Exam 1: Leukocytes Flashcards
Leukocyte
Types
-
Granulocytes
- Neutrophils
- Eosinophils
- Basophils
-
Agranulocytes (“Mononuclear leukocytes”)
- Lymphocytes
- Monocytes
All leukocytes contain…
azurophilic granules
- “Primary granules”
- Produced first during differentiation
- Specialized lysosomes
Only granulocytes contain…
specific granules
- “Secondary granules”
- Produced later during differentiation
- Non-lysosomal secretory vacuoles
Granulocyte
Morphology
3 types of granulocytes
Distinguished by LM based on:
- Nuclear lobulation ⇒ neutrophil > eosinophil > basophil
- Size and staining affinity of specific granules
Neutrophil
Morphology
-
Nucleus ⇒ 2-5 lobes connected by thin strands
- # and shape of lobes ∆ over time
- Inactivated X chromosome forms a drumstick appendage visible in some nuclei
- > 5 lobes ⇒ hypersegmented
-
Small, poorly staining specific granules
- Cytoplasm with “pebbly” appearance
Neutrophil azurophilic granules contain…
myeloperoxidase
(Used for detection during automated flow cytometry)
Neutrophil
Functions
-
Highly phagocytic
- Especially good at killing bacteria
- First leukocyte to arrive at site of acute infection or inflammation
Neutrophil
Recognition
Membrane receptors bind to material to be phagocytized ⇒ enhances specificity and rate of phagocytosis
- Some receptors bind directly to target e.g. bacteria
-
Toll-like receptors (TLRs) or “Pattern recognition receptors (PRRs)
- Binds to highly conserved molecular sequences common to many pathogens and endotoxins ⇒ Pathogen-associated molecular patterns (PAMPs)
-
Toll-like receptors (TLRs) or “Pattern recognition receptors (PRRs)
- Some receptors bind indirectly to target via opsonins
- Only recognizes bound form of opsonin
- Ex: Neutrophil Fc receptors for IgG
Neutrophil
Effector Mechanisms
Neutrophils kill via:
- Respiratory burst
- Mediators stored in azurophilic and specific granules
- Neutrophil extracellular traps (NETs)
Respiratory Burst
A measurable increase in oxygen consumption triggered by phagocytosis.
- Enzyme cascades produce ROI/ROS that are antimicrobial
- Kill by oxidizing microbial macromolecules
- First step catalyzed by NADPH oxidase
- Inactive subunits found on plasma membrane and in cytosol
- Activation forms membrane-bound multimeric enzyme
- Also found on phagosome membranes
Neutrophil
ROIs
- Superoxide anion radical (O2·-)
- Made by NADPH oxidase
- Hydroxyl radical (OH·)
- Hydrogen peroxide (H2O2)
- Hypochlorous acid (HOCl)
- Made from H2O2 by myeloperoxidase in azurophilic granules
- Dissociates to form hypochlorite (OCl-)
Neutrophil
Azurophilic Granule Content
-
Myeloperoxidase
- H2O2 → Hypochlorous acid
-
Bactericidal permeability-increasing protein (BPI)
- Damages cell membrane of gram-negative bacteria
-
Lysozyme
- Attacks cell walls of bacteria, esp. gram-positive
-
Defensins
- Small cationic proteins that permeabilize bacterial membranes
- Neutrophil elastase
Neutrophil
Specific Granule Content
-
Lysozyme
- Attack bacterial cell walls
-
Lactoferrin
- Competes for iron and copper ⇒ bacteriostatic
- Collagenase
Neutrophil Extracellular Traps
(NETs)
Extracellular webs composed of nuclear/mitochondrial DNA bound by histones and granule proteins.
- Released from activated neutrophils
- Traps microbes ⇒ killed by high local concentration of granule contents
- Helps prevent microbial spread
- In sepsis, can form within blood vessels causing problems
Neutrophil
Damage
- Mediator release into extracellular space can damage healthy tissue
- Ex. elastase and collagenase
- Release can occur during NET formation or phagocytosis
- With phagocytosis, due to premature fusion of granules with phagosome
Activated neutrophils due by ___ forming the major component of ___ which is removed by ___.
