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During the
first one and one-half units we looked at microorganisms: how they
replicate, why some are potentially more pathogenic than others, and
how we can control them with antimicrobial agents. The remainder of
Unit 2 and all of Unit 3 is devoted to the ways in which the body
defends itself against microbes and other potentially harmful cells
and molecules.
The body has
two immune systems: the innate immune system and the
adaptive immune system.
The second half of Unit 2
deals with innate immunity while Unit 3 will cover adaptive immunity.
Let's first briefly compare acquired and innate immunity.
1.
Adaptive (acquired) immunity refers to antigen-specific defense
mechanisms that take several days to become protective and are
designed to react with and remove a specific antigen. This is
the immunity one develops throughout life.
An antigen is defined as a substance
that reacts with antibody molecules and antigen receptors on
lymphocytes. An immunogen is an antigen that is
recognized by the body as nonself and stimulates an adaptive immune
response. For simplicity we will use the term antigen when
referring to both antigens and immunogens. The actual portions or
fragments of an antigen that react with antibodies and lymphocyte
receptors are called epitopes.
The body
recognizes an antigen as foreign when epitopes of that antigen bind
to B-lymphocytes and T-lymphocytes by means of epitope-specific
receptor molecules having a shape complementary to that of the epitope.
The
epitope receptor on the surface of a B-lymphocyte is called a B-cell
receptor and is actually an antibody molecule called surface
immunoglobulin (sIg). The receptor on a T-lymphocyte is called a T-cell
receptor (TCR).
It is
thought that the human body has the genetic ability to recognize 107
- 109 different epitopes. In otherwords, the body has 107
- 109 distinct clones of both B-lymphocytes and T-lymphocytes,
each with a unique B-cell receptor or T-cell receptor. In this variety
of B-cell receptors and T-cell receptors there is bound to be at least
one that has an epitope-binding site able to fit, at least to some
degree, any antigen the immune system eventually encounters. With the
adaptive immune responses, the body is able to recognize any
conceivable antigen it may eventually encounter.
Adaptive
immunity usually improves upon repeated exposure to a given infection
and involves:
-
antigen-presenting
cells (APCs) such as macrophages and dendritic cells;
-
the
activation and proliferation of antigen-specific B-lymphocytes;
-
the
activation and proliferation of antigen-specific T-lymphocytes; and
-
the
production of antibody molecules, cytotoxic T-lymphocytes (CTLs),
and cytokines.
Acquired
immunity includes humoral immunity and cell-mediated immunity and will
be the topic of Unit 3.
The downside
to the specificity of adaptive immunity is that only a few B-cells and
T-cells in the body recognize any one epitope. These few cells then
must rapidly proliferate in order to produce enough cells to mount an
effective immune response against that particular epitope, and
that typically takes several days. During this time the
pathogen could be causing considerable harm, and that is why innate
immunity is also essential.
2. Innate
immunity refers to antigen-nonspecific defense mechanisms that a host
uses immediately or within several hours after exposure to almost any
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.
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