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Innate immunity refers to antigen-nonspecific defense mechanisms that
a host uses immediately or within several hours after exposure to an
antigen
.
This is the immunity one is born with and is the initial response
by the body to eliminate microbes and prevent infection.
Unlike
adaptive immunity, innate immunity does not recognize every possible
antigen. Instead, it is designed to recognize a few highly
conserved structures present in many different microorganisms. The
structures recognized are called pathogen-associated molecular
patterns and include LPS from the gram-negative cell wall,
peptidoglycan, lipotechoic acids from the gram-positive cell wall, the
sugar mannose (common in microbial glycolipids and glycoproteins but
rare in those of humans), bacterial DNA, N-formylmethionine found in
bacterial proteins, double-stranded RNA from viruses, and glucans from
fungal cell walls. Most body defense cells have pattern-recognition
receptors for these common pathogen-associated molecular patterns
and so there is an immediate response against the invading
microorganism. Pathogen-associated molecular patterns can also be
recognized by a series of soluble pattern-recognition receptors in the
blood that function as opsonins and initiate the complement pathways.
In all, the innate immune system is thought to recognize approximately
103 molecular patterns. All of this will be discussed in
greater detail in upcoming sections.
The innate immune responses
involve:
-
phagocytic
cells (neutrophils, monocytes, and macrophages);
-
cells that
release inflammatory mediators (basophils, mast cells, and
eosinophils);
-
natural
killer cells (NK cells); and
-
molecules
such as complement proteins, acute phase proteins, and cytokines.
Examples of
innate immunity include anatomical barriers, mechanical removal,
bacterial antagonism, pattern-recognition receptors, antigen-nonspecific
defense chemicals, the complement pathways, phagocytosis, inflammation,
and fever. In the next several sections we will look at each of these
in greater detail.
We will now
take a closer look at the 3 pathways of the complement system.
The Complement System
The
complement system refers to a series of proteins circulating in the
blood and bathing the fluids surrounding tissues. The proteins
circulate in an inactive form, but in response to the recognition of
molecular components of microorganism, they become sequentially
actived, working in a cascade where in the binding of one protein
promotes the binding of the next protein in the cascade.
There are 3
complement pathways that make up the complement system: the
classical complement pathway, the lectin pathway, and the alternative
complement pathway. The pathways differ in the manner in which
they are activated and ultimately produce a key enzyme called
C3 convertase:
1. The
classical complement pathway is activated by antigen-antibody
complexes.
2. The
lectin pathway is activated by the interaction of microbial
carbohydrates with mannose-binding proteins in the plasma and
tissue fluids.
3. The
alternative complement pathway is activated by C3b binding to
microbial surfaces and to antibody molecules .
The end
results and defense benefits of each pathway, however, are the same.
All complement pathways carry out 6 beneficial innate defense
functions. They:
1. trigger inflammation
;
2. chemotactically
attract phagocytes to the infection site;
3. promote the attachment of antigens
to phagocytes (enhanced attachment or opsonization
);
4. cause lysis of gram-negative bacteria and
human cells displaying foreign epitopes
;
5. plays a role in the activation of naive
B-lymphocytes
;
and
6.
remove harmful immune
complexes from the body.
We will now
look at each of these complement pathways and see how they function to
protect the body.
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