Understanding the Immune System in One Video

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

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

in the skin

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

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

toll-like receptors

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

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

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

activation of the Kynan system

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

systemic inflammatory response

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

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

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

the antigen those cells become activated

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

protein on the infected cell

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

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