The immune system is a complex defense network comprising innate and specific components that work together to protect the body from pathogens. This overview explains the basic features of both systems, their immediate and specialized responses, and the role of the complement system.
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hi this is tom from zero to finals calm
this video is designed to give an
overview of immunology so that you
understand all the basic features of the
immune system it will also help you to
return to this video once you've already
learned the details so that you can
again fit those details back into the
bigger picture about how the immune
system works
the immune system can be categorized
into the innate immune system and the
specific immune system the innate immune
system consists of the components that
are already at the location of the
infection and respond immediately with a
generalized response the specific immune
system involves specialized T and B
cells that are waiting in places away
from the site of infection and then when
they're activated they launch a
specialized system that fights the
specific invading pathogen the specific
immune system takes a little while
longer to recognize the infection
particularly when a pathogen is invading
the body for the first time so in this
video we're going to cover both of these
systems and also talk about a system
called the complement system that works
alongside the innate and specific immune systems
systems
let's look at what happens when a
pathogen invades the body initially
there are physical barriers that need to
be broken before an infection can take
place the skin is an example of one of
these barriers most invaders will get
stuck at this barrier however
occasionally an invading pathogen will
get through such as when there's a cut
as well as the skin there's other
physical barriers such as the mucosa of
the respiratory system the
gastrointestinal tract and the urinary tract
tract
there are also chemical barriers that
help destroy harmful pathogens before
they can cause any infection such as the
hydrochloric acid in the stomach
lysozyme in sweat and tears and lactic
acid in the vagina
let's look at the initial responses of
the immune system once an invading
pathogen has broken through the physical
and chemical barriers to infection three
things happen
number one macrophages recognize the
pathogen and activate the innate immune
system number 2 dendritic cells pick up
antigens from the pathogen and then head
off in the blood and lymphatic systems
to track down the relevant T and B cells
of the specific immune system and
activate them
number three the invading pathogen
activates the complement system directly
via the lectin pathway and the
alternative pathway and we'll talk a bit
more on those later
first let's look at how the innate
immune system provides an immediate
response to infection
the first to respond to the macrophages
they recognize pathogens by specific
characteristics that occur on pathogens
but don't occur on cells of the body
these characteristics are called
pathogen associated molecular patterns
or PA MPs they recognize PA MPs using
various receptor types including
at this point I need to talk briefly
about a process called phagocytosis now
phagocytosis is the process that
macrophages and other phagocytes like
neutrophils use in order to destroy pathogens
pathogens
so what happens is once they've
recognized the pathogen they start to
wrap their cell membrane around that
pathogen and absorb them - with inside
their cell the pathogen is then left
inside something called a phagosome and
this phagosome fuses with surrounding
lysosomes that provide digestive enzymes
that then break down that pathogen
destroying it and also processing all of
the components into harmless waste products
products
if the invading pathogens are harmless
enough that the macrophages can deal
with them alone
then they're cleared and it goes no
further if the attacking army is too
great the macrophages need help so they
release cytokines which are signaling
proteins sort of like local hormones
that sound the alarm of an infection in
the local area this leads to a process
called inflammation one of the most
important actions of these cytokines is
to recruit and activate more cells of
the immune system such as macrophages
monocytes and neutrophils monocytes are
precursors to macrophages that float
around in the blood once they enter
tissues they differentiate into
macrophages and can then carry out all
the processes that macrophages normally
carry out neutrophils are another type
of phagocyte that circulate in the blood
and can enter the tissues and help by
destroying invading pathogens the
inflammatory response also involves a
number of other processes that help to
contain and fight the infection
these are phaser dilation increased
vascular permeability
Marcelle activation and degranulation
releasing more cytokines that further
stimulate the inflammatory response
activation of the clotting system and
inflamation itself actually stimulates
macrophages and neutrophils to secrete
more cytokines notably chemicals called
interleukins this is known as the acute
phase response and it leads to a more
this involves sending cytokines
specifically interleukin 1 to the brain
to tell it to produce a fever
that leads to high temperatures that are
poorly tolerated by many pathogens and
cause reduced appetite and lethargy so
that the person conserves more energy
that can be used to fight the infection
it also involves sending interleukin 6
to the liver to produce acute phase
protein that act to something called
opsin ins and remember this word because
we'll talk about ops tonin's in a minute
additionally interleukin 8 is release
that recruits and activates more
neutrophils interleukin 2 and 12
activate natural killer cells and tumor
necrosis factor-alpha is released that
does all of these effects by itself as I
just mentioned I want to take a quick
look at UPS onehans as these are very
important in the immune system ops
onehans are complex molecules that
attach themselves to pathogens and make
it easier for macrophages and
neutrophils to recognize and phagocytose
that pathogen
I think of them a bit like a fork that
Spears the pathogen so that macrophage
can attach to the other end of the four
and then use it to eat the pathogen
an example of an opsin in that you may
have heard of is c-reactive protein or
CRP CRP is produced by the liver in
response to interleukin 6 and we
actually measure the level of CRP in our
patients to assess how much inflammation
