Hello, this is professor Akiko Iwasaki, i'm an immunologist. i often hear
questions like what is the difference between antibody and immunity?
with the help from BioRender, i've put together immunology 101
Tweetorial for non-immunologists. the immune system protects the host
organism from invading pathogens like viruses, bacteria,
fungi and parasites. to do so the immune system employs two layers
the innate and adaptive immunity. innate immunity acts within minutes of
infection to remove the pathogen, this is a non-specific short-term
solution. in parallel, innate immune cells induce
activation of adaptive immune system. this takes a week or two to get going,
but once established, the adaptive immunity provides
long-term pathogen-specific defense for the host.
so let's dive a bit deeper! innate immunity starts by detection of
microbial pathogens through sensors that detect unusual
molecules or activities known as 'pattern recognition'. once
microbial patterns are detected by these sensors
they trigger immediate alarm through
secretion of cytokines and interferons.
cytokines and interferons bind to their receptors on various cells around them
to help put up guard against further invasion.
they also induce death of infected cells so as to stop
further spread of the virus. finally interferons and cytokines also induce
another set of signals that flag white blood cells
to be recruited to the site of infection to wall off
and attack the invaders. however, when cytokines are triggered without breaks,
they can cause damage to the cells responding to the cytokines
and shut down the function of the organs. this is known as cytokine storm which
mediates severe disease including covid19 during sars-cov-2 infection.
the virus infects and replicates in the
lung epithelial cells, the virus is detected by macrophages
through pattern recognition receptors which trigger the secretion of
interferons and cytokines. however this virus appears to be
oblivious to the interferons' antiviral effects,
instead the cytokines recruit more white blood cells
to swarm the tissue, creating a 'cytokine storm',
where many cells contribute to the secretion of lots of different cytokines.
some of these cytokines include those that induce fibrin deposits,
as well as damage the blood vessels for fluid to leak out
into the alveoli causing respiratory failure.
viruses block the secretion and function of cytokines and interferons
so that they can replicate and spread by evading the host innate defense system.
when this happens the body starts the second layer of
immune system - the adaptive immune response.
the T and B lymphocytes, the key players of adaptive immune system
require education by dendritic cells. dendritic cells are specialized white
blood cells that survey the tissue for potential
pathogens, once dendritic cells detect pathogens
through their pattern recognition receptors
they not only secrete cytokines, but are also
signaled to migrate to draining lymph nodes.
naive uneducated T and B cells sitting in the lymph nodes
interact with dendritic cells. if their receptors happen to match the antigen
presented by the dendritic cells, they become activated expand
and become so-called effector lymphocytes.
these effector lymphocytes migrate back to the site of infection,
and directly kill infected cells and block further viral spread,
by secreting specific antibodies. these specialized white blood cells,
T and B cells are called into action by
antigen-presenting cells. once they reach the target tissue,
T cells, trained into killer soldiers, detect
specific virus fragment on the surface of
infected cells and destroy them, eliminating the source
of viral factory. antibodies secreted by b cells bind to
the surface of the virus and block their entry into host cells.
these are called the neutralizing antibodies.
b cells also secrete non-neutralizing antibodies
that call in other white blood cells such as
natural killer cells (NK cells) to kill virus infected cells. natural killer cells
are professional killer cells but need guidance from antibodies to target their
weapons against the infected cells.
what's cool about the adaptive immune system is that both
T and B cells become memory cells, and provide immune defense for a long time.
memory t and b cells survive for years, and are able to defend against the same
pathogen during second exposure, much better than
the untrained lymphocytes. so the second time you encounter the
same virus, your memory b cells will quickly
stimulate to produce much higher levels in quality of
antibodies. similarly, memory t cells provide quick
and robust protection, especially if they are within the site
of virus entry.
some b cells become plasma cells, which secrete
tons of antibodies for years. if you were vaccinated as a child, you
are likely to have antibodies to the vaccine antigens decades later.
antibodies will bind to the invading pathogen immediately,
and block their further spread. serological assays detect the level of
antibodies to various pathogens, and is a good indicator of your previous
exposure to pathogen or vaccination.
how long the antibody lasts depends on the type of pathogens...
for measles, antibodies lasts for decades. for common cold or coronaviruses, one to two years.
however you still have memory b cells
and t cells, that can reignite antibody and killer cells
during the second exposure. you're still
much better off than when you were naive to the pathogen.
however the safest and the most reliable way to achieve
immunity is through vaccination. vaccines are designed to trigger robust
and long-lasting immunity, superior to that achieved by natural
infection. it is the only safe way to develop herd immunity.
herd immunity refers to the protection
achieved at the population level if enough people
are immune to a given pathogen. For highly
infectious viruses like sars-cov-2, majority of people in the population
must be immune in order to confer protective herd
immunity against those who are not yet immunized.
there are over a hundred vaccine candidates against SARS-CoV-2,
we hope that some of these will provide durable and effective immunity
mediated by both antibodies and T cells to help us stop the spread of COVID-19.
Thank you for listening.