rapid turn-around time, high sensitivity, and unde-
manding collection requirements of nucleic acid
studies, and because they accomplish identification
of a pathogen at the same time they demonstrate its
presence, they are likely to have an ever more
important clinical role, even when the pathogen is
readily visualized or cultured (Tang
et al.
1997).
Microbial substances not only accumulate at the
site of infection but they diffuse locally, enter the
lymph, and appear in the plasma (Figure 9.3).
Microbial substances that arise from infections that
involve blood cells, endothelial cells, or other cells
in contact with the blood, have direct access to the
plasma. During the period of time that a microbial
substance is present in the plasma at a concentration
that is measurable, it can be used to demonstrate the
presence of the pathogen. This time period is the
diagnostic window for the substance.
Hepatitis B is an infection that releases virus
particles as well as viral proteins into the plasma
(Gitlin 1997). The intact virus consists of an inner
core and outer envelope. The inner core contains
the double-stranded viral DNA, a viral DNA
polymerase, a C protein that gives rise to capsid
core antigen, HBcAg, and soluble e-antigen, HBeAg
(Conway
et al.
1998). The outer lipoprotein coat
contains the protein that represents the surface
antigen, HBsAg. Surface antigen is produced in
excess in infected hepatocytes and is in part released
as free lipoprotein particles.
Symptoms of hepatitis and laboratory findings of
liver injury and dysfunction appear approximately 2
months after exposure to the virus and, in uncompli-
cated cases, last 2 to 3 months. At that point in the
disease, hepatitis virions containing HBV DNA are
typically present in the plasma as are detectable con-
centrations of HBeAg and HBsAg. As shown in
Figure 9.4, the diagnostic windows for these
markers begin weeks prior to symptomatic disease
and extend to the end of the symptomatic period.
The diagnostic window for HBsAg is longer, lasting
up to months after the disappearance of symptoms.
The detection of HBsAg in the plasma is indicative
of hepatitis B infection; the finding of HBeAg or
HBV DNA indicates disease with a high viral repli-
cation rate and high infectivity. HBsAg is also
detectable in the plasma of carriers and patients with
chronic disease. HBeAg and HBV DNA are present
in low, often undetectable, concentrations in these
clinical states because of low viral replication rates
and correspondingly low virion release rates.
Immune response
An active humoral immune response to infection
produces a many-fold increase in the plasma concen-
tration of antibodies specific for the offending patho-
gen (Keren and Warren 1992). IgM class antibodies
appear first, usually within a week of the start of
infection and persist for a number of weeks. The
delay in the appearance of antibodies is due to the
time it takes for activation and maturation of the B
cell response. During that period, microbial anti-
gens may be present in the plasma. Once apprecia-
ble quantities of antibodies are being produced, the
circulating antigen is bound as antigen-antibody
complexes and is rapidly removed from circulation.
As the IgM antibodies subside, IgG class antibodies
appear. These antibodies slowly wax, then wane,
with detectable concentrations often persisting in the
plasma for very long periods despite resolution of
the infective process. IgM antibodies, therefore,
mark acute infection. A 4-fold increase in total
antibody titer (i.e., roughly a 16-fold increase in
plasma antibody concentration) over an interval of 2
or more weeks is also considered indicative of acute
infection. IgG antibodies mark prolonged infection
or past infection. High concentrations of IgG anti-
bodies suggest that the infection is active or recent.
The specificity of the antibodies produced in
response to a pathogen may be to the whole species
or only to the particular strain within the species
responsible for the infection. In the first case, the
dominant immunogenic epitopes are shared by all of
the species members; in the second, the immuno-
genic epitopes are specific at the strain level but not
at the species level. If all of the members of a
species are comparably virulent, the clinical goal is
to identify infection by any species member. If a
patient’s antibodies are also species-specific, testing
can be done using a single species-specific antigen.
This is the situation, for example, for
Salmonella
typhi
. If, however, the patient’s antibodies are
strain-specific, testing has to be performed using
multiple antigens, one for each relevant strain. If
the number of relevant strains is large, such testing
is not feasible. This is the situation for most of the
other species of
Salmonella
. On the other hand, if
only one strain or group in a species is clinically
relevant, then antibodies specific for that strain or
group should be produced for they can then be used
to identify the presence of that particular strain.
The antigens to which antibodies arise in infec-
tion may be structural proteins or carbohydrates of
Tissue Injury
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