there is in the body and it can be a
really good indicator of the severity of
the infection that our patient is
suffering with so that summarizes the
innate immune system a generalized
system that recognizes and responds to
an invading pathogen by causing an
inflammatory response and recruiting
cells that destroy the pathogens by phagocytosis
next let's look briefly at the
complement system the complement system
works alongside the innate and the
specific immune system to help them
destroy pathogens this involves a series
of complement proteins labeled C 1 to C
9 once a complement system is triggered
the proteins start to activate each
other in something called the complement cascade
cascade
various products of the complement
cascade have important functions such as
acting as UPS own ins triggering further
information and directly attacking and
destroying the pathogens
the complement system is triggered in
one of three ways the lectin and the
alternative pathway are activated
directly by pathogens and the classical
pathway is activated by antibody antigen
complexes that arise from the specific
immune system
and that brings us nicely on to looking
at the function of the specific immune
system the specific immune system
involves two characters the T and the B
cells and these are both types of lymphocyte
lymphocyte
they are free to float around the
lymphatic system and the blood but they
spend most of their time in the lymph
node and the mucosa associated lymphoid
tissue think of these lymphoid tissues
as army barracks that are full of
soldiers that are all sat around waiting
to get word of an enemy that they've
specifically been trained to fight each
pathogen has molecules that are unique
to them they're known as antigens
each t-cell has T cell receptors that
are specific to a single type of antigen
be cells have antibodies on their cell
membrane that just like t-cell receptors
are specific to a single type of antigen
the result is there's millions of
different T and B cells that are all
specific to a single type of antigen
when a new pathogen arrives and causes
an infection the T and B cells that are
specific to that pathogen need to be
alerted the trouble is there certain
lymphatic tissue such as lymph nodes
while the infection is happening
somewhere completely different this is
where the dendritic cell comes in these
cells are like messengers and they pick
up the antigens at the site of the
invasion display them on their cell
surface and then bring them through the
blood and lymphatic system into the
lymphatic tissue
when they're there all the T and B cells
have a look at the antigen and see
whether they recognize it when the T and
B cells are found that are specific to
so the specific immune response starts
with the dendritic cell presenting the
antigens on their HLA class 2 molecules
to the cd4 cells these cd4 cells which
are a type of t-cell then proliferate
and become T helper cells the T helper
cells present antigens on their HLA
class 1 molecule that can be recognized
by cd8 cells another type of T cell they
also secrete cytokines that are
responsible for making the cd8 cells
proliferate and differentiate into
cytotoxic T cells these T helper cells
also release cytokines that stimulate B
cells to proliferate and differentiate
into plasma cells that can release large
quantities of antibodies and memory B
cells that hang around as part of the
immune memory to respond quickly and
future inflections with that specific
pathogen the T helper cells also travel
to areas of infection and secrete
cytokines that help to recruit monocytes
and macrophages to the infected tissue
and activate the macrophages to cause
inflammation and start the process of phagocytosis
cytotoxic t-cells are responsible for
killing cells that have been infected by
pathogens such as virally infected cells
to do this they need to attach
themselves to the infected cell via the
T cell receptor and the HLA class 1
molecule expressing the relevant antigen
once they are attached to the virally
infected cell they have to killing
mechanisms that they can choose from the
first is called granule exocytosis where
they basically spray the infected cell
with enzymes that destroy the membrane
and lead to cell lysis and cell death
the second is they can activate the fast
molecule the fast molecule is like a
self-destruct switch that once it's
activated it causes the cell to undergo
apoptosis plasma cells and antibodies
are an essential part of the specific
immune system plasma cells are B cells
that have differentiated and become
antibody producing cells and their job
is to produce tons of antibodies that
are specific to the invading pathogen
these antibodies are proteins that are
shaped like a y one end is variable in
shape to match different antigens
whereas the other end is fixed in shape
and can be recognized by many cells of
the immune system
these proteins float around the blood
and attach themselves to antigens that
match their specific variable region on
these antibodies help the immune system
fight pathogens in a number of different
ways firstly they can attach themselves
to enemy toxins which themselves are
antigens and neutralize their toxic
effect secondly antibodies can attach
themselves to the receptors of viruses
and bacteria and prevent them for
carrying out their function for example
it can stop viruses from being able to
recognize cells that it may want to
invade and therefore prevent the viral
invasion of that cell thirdly antibodies
can attach themselves to pathogens then
clump together to slow the spread of
that pathogen down this is called
agglutination finally it can be very
difficult for the simplistic receptors
of macrophages and neutrophils to
recognize certain pathogens antibodies
can act as obso nians
that are highly specific to the invading
pathogen and by attaching themselves to
that pathogen acting as Epsilon's they
can help the macrophages and neutrophils
to recognize and destroy that pathogen
that summarizes an overview of the
immune system I hope you found this
video helpful it was created as part of
a set of videos that covers everything
you need to know about the immune system
for medical school and practicing as a
doctor so go ahead and check out the
rest of those videos if you like this
one then please like comment and
subscribe for more videos to help you
achieve success in your medical career
you can also check out the zero to
finals website where you'll find notes
illustrations practice questions and a
blog and I hope it's all useful for you
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