A&P II: 18.4 Leukocytes and Platelets pt.2

Which leukocyte is the largest circulating WBC?

Monocyte.

What percentage of WBCs are monocytes?

Approximately 2–8%.

What is the typical size of a monocyte?

Approximately 12–20 µm in diameter.

What does a typical monocyte look like?

It has a large kidney-shaped, horseshoe-shaped, or deeply indented nucleus.

From what lineage do monocytes arise?

The myeloid stem-cell lineage.

What happens when a monocyte leaves circulation and enters tissue?

It differentiates into a macrophage.

What is a macrophage?

A tissue phagocyte derived from a circulating monocyte.

What is the primary role of macrophages?

They phagocytose pathogens, foreign material, damaged cells, cellular debris, and worn-out erythrocytes.

What are other functions of macrophages?

They release defensins and other antimicrobial substances and release chemotactic signals that recruit additional WBCs.

What is a fixed macrophage?

A macrophage that remains in a particular tissue.

What is a free macrophage?

A macrophage that migrates through tissue fluid to sites where it is needed.

What is a microglial cell?

A specialized macrophage located in nervous tissue.

Why do macrophages phagocytose worn-out erythrocytes?

To remove old RBCs and allow recycling of components, especially iron.

Why are macrophage chemotactic signals important?

They attract additional leukocytes to sites of infection or tissue injury.

What is monocytosis?

An increased monocyte count.

What conditions can cause monocytosis?

Viral or fungal infections, tuberculosis, leukemia, and some chronic inflammatory disorders.

What can a low monocyte count indicate?

Bone-marrow suppression.

How long do most leukocytes survive?

Hours to days; in severe infection, some may survive only minutes.

Which WBCs can live for years?

Memory B cells and memory T cells.

Where are most leukocytes formed?

In red bone marrow.

What regulates leukocyte formation in bone marrow?

Cytokines, particularly colony-stimulating factors and interleukins.

What are cytokines?

Chemical signaling molecules that regulate immune-cell communication, proliferation, differentiation, inflammation, and immune responses.

What are colony-stimulating factors?

Cytokines that stimulate blood-cell progenitors to proliferate and differentiate into particular leukocyte lineages.

What are interleukins?

Cytokine signaling molecules involved in immune communication, cell proliferation, differentiation, inflammation, and hematopoiesis.

Where can lymphocytes continue developing and proliferating?

In lymphatic tissues, including germinal centers.

What are germinal centers?

Regions in lymphatic tissues where lymphocytes can develop, proliferate, and participate in immune responses.

What is leukopenia?

An abnormally low WBC count or reduced WBC production.

What is the major consequence of leukopenia?

Reduced ability to defend against infection.

What is leukocytosis?

An abnormally elevated WBC count or excessive leukocyte proliferation.

Does leukocytosis always mean effective immunity?

No. The increased WBCs may be abnormal, immature, or poorly functioning, especially in malignancies.

Why can both leukopenia and leukocytosis increase infection risk?

Leukopenia means too few WBCs; leukocytosis may involve many abnormal or nonfunctional WBCs.

What is leukemia?

Cancer involving abnormal, excessive production or accumulation of leukocytes or their precursors.

What is myelocytic leukemia?

Leukemia involving cells of the myeloid lineage.

What is lymphocytic leukemia?

Leukemia involving cells of the lymphoid lineage.

What is chronic leukemia?

Leukemia in which relatively mature WBCs accumulate because they fail to die normally.

What is acute leukemia?

Leukemia characterized by excessive production of immature WBCs.

Why can leukemia increase infection risk even if total WBC count is high?

The leukocytes may be abnormal or immature and unable to provide effective immune defense.

Why can leukemia impair other blood-cell production?

Abnormal WBC proliferation can crowd or disrupt bone marrow and interfere with normal blood-cell formation.

What is lymphoma?

Cancer in which malignant B lymphocytes and/or T lymphocytes accumulate in lymphatic tissues.

Where can lymphoma masses develop?

In lymph nodes, spleen, liver, and other tissues.

How does lymphoma differ from leukemia?

Lymphoma involves malignant lymphocytes accumulating as masses in lymphatic tissues; leukemia involves abnormal leukocyte production or accumulation in bone marrow and blood.

Are platelets complete cells?

No. Platelets are small membrane-bound cytoplasmic fragments, not complete cells.

What is another name sometimes used for platelets?

Thrombocytes.

Why can the term thrombocyte be misleading?

