How do granulocytes and macrophages detect enemies?

John Richard Seavitt jrseavit at artsci.wustl.edu
Thu Oct 2 13:40:16 EST 1997


On 1 Oct 1997, Axel Boldt wrote:

> But how did the macrophage know whom to digest in the
> first place? Since they are not antigen specific, they have to use
> some criteria to distinguish between friend and foe, no?

They do indeed, and I'll break them down into two groups, though there is
some overlap.  Foreign material from infections in the body is often bound
up by antibodies in the blood and lymph.  These antibodies are generally
there as 'memories' of previous infections; and there are additional serum
proteins (collectively refered to as complment) that can bind infection
organisms though prebound antibodies OR independently.

Phagocytic cells have proteins on their surfaces that will bind both
antibodies or complement proteins, and this binding triggers phagocytosis.

Phagocytes also have proteins on their surface that bind, for example
bacteria, in specific ways.  This is not the 'specificity' of antigen
receptor recombination, but rather based on certain differences bewteen
bacterial cells and mammalian cells.  For example, bacterial cell walls
have a component called LPS, which phagocytes have receptors for. 
However, no human cells have LPS, and thus this is a way to distinguish
the two.  There are some additional blood proteins that have affinity for
various bacterial products, and for which phagocytes have specific
receptors. 

We refer to these systems by which phagocytes can identify material of
specific interest (for phagocytosis, degradation, and antigen
presentation) as 'nonspecific' or 'innate' immunity, since it does not
utilize the recombination used to generate novel and specific antigen
receptors we see in T and B cells.  However, the latter takes time to
develop in response to an infection (and requires the antigens be
presented to it in the first place), so it is at least as important to
your health.  Individuals with genetic defects resulting in a loss of
complement components have recurring bacterial infections, and often die
before adulthood.

John




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