A&P II: CHAPTER 18.4 leukocytes and platelets

What are leukocytes?

Leukocytes, or white blood cells (WBCs), are the blood's primary defense cells. They are generally colorless unless stained for laboratory examination.

What do leukocytes defend the body against?

Bacteria, viruses, fungi, parasites, infected cells, damaged cells, abnormal cells, potentially cancerous cells, dead cells, and cellular debris.

What are the major functions of leukocytes?

Destroy microorganisms, attack infected/damaged/abnormal cells, remove cellular debris and dead cells, and participate in inflammation and immune communication.

What is the normal total leukocyte count?

Approximately 5,000–10,000 cells/µL of blood.

How do WBCs compare with erythrocytes in abundance and size?

WBCs are far less numerous than erythrocytes but are generally larger.

Why are leukocytes considered complete cells?

They contain a nucleus and organelles.

What are the five major types of circulating leukocytes?

Neutrophils, eosinophils, basophils, lymphocytes, and monocytes.

Why can leukocytes leave the bloodstream?

They must enter tissues to respond to most infections, inflammation, and injuries.

What is emigration?

The movement of a leukocyte out of a blood vessel and into tissue fluid.

What is another term for emigration?

Diapedesis.

What is diapedesis?

The process in which a WBC squeezes between adjacent cells of a blood-vessel wall to enter tissue fluid.

How do leukocytes move once they have entered tissues?

By amoeboid motion.

What is positive chemotaxis?

Movement of leukocytes toward a chemical signal.

What produces the chemical signals involved in positive chemotaxis?

Injured or infected cells and other leukocytes release chemical attractants.

What is the purpose of positive chemotaxis?

It draws WBCs to the affected site of infection, inflammation, or injury.

During a bacterial skin infection, what attracts neutrophils from nearby capillaries into tissue?

Chemical signals released by bacteria and damaged tissue.

How long do many leukocytes survive?

Hours to days.

How long can some leukocytes survive during severe infection?

Only minutes.

Which leukocytes may survive for years?

Memory B cells and memory T cells.

Why can memory B and T cells survive for years?

They allow faster immune responses when the body encounters the same antigen again.

What is a differential leukocyte count?

A measurement of the percentage of each WBC type in a blood sample.

Why is a differential leukocyte count clinically useful?

Changes in relative WBC numbers can suggest infection, allergy, parasites, inflammation, marrow suppression, or hematologic cancer.

What are the two major ways leukocytes are classified?

By visibility of cytoplasmic granules on a stained smear and by hematopoietic stem-cell lineage.

What are granulocytes?

Leukocytes with numerous visible cytoplasmic granules and usually lobed nuclei.

Which cells are granulocytes?

Neutrophils, eosinophils, and basophils.

What lineage produces granulocytes?

The myeloid stem-cell lineage.

What are agranulocytes?

Leukocytes with fewer, less visible cytoplasmic granules and nuclei that are not distinctly lobed.

Which cells are agranulocytes?

Lymphocytes and monocytes.

What lineage produces lymphocytes?

The lymphoid stem-cell lineage.

What lineage produces monocytes?

The myeloid stem-cell lineage.

Do agranulocytes completely lack cytoplasmic granules?

No. Their granules are smaller, fewer, and less visible on routine stained blood smears.

Why are granulocytes named neutrophils, eosinophils, and basophils?

Their names reflect the way their cytoplasmic granules stain.

What are the usual structural features of granulocytes?

Visible cytoplasmic granules and generally lobed nuclei.

What percentage of WBCs are neutrophils?

Approximately 50–70%.

Which leukocyte is most abundant in circulation?

Neutrophil.

What is the typical diameter of a neutrophil?

About 10–12 µm.

What is the appearance of neutrophil granules?

Fine, pale-lilac granules.

How many lobes does a mature neutrophil nucleus typically have?

Two to five lobes.

What does polymorphonuclear leukocyte mean?

A leukocyte with a multilobed nucleus; it is a common name for a neutrophil.

What are other names for neutrophils?

Polymorphonuclear leukocytes (PMNs) or polys.

What are bands?

Immature neutrophils with incompletely formed nuclear lobes.

What is the main function of neutrophils?

Rapid phagocytosis, especially of bacteria.

Why are neutrophils important in acute bacterial infection?

They are among the earliest responders, rapidly migrate into infected tissue, and efficiently engulf and destroy bacteria.

How do neutrophils find infection sites?

They move toward chemical attractants by positive chemotaxis.

What do neutrophils phagocytose?

