3.5 & 3.7 - White Blood Cells

Question 1

Origin of white blood cells (leukocytes)

Answer 1

• multipotent HSC gives rise to a myeloblast, which in turn can give rise to granulocytes and monocytes
• granulocytes refer to neutrophils, basophils and eosinophils, which have granules present in the cytoplasm that contain agents (proteolytic enzymes) essential for their microbicidal function
• signalling through myeloid growth factors e.g. G-CSF, M-CSF, GM-CSF is essential for the proliferation and survival of myeloid cells
• cell division occurs in myeloblasts, promyelocytes and myelocytes (does not occur in metamyelocytes or band forms)

Question 2

Neutrophil

Answer 2

• the neutrophil granulocyte survives 7-10 hours in the circulation before migrating to tissues
• the nucleus of the mature neutrophil is segmented / lobulated
• main function is defence against infection; it phagocytoses and kills microorganisms by two mechanisms:
  1. superoxide dependent: the release of reactive oxygen species known as the ‘respiratory burst’ provides a substrate for the enzyme myeloperoxidase –> production of toxic acidic substances
  2. oxygen-independent: a variety of antimicrobial agents are released e.g. defensins, gelatinases
• neutrophils released from bone marrow –> peripheral blood –> tissues
• first step in neutrophil migration to tissues is chemotaxis
• neutrophils become marginated in the vessel lumen, adhere to the endothelium and migrate into tissues
• phagocytosis of microorganisms occurs following cytokine priming

Question 3

Eosinophil

Answer 3

• a myeloblast can also give rise to eosinophil granulocytes
• the eosinophil spends less time in the circulation than the neutrophil
• main function is defence against parasitic infection
• important in regulation of Type 1 (immediate) hypersensitivity reactions - inactivate the histamine and leukotrienes released by basophils and mast cells
• bilobed nuclei (not multilobed)

Question 4

Basophil

Answer 4

• a myeloblast can also give rise to basophil granulocytes
• its granules contain stores of histamine and heparin, as well as proteolytic enzymes
• lobulated nucleus harder to see, larger granules and dark stained
  Basophils involved in a variety of immune and inflammatory responses e.g:
• mediation of the immediate-type hypersensitivity reaction in which IgE-coated basophils release histamine and leukotrienes
• modulation of inflammatory responses by releasing heparin and proteases
• mast cells are similar to basophils, but reside in tissues rather than the circulation

Question 5

Monocyte

Answer 5

• myeloid stem cell gives rise to monocyte precursors and thence monocytes
• spend several days in circulation
• migrate to tissues where they develop into macrophages and other specialised cells that have a phagocytic and scavenging function
• macrophages also store and release iron
• nucleus looks like a kidney bean
  Main functions:
• phagocytosis of microorganisms covered with antibody and complement
• phagocytosis of bacteria / fungi
• antigen presentation to lymphoid and other immune cells

Question 6

Origin of white cells: lymphocytes

Answer 6

• multipotent HSC also gives rise to a lymphoid stem cell
• lymphoid stem cell gives rise to T cells, B cells and natural killer (NK) cells
• lymphocytes recirculate to lymph nodes and other tissues and then back to the bloodstream
• intravascular lifespan very variable
• nucleus fills most of the space and is single lobed

Question 7

B lymphocytes

Answer 7

• originate in foetal liver and bone marrow
• development involves Ig heavy and light chain gene rearrangement - allows specific antibodies to be released
• leads to production of surface Igs against many different antigens - humoral immunity
• subsequent maturation requires exposure to antigens in lymphoid tissue e.g. lymph nodes
• results in recognition of self and non-self antigens by mature B cells and production of specific Igs and antibodies

Question 8

T lymphocytes and NK cells

Answer 8

• lymphocyte progenitors migrate from foetal liver to the thymus leading to development of T lymphocytes
• involved in cell-mediated immunity
• NK cells are part of the innate immune system - they can kill tumour cells and virus-infected cells
• cannot differentiate between T and B cells - in order to do this, must stain with antibody

Question 9

White cell abnormalities

Answer 9

• changes can be numerical, morphological (type/function) or both
• transient (not permanent) leucocytosis suggests a reactive (secondary) cause, and occurs when a normal bone marrow responds to an external stimulus e.g. infection, inflammation, infarction
• persistent leucocytosis suggests a primary blood cell disorder - leucocyte count is abnormal due to acquired somatic DNA damage affecting a haematopoietic precursor cell giving rise to blood cancers e.g. leukaemia, lymphoma or myeloma

Question 10

Causes of white cell abnormalities

Answer 10

Leucocytosis - too many white cells - but which type of white cell is increased?

