Leukocytes function in body defenses. They squeeze out of the walls of blood vessels through emigration or diapedesis, then may move through tissue fluid or become attached to various organs where they fight against pathogenic organisms, diseased cells, or other threats to health. Granular leukocytes, which include neutrophils, eosinophils, and basophils, originate with myeloid stem cells, as do the agranular monocytes. The other agranular leukocytes, NK cells, B cells, and T cells, arise from the lymphoid stem cell line. The most abundant leukocytes are the neutrophils, which are first responders to infections, especially with bacteria. About 20–30 percent of all leukocytes are lymphocytes, which are critical to the body’s defense against specific threats. Leukemia and lymphoma are malignancies involving leukocytes. Platelets are fragments of cells known as megakaryocytes that dwell within the bone marrow. While many platelets are stored in the spleen, others enter the circulation and are essential for hemostasis; they also produce several growth factors important for repair and healing.
agranular leukocytes
leukocytes with few granules in their cytoplasm; specifically, monocytes, lymphocytes, and NK cells
B lymphocytes
(also, B cells) lymphocytes that defend the body against specific pathogens and thereby provide specific immunity
basophils
granulocytes that stain with a basic (alkaline) stain and store histamine and heparin
defensins
antimicrobial proteins released from neutrophils and macrophages that create openings in the plasma membranes to kill cells
diapedesis
(also, emigration) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues
emigration
(also, diapedesis) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues
eosinophils
granulocytes that stain with eosin; they release antihistamines and are especially active against parasitic worms
granular leukocytes
leukocytes with abundant granules in their cytoplasm; specifically, neutrophils, eosinophils, and basophils
leukemia
cancer involving leukocytes
leukocyte
(also, white blood cell) colorless, nucleated blood cell, the chief function of which is to protect the body from disease
leukocytosis
excessive leukocyte proliferation
leukopenia
below-normal production of leukocytes
lymphocytes
agranular leukocytes of the lymphoid stem cell line, many of which function in specific immunity
lymphoma
form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues
lysozyme
digestive enzyme with bactericidal properties
megakaryocyte
bone marrow cell that produces platelets
memory cell
type of B or T lymphocyte that forms after exposure to a pathogen
monocytes
agranular leukocytes of the myeloid stem cell line that circulate in the bloodstream; tissue monocytes are macrophages
natural killer (NK) cells
cytotoxic lymphocytes capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers; provide generalized, nonspecific immunity
neutrophils
granulocytes that stain with a neutral dye and are the most numerous of the leukocytes; especially active against bacteria
polymorphonuclear
having a lobed nucleus, as seen in some leukocytes
positive chemotaxis
process in which a cell is attracted to move in the direction of chemical stimuli
T lymphocytes
(also, T cells) lymphocytes that provide cellular-level immunity by physically attacking foreign or diseased cells
thrombocytes
platelets, one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes
thrombocytopenia
condition in which there are too few platelets, resulting in abnormal bleeding (hemophilia)
thrombocytosis
condition in which there are too many platelets, resulting in abnormal clotting (thrombosis)
Figure 4 Are you able to recognize and identify the various formed elements? You will need to do this is a systematic manner, scanning along the image. The standard method is to use a grid, but this is not possible with this resource. Try constructing a simple table with each leukocyte type and then making a mark for each cell type you identify. Attempt to classify at least 50 and perhaps as many as 100 different cells. Based on the percentage of cells that you count, do the numbers represent a normal blood smear or does something appear to be abnormal?
Figure 4 This should appear to be a normal blood smear.
1. The process by which leukocytes squeeze through adjacent cells in a blood vessel wall is called ________.
A) leukocytosis
B) positive chemotaxis
C) emigration
D) cytoplasmic extending
C
2. Which of the following describes a neutrophil?
A) abundant, agranular, especially effective against cancer cells
B) abundant, granular, especially effective against bacteria
C) rare, agranular, releases antimicrobial defensins
D) rare, granular, contains multiple granules packed with histamine
B
3. T and B lymphocytes ________.
A) are polymorphonuclear
B) are involved with specific immune function
C) proliferate excessively in leukopenia
D) are most active against parasitic worms
B
4. A patient has been experiencing severe, persistent allergy symptoms that are reduced when she takes an antihistamine. Before the treatment, this patient was likely to have had increased activity of which leukocyte?
A) basophils
B) neutrophils
C) monocytes
D) natural killer cells
A
5. Thrombocytes are more accurately called ________.
A) clotting factors
B) megakaryoblasts
C) megakaryocytes
D) platelets
D
1. One of the more common adverse effects of cancer chemotherapy is the destruction of leukocytes. Before his next scheduled chemotherapy treatment, a patient undergoes a blood test called an absolute neutrophil count (ANC), which reveals that his neutrophil count is 1900 cells per microliter. Would his healthcare team be likely to proceed with his chemotherapy treatment? Why?
A neutrophil count below 1800 cells per microliter is considered abnormal. Thus, this patient’s ANC is at the low end of the normal range and there would be no reason to delay chemotherapy. In clinical practice, most patients are given chemotherapy if their ANC is above 1000.
2. A patient was admitted to the burn unit the previous evening suffering from a severe burn involving his left upper extremity and shoulder. A blood test reveals that he is experiencing leukocytosis. Why is this an expected finding?
Any severe stress can increase the leukocyte count, resulting in leukocytosis. A burn is especially likely to increase the proliferation of leukocytes in order to ward off infection, a significant risk when the barrier function of the skin is destroyed.