It can imply that a platelet is a complete cell, but platelets are cytoplasmic fragments from megakaryocytes.

What cells produce platelets?

Megakaryocytes.

Where are megakaryocytes found?

In red bone marrow.

From what lineage do megakaryocytes arise?

The myeloid stem-cell lineage.

What is a megakaryoblast?

An immature precursor cell that develops into a megakaryocyte.

What is a megakaryocyte?

A very large bone-marrow cell that produces platelets by shedding cytoplasmic fragments.

What hormone stimulates megakaryocyte development and platelet production?

Thrombopoietin.

Where is thrombopoietin produced?

In the liver and kidneys.

What does thrombopoietin stimulate?

Megakaryoblast proliferation and development into megakaryocytes.

How large is a megakaryocyte?

Approximately 50–100 µm in diameter.

What structural features characterize megakaryocytes?

They have a very large lobed nucleus and extensive cytoplasm.

How are platelets formed from megakaryocytes?

Megakaryocytes extend cytoplasmic projections through marrow capillary walls, and the projections fragment to become circulating platelets.

Why does platelet formation occur at marrow capillaries?

The platelet fragments can enter the bloodstream directly after separating from megakaryocyte projections.

How many platelets can one megakaryocyte release?

Approximately 2,000–3,000 platelets.

What happens to the remaining megakaryocyte after platelet release?

Macrophages consume the remaining megakaryocyte material.

What is the approximate diameter of a platelet?

Approximately 2–4 µm.

What typical platelet count is listed in OpenStax A&P 2e Section 18.4?

Approximately 150,000–160,000 platelets/µL of blood.

Where are about one-third of platelets stored?

In the spleen.

Why is platelet storage in the spleen useful?

The spleen can release platelet reserves when blood-vessel damage increases the need for hemostasis.

How long do platelets circulate?

Approximately 10 days.

How are old platelets removed from circulation?

They are phagocytized by macrophages.

What is hemostasis?

The process that stops bleeding after blood-vessel damage.

What is the major role of platelets in hemostasis?

They participate in clot formation and help limit blood loss after vessel injury.

What additional role do platelets have besides hemostasis?

They release growth factors that support tissue growth and repair, especially connective-tissue repair.

Why are platelets important even though they are not complete cells?

They are essential for limiting blood loss and initiating repair after blood-vessel damage.

What is thrombocytopenia?

An abnormally low platelet count.

What is the major consequence of thrombocytopenia?

Impaired hemostasis and increased bleeding risk.

What is thrombocytosis?

An abnormally high platelet count.

What is the major consequence of thrombocytosis?

Increased risk of thrombosis, or unwanted clot formation.

What is thrombosis?

Formation of a blood clot within a blood vessel when it is not appropriately needed.

What is the key contrast between thrombocytopenia and thrombocytosis?

Thrombocytopenia increases bleeding risk; thrombocytosis increases unwanted clotting risk.

Which WBC type is most associated with bacterial infection?

Neutrophil.

Which WBC type is most associated with parasite defense and allergy modulation?

Eosinophil.

Which WBC type releases histamine and heparin?

Basophil.

Which WBC type includes B cells, T cells, and NK cells?

Lymphocyte.

Which WBC becomes a macrophage in tissue?

Monocyte.

What is the WBC abundance order from most to least common?

Neutrophils, lymphocytes, monocytes, eosinophils, basophils.

What mnemonic helps remember WBC abundance from most to least common?

Never Let Monkeys Eat Bananas: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils.

What is the key difference between granulocytes and agranulocytes?

Granulocytes have conspicuous granules and lobed nuclei; agranulocytes have less-visible granules and non-lobed or indented nuclei.

What is the key difference between neutrophils and monocytes?

Neutrophils are rapid early phagocytes, especially against bacteria; monocytes become macrophages for sustained tissue phagocytosis and WBC recruitment.

What is the key difference between eosinophils and basophils?

Eosinophils target parasites and moderate allergy-related inflammation; basophils promote inflammation with histamine and release heparin.

What is the key difference between B cells and T cells?

B cells can become plasma cells that produce antibodies; T cells provide cell-mediated immunity and coordinate or attack infected and abnormal cells.

What is the key difference between NK cells and B/T cells?

NK cells perform innate nonspecific killing; B and T cells provide antigen-specific adaptive immunity.

What is the key difference between leukocytes and platelets?

Leukocytes are complete nucleated defense cells; platelets are megakaryocyte-derived cytoplasmic fragments that support hemostasis and repair.