Bacteria and cellular debris.

What does lysozyme from neutrophils do?

It damages bacterial cell walls.

What oxidants can neutrophils release to kill pathogens?

Oxidants including hydrogen peroxide.

What are defensins?

Proteins that damage bacterial and fungal membranes.

What is neutrophilia?

An increased neutrophil count.

What conditions are associated with increased neutrophils?

Bacterial infection, inflammation, burns, or severe stress.

What is neutropenia?

A decreased neutrophil count.

What can cause decreased neutrophils?

Drug toxicity or bone-marrow disorders.

Why does neutropenia increase susceptibility to infection?

Fewer neutrophils are available for rapid phagocytic defense.

What percentage of WBCs are eosinophils?

Approximately 2–4%.

What is the typical diameter of an eosinophil?

About 10–12 µm.

What color do eosinophil granules stain?

Red-orange with eosin.

How many lobes does an eosinophil nucleus usually have?

Two to three lobes.

What are the main functions of eosinophils?

They defend against parasitic worms, release parasite-toxic substances, moderate inflammation with antihistamine effects, and phagocytose antigen-antibody complexes.

What types of parasites are eosinophils especially important against?

Parasitic worms, especially helminths.

How do eosinophils help defend against parasites?

They release substances toxic to parasites.

How do eosinophils moderate inflammatory responses?

They release molecules with antihistamine effects.

What does it mean that eosinophils have antihistamine effects?

They release molecules that oppose histamine activity.

Can eosinophils phagocytose?

Yes, especially antigen-antibody complexes.

What is an antigen-antibody complex?

A target antigen bound to an antibody.

What is eosinophilia?

An increased eosinophil count.

What conditions can increase eosinophils?

Allergies, parasitic-worm infections, and some autoimmune disorders.

What can decrease eosinophil count?

Stress or drug toxicity.

What percentage of WBCs are basophils?

Less than 1%.

Which leukocyte is the least common in circulation?

Basophil.

What is the typical diameter of a basophil?

About 8–10 µm.

What color are basophil granules?

Large dark-blue to purple granules.

Why can a basophil nucleus be hard to see?

Its large dark granules can obscure its two-lobed nucleus.

What are the main functions of basophils?

They promote inflammation by releasing histamine and release heparin, an anticoagulant.

What does histamine released by basophils do?

It promotes inflammation.

What does heparin released by basophils do?

It opposes clot formation as an anticoagulant.

Why are basophils important in inflammatory responses?

Their histamine release intensifies inflammation.

How are basophils and mast cells similar?

Both have similar functions, including histamine release.

Are basophils and mast cells the same cell type?

No. They are distinct cells and do not share the same developmental lineage.

Why is it inaccurate to call mast cells basophils that left the bloodstream?

Mast cells and basophils are separate cell types that arise from different lineages.

What is basophilia?

An increased basophil count.

What conditions can increase basophil counts?

Allergies, parasitic infection, or hypothyroidism.

What conditions can decrease basophil counts?

Pregnancy, stress, or hyperthyroidism.

What percentage of WBCs are lymphocytes?

Approximately 20–30%.

What is the primary role of lymphocytes?

They participate in innate and adaptive immune responses and provide immune memory.

From what lineage do lymphocytes arise?

The lymphoid stem-cell lineage.

Where are lymphocytes initially produced?

In bone marrow.

Where do lymphocytes complete important development stages?

In lymphatic tissues.

What is a common structural appearance of circulating lymphocytes?

A large, round nucleus with a relatively small rim of cytoplasm.

What can lymphocytes do by mitosis?

Form clones with the same immune specificity.

What are the three major types of lymphocytes?

Natural killer cells, B lymphocytes, and T lymphocytes.

What is innate immunity?

Nonspecific immune defense that does not require prior exposure to a particular pathogen.

What is adaptive immunity?

Specific immune defense directed against particular antigens.

What is the primary role of natural killer cells?

They recognize and kill cells with abnormal or nonself surface markers.

What are common targets of natural killer cells?

Many virus-infected cells and cancer cells.

Where do B cells mature?

In the bone marrow.

What kind of immunity do B cells participate in?

Adaptive, or specific, immunity.

What can an activated B cell become?

A plasma cell.

What do plasma cells produce?

Antibodies, also called immunoglobulins.

Do all lymphocytes produce antibodies?

No. Antibodies are produced by plasma cells derived from activated B cells; T cells and NK cells do not produce antibodies.

Where do T cells mature?

In the thymus.