• neutrophilia
• eosinophilia
• basophilia
• lymphocytosis
• monocytosis

Leukopenia - reduction in total number of white cells

• neutropenia - reduction in neutrophil count
• lymphopenia - reduction in lymphocyte count
• leucocytosis and leukopenia usually result from changes in neutrophil count since this is usually the most abundant leucocyte in circulation

Question 11

Neutrophilia

Answer 11

• too many neutrophils
• causes: infection (particularly bacterial), inflammation, infarction or other tissue damage
• normal feature in pregnancy and may be seen following exercise (caused by rapid shift of neutrophils from the marginated pool to the circulating pool) and after administration of corticosteroids
• may be accompanied by toxic changes and ‘left shift’ - the presence of early myeloid cells e.g. metamyelocytes
• left shift - increase in non-segmented neutrophils, or that there are neutrophil precursors in the blood
• toxic granulation is heavy coarse granulation of neutrophils - bigger, more prominent granules and vacuolation
• chronic myeloid/granulocyte leukaemia (CML) = myeloproliferative disorder - primary blood cancer associated with neutrophilia, basophilia and left shift

Question 12

Neutropenia

Answer 12

• too few neutrophils
• can occur in a large number of conditions and following chemotherapy / radiotherapy
• can also result from autoimmune disorders, severe bacterial infections, certain viral infections and drugs (some anticonvulsants, antipsychotics and antimalarials)
• can have a physiological basis e.g. benign ethnic neutropenia in people of African or Afro-Caribbean ancestry
• very low neutrophil count increases risk of serious infection and the need for urgent treatment with intravenous antibiotics
• look at differential WBC count as well as just total WBC count

Question 13

Hypersegmented neutrophil

Answer 13

• normal neutrophil should have between 3 and 5 lobes
• neutrophil hypersegmentation means there is an increase in the average number of neutrophil lobes or segments (‘right shift’)
• usually due to lack of vitamin B12 or folic acid (megaloblastic anaemia)

Question 14

Eosinophilia

Answer 14

• too many eosinophils
• usually due to allergy or parasitic infection - asthma, eczema, drugs
• can occur in some forms of leukaemia e.g. CML

Question 15

Basophilia

Answer 15

• too many basophils
• uncommon finding - usually due to leukaemia or a related condition

Question 16

Monocytosis

Answer 16

• causes: infection (particularly chronic i.e. long bacterial infection) or chronic inflammation
• some types of leukaemia

Question 17

Lymphocytosis

Answer 17

• too many lymphocytes
• often a response to viral infection (transient)
• can result from lymphoproliferative disorder e.g. chronic lymphocytic leukaemia (persistent)

Question 18

Lymphopenia

Answer 18

• decrease in number of circulating lymphocytes
• defined as a total lymphocyte count < 1 x 10^9/l
• in normal blood, most lymphocytes are CD4+ T cells
• causes include: HIV infection, chemotherapy, radiotherapy, corticosteroids
• patients with severe infection may develop a transient low lymphocyte count

Question 19

Leukaemia

Answer 19

• a cancer of the blood - a bone marrow disease
• leukaemias are described as being myeloid or lymphoid according to whether the causative acquired mutation in the bone marrow is in a myeloid or lymphoid progenitor, and acute or chronic
• mutations lead the progeny to show abnormalities in proliferation, differentiation or cell survival leading to steady expansion of the leukemic clone
• the leukemic cells replace normal HSCs in the bone marrow and may overspill into the blood

Question 20

Why does leukaemia occur?

Answer 20

• results from a number of somatic mutations occurring in a primitive cell (HSC) that, as a result, has a growth or survival advantage over normal cells
• these mutations may be spontaneous, random or result from exposure to mutagens e.g. chemo/radiotherapy
• the single cell gives rise to a clone that steadily replaces normal cells
• may not require usual growth factors
• disturbance in proliferation and/or maturation
• failure of apoptosis
• mutations concerned are in oncogenes and sometimes in tumour suppressor genes

Question 21

How is leukaemia classified?

