Most leukocytes are located in peripheral tissues rather than in the
blood, only about 2% are actually in the blood. They travel through blood
looking for disturbances in your body's normal functioning. If they detect
the appropriate signal, demonstrate both chemotaxis (the ability to move
towards the signal) and emigration or diapedesis (they can fit between
small spaces between cells and pass through capillary walls due to their
ability to move in an amoeboid fashion). Microbe toxins, colony stimulating
factors, & kinins (produced by damaged tissues) may attract leukocytes
(especially neutrophils & macrophages).
Some leukocytes (neutrophils, macrophages, monocytes) phagocytize (engulf)
microbes and may kill with chemicals such as lysozyme, defensins, &
strong oxidants. Eosinophils undergo the exocytosis of these chemicals
onto parasites that are too large for phagocytosis. Basophils facilitate
the inflammatory response. Most leukocytes live only a few days and during
an infection they may only survive a few hours. Because they are so short-lived,
75% of blood cells made by the bone marrow (perhaps 4-6 million per second
Monocyte (which mature into macrophages)
The types of white blood cells have fixed percentages in the blood and
a differential white blood cell count examines these percentages. Changes
from normal values may be useful in diagnosing a patient's problem:
--neutrophils increase due to burns, stress, inflammation, & bacterial
infections; decrease after radiation and in B12 deficiencies
--eosinophils increase due to allergies, parasitic infections, and
autoimmune diseases; decrease in stress and Cushings syndrome
--basophils: increase in allergies, hypersensitivities, cancers, hypothyroidism;
decrease during pregnancy, ovulation, stress, hyperthyroidism
--lymphocytes: increase in viral infections, immune disorders, some
--monocytes: increase in viral or fungal infections, tuberculosis
Leukemia is the uncontrolled production and accumulation of immature
leukocytes; since this decreases the numbers of red blood cells and
platelets made, leukemia patients are in danger of anemia, hemorrhaging,
Note the abnormally high numbers of white blood cells (staining purple)
in the blood from a leukemia patient in the following images.
In the bone marrow, very active cells known as metamegakaryocytes shed
bits of cytoplasm surrounded by cell membrane into the bloodstream, these
are known as platelets. The average metamegakaryocyte produces 1000-1500
platelets and there are 250,000-400,000 platelets per ml of blood. They
are only 2-4 um in size and have no nucleus. They are full of granules
that have chemicals to promote clotting. They live only about 5-9 days
and are visible as the small purple cells between the red blood cells
in the following images..
Normally, platelets are repelled from each other and the blood vessel
Platelets have 3 functions after bleeding begins: they form a platelet
plug to block the break in the blood vessel, they induce coagulation (form
a fibrin clot), and they help maintain endothelial lining of blood vessels.
After vascular damage has occurred, platelets release their granules,
form a platelet plug (in the first image below), and induce the blood
protein fibrinogen to form the fibrin of the blood clot (second image).
Fibrinolysis is the dissolution of clots; no clot is meant to last forever.
Clots which have formed inappropriately can be dissolved as well.
Anything floating in the blood (even an air bubble)
that may potentially block a blood vessel is called an embolus. Inappropriate
clots may form such plugs in undamaged blood vessels in a process known
Thrombolytic agents (kallikrein, tPA, streptokinase) can be injected
into the body to dissolve clots. Anticoagulants (such as heparin and
warfarin) prevent blood from clotting in the first place
Platelets may start the formation of clots in response to irregular
endothelial surfaces, artificial heart valves, or endothelial lipid
Low daily doses of aspirin decreases the formation of a thrombus (by
inhibiting the clotting factor thromboxane) and thus decrease the risk
of heart attack and stroke.
Hemophilia is a genetic disease caused by the inability to make a clotting
factor. Hemophiliacs are in danger of hemorrhage if they are cut. There
are different kinds of hemophilia but each involves the lack of a clotting
factor. Hemophilia types A and B are sex-linked while type C isn't.
Tragically, most hemophiliacs who received blood transfusions through
1982-85 were contaminated with HIV.