Basics Of Haematology For USMLE

Erythrocyte (Eryth = red; cyte = cell)

• It carries O2 to tissues and CO2 to the lungs.
• It is anucleate and lacks organelles. 
• It is biconcave and has a large surface area-to-volume ratio for rapid gas exchange. 
• The life span of erythrocytes is 120 days.
• The source of energy is glucose (90% used in glycolysis, 10% used in HMP shunt).
• The membrane contains a Cl−/HCO3 antiporter, which allows RBCs to export HCO3 and transport CO2 from the periphery to the lungs for elimination.
• Anisocytosis is of varying sizes.
• Poikilocytosis is an increase in abnormal red blood cells of any shape.

Thrombocyte (Platelet)

• It is involved in first-degree hemostasis. 
• Small cytoplasmic fragments derived from megakaryocytes.
• The life span is of 8–10 days. 
• When activated by endothelial injury, aggregates with other platelets and interacts with fibrinogen to form a platelet plug. 
• Contains dense granules (ADP, Ca2+) and α granules (vWF, fibrinogen, fibronectin). Approximately 1⁄3 of the platelet pool is stored in the spleen.

Leukocyte (Leuk = white; cyte = cell)

• It is divided into granulocytes (neutrophils, eosinophils, basophils, mast cells) and mononuclear cells (monocytes, lymphocytes).
• WBC differential count from highest to lowest:

Neutrophils (~ 60%)

Lymphocytes (~ 30%)

Monocytes (~ 6%)

Eosinophils (~ 3%)

Basophils (~ 1%)

Neutrophil

• This is an acute inflammatory response cell. 
• It is increased in bacterial infections. 
• It is phagocytic. 
• It has a multilobed nucleus. 
• Specific granules contain leukocyte alkaline phosphatase (LAP), collagenase, lysozyme, and lactoferrin. 
• Azurophilic granules (lysosomes) contain proteinases, acid phosphatase, myeloperoxidase, and β-glucuronidase.
• Hypersegmented neutrophils (nucleus has 6+lobes) are seen in vitamin B12/ folate deficiency.
• Important neutrophil chemotactic agents: C5a, IL-8, LTB4, kallikrein platelet-activating factor.

Lymphocyte

• Refers to B cells, T cells, and NK cells. B cells and T cells mediate adaptive immunity. 
• NK cells are part of the innate immune response. 
• Round, densely staining nucleus with a small amount of pale cytoplasm.

B cell 

• Part of the humoral immune response. 
• It originates from stem cells in the bone marrow and matures in the marrow. 
• Migrates to peripheral lymphoid tissue (follicles of lymph nodes, white pulp of spleen,
• unencapsulated lymphoid tissue). 
• When antigen is encountered, B cells differentiate into plasma cells (which produce antibodies) and memory cells. 

T cell 

• Mediates cellular immune response. 
• It originates from stem cells in the bone marrow but matures in the thymus. 
• T cells differentiate into cytotoxic T cells (express CD8, recognise MHC I), helper T cells (express CD4, recognise MHC II), and regulatory T cells.
• CD28 (costimulatory signal) is necessary for T-cell activation. 
• The majority of circulating lymphocytes are T cells (80%).
• CD4+ helper T cells are the primary target of HIV.

Monocyte (Mono = one (nucleus); cyte = cell)

• Differentiates into macrophages in tissues.
• Large, kidney-shaped nucleus. 
• Extensive “frosted glass” cytoplasm.
• Found in blood.

Eosinophil (Eosin = pink dye; philic = loving)

• Defends against helminthic infections (major basic protein). 
• Bilobate nucleus. 
• They are packed with large eosinophilic granules of uniform size. 
• Highly phagocytic for antigen-antibody complexes.
• Produces histaminase, a major basic protein (MBP, a helminthotoxin), eosinophil
• peroxidase, eosinophil cationic protein, and eosinophil-derived neurotoxin.
• Causes of eosinophilia 

1. Neoplasia
2. Asthma
3. Allergic processes
4. Chronic adrenal insufficiency
5. Parasites (invasive)

Macrophage (Macro = large; phage = eater)

• Phagocytoses bacteria, cellular debris, and senescent RBCs. 
• Long life in tissues.
• Macrophages differentiate from circulating blood monocytes.
• It is activated by γ-interferon.
• It can function as an antigen-presenting cell via MHC II.
• Found in tissue. The name differs for each tissue type (e.g., Kupffer cells in the liver and histiocytes in
• connective tissue).
• It is an important component of granuloma formation (eg, TB, sarcoidosis).
• Lipid A from bacterial LPS binds CD14 on macrophages to initiate septic shock.

Mast cell 

• Mediates allergic reactions in local tissues.
• Mast cells contain basophilic granules and originate from the same precursor as basophils
• but are not the same cell type. 
• It can bind the Fc portion of IgE to the membrane. 
• IgE crosslinks upon antigen binding, leading to degranulation and release of histamine, heparin, tryptase, and eosinophil chemotactic factors.
• It is involved in type I hypersensitivity reactions.
• Cromolyn sodium prevents mast cell degranulation (used for asthma prophylaxis).

Dendritic cell

• Highly phagocytic antigen-presenting cell (APC). 
• Functions as a link between innate and adaptive immune systems. 
• Expresses MHC class II and Fc receptors on the surface. 
• It is called Langerhans cell in the skin.

Plasma cell

• Produces large amounts of antibodies specific to a particular antigen. 
• “Clock-face” chromatin distribution, eccentric nucleus, abundant RER, and well-developed Golgi apparatus. 
• It is found in the bone marrow and does not typically circulate in peripheral blood.
• Multiple myeloma is a plasma cell cancer.

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