641
CHAPTER SIXTEEN
Lymphatic System and Immunity
nosuppressive drugs were typically given for the rest of the
recipient’s life. That view is changing, as cases accumulate of
patients who received well-matched transplants and are sur-
viving
without
continued immunosuppressive therapy. These
patients received, along with standard transplants such as kid-
neys, bone marrow stem cells from the donors. Apparently the
infl ux of donor cells establishes a “stable coexistence” with
the recipient’s cells—they accept each other. When this bal-
ance is disturbed, tissue rejection or graft-versus-host disease
is the outcome. Researchers hypothesize that bombarding the
recipient’s body with immunosuppressive drugs immediately
after the transplant can disturb this process. The emerging
new view is to give the drugs
before
the transplant, and in
some cases minimize their use after.
Autoimmunity
The immune system can fail to distinguish self from nonself,
producing antibodies, called
autoantibodies,
and cytotoxic
T cells that attack and damage the body’s tissues and organs.
This attack against self is called
autoimmunity.
The signs
and symptoms of autoimmune disorders refl
ect the affected
cell types. In autoimmune hemolytic anemia, autoantibodies
destroy red blood cells. Autoimmune ulcerative colitis harms
colon cells and severe abdominal pain results.
Table 16.11
lists some autoimmune disorders.
Why might the immune system attack body tissues?
Perhaps a virus, while replicating in a human cell, “borrows”
proteins from the host cell’s surface and incorporates them
onto its own surface. When the immune system “learns”
the surface of the virus to destroy it, it also learns to attack
the human cells that normally bear the particular protein.
Another explanation of autoimmunity is that T cells never
learn in the thymus to distinguish self from nonself.
A third possible route of autoimmunity is when a nonself
antigen coincidentally resembles a self antigen. For example,
damage to heart valve cells in acute rheumatic fever is due
to attack by antibodies present from a recent throat infection
with group A streptococcus bacteria. The surfaces of the cells
that make up the heart valve resemble those of the bacteria.
Some disorders thought to be autoimmune may have
a stranger cause—fetal cells persisting in a woman’s circula-
tion for decades. In response to an as yet unknown trigger, the
fetal cells, perhaps “hiding” in a tissue such as skin, emerge,
A
delayed-reaction allergy
(type IV) may affect any-
one. It results from repeated exposure of the skin to certain
chemicals—commonly, household or industrial chemicals or
some cosmetics. Eventually the foreign substance activates T
cells, many of which collect in the skin. The T cells and the
macrophages they attract release chemical factors, which, in
turn, cause eruptions and infl ammation of the skin (derma-
titis). This reaction is called
delayed
because it usually takes
about forty-eight hours to occur.
Transplantation and Tissue Rejection
When a car breaks down, replacing the damaged or malfunc-
tioning part often ±
xes the trouble. The same is sometimes
true for the human body. Transplanted tissues and organs
include corneas, kidneys, lungs, pancreases, bone marrow,
pieces of skin, livers, and hearts. A transplant is risky. The
recipient’s cells may recognize the donor’s tissues as foreign
and attempt to destroy the transplanted tissue in a
tissue
rejection reaction.
The transplanted tissue may also pro-
duce molecules that harm the recipient’s tissue, a response
called graft-versus-host disease (GVHD).
Tissue rejection resembles the cellular immune response
against a foreign antigen. The greater the antigenic differ-
ence between the cell surface molecules (MHC antigens,
discussed earlier in this chapter on page 630) of the recipi-
ent tissues and the donor tissues, the more rapid and severe
the rejection reaction. Matching the cell surface molecules of
donor and recipient tissues can minimize the rejection reac-
tion. This means locating a donor whose tissues are antigeni-
cally similar to those of the person needing a transplant.
The four major types of grafts (transplant tissue) include
Isograft.
Tissue is from an identical twin.
Autograft.
Tissue is taken from elsewhere in a person’s
body. (Technically, this is not a transplant because it is
within an individual.)
Allograft.
Tissue comes from another person who is not
an identical twin.
Xenograft.
Tissue comes from a different species, such
as pigs and baboons.
Table 16.10
presents examples of transplants.
Transplanting a body part, such as a hand or face, is much more dif-
F
cult than transplanting tissue or an organ, because a greater diver-
sity of cell and tissue types must be replaced, and each has di±
erent
effects on the recipient’s immune system. A hand transplant, for
example, requires skin, muscle, nerve, and bone.
Immunosuppressive drugs are used to reduce the recipi-
ent immune system’s rejection of transplanted tissue. These
drugs suppress T cell or antibody production, thereby damp-
ening the cellular and humoral immune responses, but often
have severe side effects, including infection, kidney dam-
age, and cancer. Since their use began in the 1980s, immu-
TABLE
16.10
|
Transplant Types
Type
Donor
Example
Isograft
Identical twin
Bone marrow transplant from a healthy
twin to a twin who has leukemia
Autograft
Self
Skin graft from one part of the body to
replace burned skin
Allograft
Same species
Kidney transplant from a relative or closely
matched donor
Xenograft
Di±
erent species
Heart valves from a pig
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