Each platelet lacks a nucleus and is less than half the
size of a red blood cell. It is capable of ameboid movement
and may live for about ten days. In normal blood, the
varies from 130,000 to 360,000 platelets per microliter.
Platelets help repair damaged blood vessels by sticking
to broken surfaces. They release
smooth muscles in the vessel walls, reducing blood ﬂ ow.
summarizes the characteristics of blood cells and
What is the normal human blood platelet count?
What is the function of blood platelets?
may change in particular diseases. The number of neutro-
phils, for instance, usually increases during bacterial infec-
tions, and eosinophils may become more abundant during
certain parasitic infections and allergic reactions. In HIV
infection and AIDS, the numbers of a type of lymphocyte
called helper T cells plummet.
lists some disorders that alter the numbers
of particular types of white blood cells. Clinical Application
14.2 examines leukemia, cancer of white blood cells.
What is the normal human white blood cell count?
Distinguish between leukocytosis and leukopenia.
What is a diF
erential white blood cell count?
letz), also called
are not complete cells. They arise from very large cells in the
red bone marrow, called
that fragment like a shattered plate, releasing small sections
of cytoplasm—platelets—into the circulation. The larger frag-
ments of the megakaryocytes shrink and become platelets
as they pass through the blood vessels of the lungs (see F g.
14.13). Megakaryocytes, and therefore platelets, develop from
hematopoietic stem cells (see F g. 14.4) in response to the hor-
Abnormal White Blood
White Blood Cell
Hairy cell leukemia, whooping cough,
Tapeworm infestation, hookworm infestation,
Typhoid fever, malaria, tuberculosis
Too few helper T cells
Neutrophils move through
blood vessel walls and
migrate toward bacteria
bacteria by phagocytosis
Bacteria are introduced
into the dermis
vessels to dilate
When bacteria invade the tissues, leukocytes migrate into the region and destroy the microbes by phagocytosis.