab regions, leaving their
Fc tails available to bind to Fc receptors on the surface of
cells such as macrophages. Antibodies can form a bridge
between the invader and the phagocyte, bringing the
invader in close, and preparing it for phagocytosis.

When a phagocyte’s Fc receptors bind to antibodies that
are opsonizing an invader, the appetite of the phagocyte increases, making it even more
phagocytic.

Antibodies can actually bind to a virus while it is still outside of a cell, and can keep the
virus either from entering the cell or from reproducing once it has entered 
neutralizing antibodies.

T CELLS
Once a virus gets into a cell, antibodies cannot get to it, so the virus is safe to make
thousands of copies of itself. To deal with this problem, the immune system evolved the
killer T cell.

T cells are very similar to B cells in appearance. Like B cells, T cells are produced in the
bone marrow and on their surface they display antibody-like molecules called T cell
receptors (TCR). T cells also employ the principle of clonal selection: When a T cell’s
receptors bind to their cognate antigen, the T cell proliferates to build up a clone of T cells
with the same specificity.

Whereas the B cells mature in the bone marrow, T cells mature in the thymus. And
whereas B cells make antibodies that can recognize any organic molecule, T cells
specialize in recognizing protein antigens. In addition, a B cell can secrete its receptors in
the form of antibodies, but a T cell’s receptors remain tightly glued to its surface. Perhaps
most importantly, a B cell can recognize an antigen “by itself,” whereas a T cell will only
recognize an antigen if it is “properly presented” by another cell.

Three main types of T cells: killer T cells (cytotoxic lymphocytes or
CTLs), helper T cells, and regulatory T cells. The killer T cells is a potent
weapon that can destroy virus-infected cells. The 2nd type of the T cells
is the helper T cell (Th cell). It directs to action by secreting
chemical messengers (cytokines). These cytokines have names like
interleukin 2 (IL-2) and interferon gamma (IFN-γ). The 3rd type is the
regulatory T cell (Treg). The role of this type of T cell is to keep the
immune system them from overreacting or from reacting
inappropriately

Special proteins called major histocompatibility complex (MHC) do
the “presenting” and T cells use their receptors to “view” this
presented antigen. There are two classes of MHC molecules.

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,  Class I MHC molecules are found in varying amounts on the surface of most
cells in the body. They function as “billboards” which inform killer T cells about
what is going on inside these cells. Killer T cells can use their receptors to look into
the cell to discover that it has been infected and that it should be destroyed.
 Class II MHC molecules also function as ‘billboards’, but this display is intended
for the enlightenment of helper T cells. Only certain cells in the body make class II
MHC molecules, and these are called antigen presenting cells (APCs).
Macrophages are excellent APCs.
 So class I MHC molecules alert killer T cells when
something isn’t right inside a cell, and class II MHC
molecules displayed on APCs inform helper T cells that
problems exist outside of cells

Although a class I MHC molecule is made up of one long chain
(the heavy chain) plus a short chain (β2-microglobulin), and a
class II MHC molecule has two long chains (α and β), you’ll notice
that these molecules are rather similar in appearance.

The first step in the activation of a helper T cell is recognition of
its cognate antigen displayed by class II MHC molecules on the
surface of an APC. A second signal is also required for activation.
This 2nd signal is non-specific, and it involves a protein on the
surface of an APC that plugs into its receptor on the surface of
the helper T cell.

The immune system includes “meeting places” – the secondary
lymphoid organs. The best known secondary lymphoid organ is
the lymph node. B cells and T cells circulate from node to node
looking for the antigens for which they are “fated”. Bringing T cells, B cells, APCs and
antigen together within the small volume of a lymph node greatly increases the
probability that they will interact and efficiently activate the adaptive immune system/

“leftover” B and T cells are called memory cells. Memory cells are easier to activate. As a
result of this immunological memory, during a second attack, the adaptive system usually
cam spring into action so quickly that you never even experience any symptoms.