Answer 21

• leukaemia differs from many other cancers in that the abnormal cells circulate in the bloodstream and migrate into various tissues
• concepts of local invasion and metastasis do not apply
• terms malignant and benign do not apply
• the terms chronic (long term) and acute (severe and sudden in onset) are used
• nature of mutation determines whether a leukaemia is acute or chronic
  The main types of leukaemia are:
• acute lymphoblastic leukaemia (ALL) - progenitors acquire mutations, often in genes encoding transcription factors = affects ability of cells to mature, while proliferation continues = accumulation of blast cells
• acute myeloid leukaemia (AML) - blast cells
• chronic lymphocytic leukaemia (CLL) - steady expansion of clone of cells that are functionally useless = replacement of normal cells by leukemic clone
• chronic myeloid leukaemia (CML) - mature myeloid cells
• different types of leukaemia differ in aetiology, nature of the mutational events, age of onset, clinical and haematological features and prognosis

Question 22

Acute vs chronic leukaemias

Answer 22

• acute - increase in immature cells (myeloblasts or lymphoblasts) with a failure of these to develop into mature leukocytes
• chronic - cells are mature but abnormal - granulocytes (CML) or lymphocytes (CLL)
• acute - bone marrow infiltrated by blast cells = impaired haemopoiesis - blast cells also circulate in peripheral blood and can be seen on the blood film
• if acute leukaemias not treated, the disease is aggressive and patients die quickly
• chronic - disease and deterioration go on for a long period of time
• CML - mature end cells still able to function
• CLL - lymphocytes are functionally useless and there is a loss of normal immune function

Question 23

What is infectious mononucleosis (glandular fever)?

Answer 23

• when lymphocytosis is due to a viral infection there are often ‘atypical’ lymphocytes
• blood film shows ‘atypical’ lymphocytes with features e.g:
• lymphocyte with intensely basophilic (blue) cytoplasm
• scalloped margins and ‘hugging’ of the surrounding red blood cells is a characteristic finding, resulting from Epstein-Barr virus infection

Question 24

What is chronic lymphocytic leukaemia (CLL)?

Answer 24

• blood film contains mature CLL lymphocytes (stained purple) and squashed CLL lymphocytes - known as a ‘smear’ or ‘smudge’ cell
• lymphoproliferative disorder
• most common cause of persistent lymphocytosis in elderly
• characterising the profile of cell surface markers expressed by lymphocytes using flow cytometry helps confirm diagnosis
• CLL is staged according to the degree of lymph node/liver/spleen involvement, and whether Hb and platelet count are reduced

Question 25

Acute lymphoblastic leukaemia - cytogenic and molecular genetic analysis

Answer 25

• cytogenic/molecular analysis is useful for managing the individual patient as it gives us information about prognosis
• advances knowledge of leukaemia because it has permitted discovery of leukaemogenic mechanisms and the development of targeted treatment

Question 26

What are some treatment approaches for acute lymphoblastic leukaemia?

Answer 26

• supportive - red blood cells, platelets, antibiotics
• systemic chemotherapy
• intrathecal (around spinal cord) chemotherapy

Question 27

What clinical features may be found in leukaemia? Accumulation of abnormal cells leading to:

Answer 27

• leukocytosis
• bone pain (if acute)
• hepatomegaly
• splenomegaly
• lymphadenopathy (if lymphoid)
• thymic enlargement (if T lymphoid)
• skin infiltration

Question 28

What clinical features may be found in leukaemia? Metabolic effects of leukaemic cell proliferation

Answer 28

• hyperuricaemia and renal failure
• weight loss
• low grade fever
• sweating

Question 29

What clinical features may be found in leukaemia? ‘Crowding out of normal haemopoiesis

Answer 29

• fatigue, lethargy, pallor, breathlessness (caused by anaemia)
• fever and other features of infection (caused by neutropenia)
• bruising, petechiae, bleeding (caused by thrombocytopenia)

Question 30

What clinical features may be found in leukaemia? Loss of immune function?

Answer 30

• loss of normal T and B cell function
• feature of CLL