and platelets are involved in each. Two of these mech-
anisms are shown in Fig. 11–6.
1.Vascular spasm—when a large vessel such as an
artery or vein is severed, the smooth muscle in its
wall contracts in response to the damage (called the
myogenic response). Platelets in the area of the
rupture release serotonin, which also brings about
vasoconstriction. The diameter of the vessel is
thereby made smaller, and the smaller opening may
then be blocked by a blood clot. If the vessel did
not constrict first, the clot that forms would quickly
be washed out by the force of the blood pressure.
2.Platelet plugs—when capillaries rupture, the
damage is too slight to initiate the formation of a
blood clot. The rough surface, however, causes
platelets to change shape (become spiky) and
become sticky. These activated platelets stick to the
edges of the break and to each other. The platelets
form a mechanical barrier or wall to close off the
break in the capillary. Capillary ruptures are quite
frequent, and platelet plugs, although small, are all
that is needed to seal them.
Would platelet plugs be effective for breaks in
larger vessels? No, they are too small, and though
they do form, they are washed away (until a clot
begins to form that can contain them). Would vas-
cular spasm be effective for capillaries? Again, the
answer is no, because capillaries have no smooth
muscle and cannot constrict at all.264 Blood
BOX11–5 WHITE BLOOD CELL TYPES: HLA
the HLA types of a donated organ to see if one or
several will match the HLA types of the potential
recipient. If even one HLA type matches, the chance
of rejection is lessened. Although all transplant
recipients (except corneal) must receive immuno-
suppressive medications to prevent rejection, such
medications make them more susceptible to infec-
tion. The closer the HLA match of the donated
organ, the lower the dosage of such medications,
and the less chance of serious infections. (The
chance of finding a perfect HLA match in the gen-
eral population is estimated at 1 in 20,000.)
There is yet another aspect of the importance of
HLA: People with certain HLA types seem to be
more likely to develop certain non-infectious dis-
eases. For example, type 1 (insulin-dependent) dia-
betes mellitus is often found in people with HLA
DR3 or DR4, and a form of arthritis of the spine
called ankylosing spondylitis is often found in those
with HLA B27. These are notgenes for these dis-
eases, but may be predisposing factors. What may
happen is this: A virus enters the body and stimu-
lates the immune system to produce antibodies.
The virus is destroyed, but one of the person’s own
antigens is so similar to the viral antigen that the
immune system continues its activity and begins to
destroy this similar part of the body. Another possi-
bility is that a virus damages a self-antigen to the
extent that it is now so different that it will be per-
ceived as foreign. These are two theories of how
autoimmune diseases are triggered, which is the
focus of much research in the field of immunology.Human leukocyte antigens (HLA)are antigens
on WBCs that are representative of the antigens
present on all the cells of an individual. These are
our “self” antigens that identify cells that belong in
the body.
Recall that in the ABO blood group of RBCs,
there are only two antigens, A and B, and four
possible types: A, B, AB, and O. HLA antigens are
also given letter names. HLA A, B, and C are called
class I proteins, with from 100 to more than 400
possibilities for the specific protein each can be. The
several class II proteins are given various D designa-
tions and, again, there are many possibilities for
each. Each person has two genes for each HLA type,
because these types are inherited, just as RBC types
are inherited. Members of the same family may
have some of the same HLA types, and identical
twins have exactly the same HLA types.
The purpose of the HLA types is to provide a
“self” comparison for the immune system to use
when pathogens enter the body. The T lympho-
cytes compare the “self” antigens on macrophages
to the antigens on bacteria and viruses. Because
these antigens do not match ours, they are recog-
nized as foreign; this is the first step in the destruc-
tion of a pathogen.
The surgical transplantation of organs has also
focused on the HLA. The most serious problem for
the recipient of a transplanted heart or kidney is
rejection of the organ and its destruction by the
immune system. You may be familiar with the term
tissue typing. This process involves determining