apoptosis
pus
macrophages
Acute Inflammation
Clinical Indicators
Largely due to neutrophils:
-
Leukocytosis
- ↑ number of leukocytes in the blood
- Due to increased number of neutrophils leaving bone marrow in response to inflammatory mediators
-
Left shift
- Neutrophil age distribution shifts towards less mature forms
- Occurs when reserve of mature neutrophils in bone marrow exhausted
Acute Inflammation
Course
-
Edema
- Occurs within a few hours
- Due to increased vessel permeability by histamine and other mediators
-
Early phase
-
Neutrophils peak at ~ day 1 ⇒ “first wave”
- Secrete chemotactic factors that attract monocytes
-
Neutrophils peak at ~ day 1 ⇒ “first wave”
-
Late stage
- Monocytes peak within ~ 2 days ⇒ “second wave”
- Differentiate into macrophages
- Phagocytize dead neutrophils, damaged tissue, and debris
Eosinophil
Morphology
- 2-3 lobed heterochromatic nucleus
- Large and eosinophilic specific granules
- 30+ mediators
- Cationic protins most abundant ⇒ important in parasitic infections
- Ex. Major basic protein
Eosinophil
Functions
-
Kill larvae of certain parasitic worms (helminths)
- Binds to surface of larva and secrete damaging substances
- Major basic protein
- ROS from respiratory burst
- Binds to surface of larva and secrete damaging substances
- Phagocytize Ag-Ab complexes
- Modulate activity of basophils and mast cells
Eosinophil
Damage
Activity can damage host tissues:
-
Asthma exacerbation
- Damages respiratory tract
- Stimulates excess mucus production
- Kills ciliated cells
- Induces lung remodeling
- ↑ CT and SM in airways
- Stiffens airways and makes them hypercontractible
- Damages respiratory tract
- Atopic dermatitis
Basophil
Morphology
- Nucleus ⇒ usually 2 poorly defined lobes
-
Large basophilic specific granules
- Often obscures nucleus
- Appears homogenous or grainy
- Can look like membrane arranged in stacks or whorls ⇒ myelin figures
Basophil
Activation
IgE cross-linking:
- IgE made following Ag exposure
- Binds to high affinity IgE receptors on basophils ⇒ “primed”
- Subsequent Ag exposure ⇒ Ag binds IgE on primed basophil ⇒ IgE molecules cross-linked by Ag ⇒ activation
Basophil activation leads to…
-
Degranulation ⇒ mediators released from specific granules
- Histamine
- Heparan sulfate
- Eosinophil chemotactic factor (ECF)
- Neutrophil chemotactic factor (NCF)
-
Synthesis and secretion of cytokines
- IL-4
- IL-13
- Arachidonic acid pathway activation by phospholiases
Basophil
Functions
- Basophils & mast cells regulate blood vessels
- More common in extravascular CT than blood
- Important in defense against ectoparasites (e.g. ticks)
- Mediator functions:
-
Vasodilation & ↑ vessel permeability
- Post-capillary venules > capillaries
- Promotes diapedesis and Ab/complement extravascularization
-
↑ mucus secretion
- Helps trap Ag
-
Bronchoconstriction
- ↑ airflow velocity easing transport of particles trapped in mucus
- Class switching ⇒ ↑ IgE production
- Chemotaxis of eosinophils, neutrophils, monocytes, and macrophages
- Pruritis
-
Vasodilation & ↑ vessel permeability
Basophil
Damage
Excessive basophil/mast cell activation results in:
- Edema
-
Excessive mucus secretion
- Clogs bronchiolar lumens
- Causes nasal congestion
- Excessive bronchoconstriction
Excessive basophil or mast cell activity can result in a…
Type I hypersensitivity
(Immediate hypersensitivity)
Which can manifest as:
- Hay fever
- Asthma
- Urticaria
- Anaphylaxis
Basophils arise from a ___ when stimulated by ___ and mature in the ___.
myeloid progenitor
IL-4
bone marrow
Monocyte
Morphology
Size and morphology highly variable:
- Irregularly shaped nucleus
- U or horsehoe shaped
- Folded on itself
- Flower petal pattern
- Not lobulated
- Lacy “moth-eaten” chromatin
- Sharp boundary between euchromatin & heterochromatin
- Nucleus and cytoplasm pale staining
- Few azurophilic granules
- No specific granules
- Well-developed golgi
- Can make azurophilic granules throughout lifespan
Monocyte
Differentiation
- Circulates in blood for up to several days before entering extravascular tissues
- In tissues, differentiates into macrophages
- Macrophages can be long-lived (several months)
Resident Macrophage
Classifications
Macrophages in certain tissues/organs have characteristic morphology, functions, and names:
- Histiocytes (“Ordinary” macrophages) ⇒ CT
- