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Sistema imune

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As	
   células	
   do	
  
sistema	
  imune	
  sabem	
  a	
  diferença	
  entre	
  os	
  vírus	
  da	
  caxumba	
  
e	
   do	
   sarampo	
   porque	
   elas	
   as	
   memorizam	
   como	
   2	
   coisas	
  
completamente	
  diferentes.	
  
	
   	
   	
  A	
  capacidade	
  do	
  sistema	
  imune	
  de	
  fazer	
  isto	
  é	
  conhecida	
  
pelo	
  bem	
  diucil	
  termo	
  "especificidade	
  anFgênica".	
  
	
  	
  	
  Então,	
  como	
  é	
  exatamente	
  que	
  o	
  sistema	
  imune	
  discrimina	
  os	
  patógenos?	
  
	
   	
  O	
  papel	
   de	
  disFnguir	
   entre	
  diferentes	
  patógenos	
  
pertence	
   aos	
   linfócitos.	
   Tanto	
   os	
   B	
   quanto	
   os	
   T	
  
possuem	
   ferramentas	
   especiais	
   que	
   cobrem	
   a	
  
superucie	
   inteira	
  da	
  célula	
  para	
  separar	
  patógenos.	
  
Estas	
   ferramentas	
   são	
   chamadas	
   receptores	
   de	
  
anigenos	
   e	
   se	
   parecem	
   com	
   minúsculos	
   bastões	
  
com	
  pequenos	
  buracos	
  em	
  suas	
  extremidades.	
  
	
   	
   Alguns	
   dos	
   buracos	
   são	
   formados	
   para	
   encaixar	
  
exatamente	
   os	
   vírus	
   do	
   sarampo,	
   enquanto	
  outros	
  
são	
   formados	
   para	
   se	
   combinar	
   com	
   os	
   vírus	
   da	
  
caxumba	
   ou	
   algum	
   outro	
   patógeno	
   somente.	
   O	
  
sistema	
  imune	
  saberá	
  se	
  um	
  determinado	
  patógeno	
  
já	
   entrou	
   no	
   organismo	
   antes,	
   e	
   será	
   capaz	
   de	
  
idenFficá-­‐lo,	
   com	
   base	
   em	
   quais	
   de	
   suas	
   células	
  
possuem	
  receptores	
  de	
  angenos	
  que	
  combinam.	
  	
  
abertura	
  
receptor	
  de	
  
angeno	
  
linfócito	
  B	
  
20
The immune system can identify what pathogen has infected your
body and decide on the best means to deal with it. Earlier, we learned
that because of immune memory, people who’d had mumps once
couldn’t get sick with it again. But this would not stop them from get-
ting sick from something else, like measles. The cells of the immune
system can tell the difference between mumps virus and measles
virus because the cells memorise them as two entirely different
things.
The immune system’s ability to do this is known by the rather diffi-
cult term of antigen specificity.
The job of distinguishing among different
pathogens belongs to the lymphocytes. Both T
and B cells have special tools for telling
pathogens apart that cover the entire surface of
the cell. These tools are called antigen receptors
and they look like tiny rods with small holes at the
end.
Some of the holes are shaped to fit the measles
virus exactly, while others are shaped to match
the mumps virus or some other pathogen only.
The immune system will know if a pathogen has
entered the body before, and be able to identify
it, based on whether any of its cells possess anti-
gen receptors that match.
So how exactly does the immune system tell pathogens apart?
2 . How the Immune System Works
How the immune system distinguishes among pathogens
B cell
antigen 
receptor
opening
���?���Q��QXD ���������� ����� 5HR 3EITE ��
20
The immune system can identify what pathogen has infected your
body and decide on the best means to deal with it. Earlier, we learned
that because of immune memory, people who’d had mumps once
couldn’t get sick with it again. But this would not stop them from get-
ting sick from something else, like measles. The cells of the immune
system can tell the difference between mumps virus and measles
virus because the cells memorise them as two entirely different
things.
The immune system’s ability to do this is known by the rather diffi-
cult term of antigen specificity.
The job of distinguishing among different
pathogens belongs to the lymphocytes. Both T
and B cells have special tools for telling
pathogens apart that cover the entire surface of
the cell. These tools are called antigen receptors
and they look like tiny rods with small holes at the
end.
Some of the holes are shaped to fit the measles
virus exactly, while others are shaped to match
the mumps virus or some other pathogen only.
The immune system will know if a pathogen has
entered the body before, and be able to identify
it, based on whether any of its cells possess anti-
gen receptors that match.
So how exactly does the immune system tell pathogens apart?
2 . How the Immune System Works
How the immune system distinguishes among pathogens
B cell
antigen 
receptor
opening
���?���Q��QXD ���������� ����� 5HR 3EITE ��
20
The immune system can identify what pathogen has infected your
body and decide on the best means to deal with it. Earlier, we learned
that because of immune memory, people who’d had mumps once
couldn’t get sick with it again. But this would not stop them from get-
ting sick from something else, like measles. The cells of the immune
system can tell the difference between mumps virus and measles
virus because the cells memorise them as two entirely different
things.
The immune system’s ability to do this is known by the rather diffi-
cult term of antigen specificity.
The job of distinguishing among different
pathogens belongs to the lymphocytes. Both T
and B cells have special tools for telling
pathogens apart that cover the entire surface of
the cell. These tools are called antigen receptors
and they look like tiny rods with small holes at the
end.
Some of the holes are shaped to fit the measles
virus exactly, while others are shaped to match
the mumps virus or some other pathogen only.
The immune system will know if a pathogen has
entered the body before, and be able to identify
it, based on whether any of its cells possess anti-
gen receptors that match.
So how exactly does the immune system tell pathogens apart?
2 . How the Immune System Works
How the immune system distinguishes among pathogens
B cell
antigen 
receptor
opening
���?���Q��QXD ���������� ����� 5HR 3EITE ��
patógeno	
  
Receptor	
  de	
  Angeno	
  
Perfeito!	
  
Não	
  
combina…	
  
Forma	
  
errada…	
  
Com-­‐
bina!	
  
E	
  este	
  aqui?	
  
Que	
  combinação	
  perfeita!	
  
click	
  
	
   	
   Tanto	
  os	
   linfócitos	
  B	
  quanto	
  os	
  T	
  possuem	
  
receptores	
   que	
   reconhecem	
   diferentes	
  
patógenos,	
  mas	
  as	
  suas	
  formas	
  e	
  funções	
  são	
  
um	
   pouquinho	
   diferentes.	
   Os	
   receptores	
   de	
  
angenos	
  da	
  célula	
  B	
  têm	
  o	
  formato	
  da	
  letra	
  
Y	
   e	
   possuem	
   um	
   buraco	
   na	
   extremidade	
   de	
  
cada	
  braço.	
  	
  
	
  	
  Já	
  os	
  receptores	
  de	
  células	
  T	
  assemelham-­‐se	
  a	
  bastões	
  e	
  têm	
  
somente	
  um	
  buraco	
  na	
  extremidade.	
  
	
   	
  Dissemos	
  anteriormente	
  que	
  os	
   linfócitos	
  B	
   se	
   livram	
  de	
  
patógenos	
  produzindo	
  anFcorpos	
  que	
  os	
  neutralizam.	
  
	
  	
  	
  Na	
  realidade,	
  estes	
  anFcorpos	
  parecem	
  exatamente	
  como	
  	
  
	
  	
  	
  receptores	
  de	
  angeno	
  que	
  foram	
  cortados	
  de	
  uma	
  célula	
  	
  
	
  	
  	
  em	
  sua	
  base.	
  E	
  eles	
  têm	
  exatamente	
  as	
  mesmas	
  aberturas	
  	
  
	
  	
  	
  	
  que	
  os	
  receptores	
  de	
  linfócitos	
  B.	
  Quando	
  alguém	
  contrai	
  	
  
	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  caxumba,	
  somente	
  as	
  células	
  B	
  que	
  têm	
  receptores	
  de	
  	
  	
  
	
  	
  	
  	
  	
  angeno	
  para	
  o	
  vírus	
  da