Chapter 2 – the Innate Immune System
The complement system is composed of about 20 different proteins that work together
to destroy invaders and to signal other immune system players that the attack is on. The
proteins that make up the complement system are produced mainly by the liver and are
present at high concentrations in blood and tissues. The complement system must be
activated before it can function, and there are three ways this can happen. The 1 st is the
classical pathway, which depends on antibodies for activation.

The 2nd way is the alternative pathway. The most abundant complement protein is C3,
which of the molecules are continually being broken into two smaller proteins. One of the
protein fragments created by this ‘spontaneous’ cleavage, C3b, is very reactive and can
bind to either of two common chemical groups (amino or hydroxyl groups). If C3b doesn’t
find one of these chemical groups to react with within about 60 microseconds, it is
neutralized by binding to a water molecule. This means that the spontaneously clipped
C3 molecule has to be right up close to the surface of the invading cell in order for the
complement cascade to continue. Once C3b is stabilized by reacting with a molecule on
the cell surface, another complement protein, B, binds to C3b. Then complement protein
D comes along and clips off part of B to yield C3bBb.

3

, C3bBb acts like a “chain saw” that can cut other C3 proteins and convert them to C3b.
Consequently, C3 molecules that are in the neighbourhood
don’t have to wait for spontaneous clipping events to convert
them to C3b – the C3bBb molecule (called a convertase) can
do the job very efficiently. And once another C3 molecule has
been clipped, it too can bind to an amino or hydroxyl group on
the surface of the bacterium. This sets up a positive feedback
loop.

Once C3b is bound to the surface of a bacterium, the complement cascade can proceed
further. The C3bBb chain saw can bind to another molecule of C3b,
and together they can clip a complement protein, C5, into two
pieces. One of these pieces, C5b, can then combine with other
complement proteins (C6, C7, C8, and C9) to make a membrane
attack complex (MAC). To form this structure, C5b, C6, C7, and
C8 form a “stalk” that anchors the complex in the bacterial cell
membrane. Then C9 proteins are added to make a channel that
opens up a hole in the surface of the bacterium.

The complement system works very fast. If a cell surface is not protected, it will be
attacked by a complement. Any unprotected surface will be a target.

The 3rd pathway is the lectin activation pathway. The central player in this pathway is
a protein that is produced mainly by the liver, and which is present in moderate
concentrations in the blood and tissues. This protein is called mannose-binding lectin
(MBL). A lectin is a protein that is able to bind to a CHO molecule, and mannose is a
carbohydrate molecule found on the surface of many common pathogens.

 The innate system mainly focuses on patterns of carbohydrates and fats that are
found on the surface of common pathogens, but not on the surface of human cells
 The way mannose-binding lectin works to active the complement system: In the
blood, MBL binds to another protein called MASP. Then, when the mannose-binding
lectin grabs its target (mannose on the surface of a bacterium, for example), the
MASP protein functions like a convertase to clip C3 complement proteins to make
C3b. Because C3 is so abundant in the blood, this happens very efficiently. The
C3b fragments can then bind to the surface of the bacterium, and the complement
chain reaction we just discussed will be off and running.
 whereas the alternative activation pathway is spontaneous, and can be visualized
as complement grenades going off randomly here and there to destroy any
unprotected surface, lectin activation can be thought of as complement “smart
bombs” that are targeted by mannose-binding lectins

Other complement system functions:

1. When C3b has attached itself to the surface of an invader, it can be clipped by a
serum protein to produce a smaller fragment, iC3b. The “i” prefix denotes that this
cleaved protein is now inactive for making MACs.
2. Decorating surfaces of invaders, thereby acting like a “poor man’s antibody” in
opsonization.
3. Fragments of complement proteins can serve as chemoattractant – chemicals that
recruit other immune system players to the battle site.

Macrophages are present in most tissues before birth, so are already “on duty” when a
baby is born. Resting macrophages in tissue are garbage collectors, to keep our tissue
free of debris.

PROFESSIONAL PHAGOCYTES: MACROPHAGES

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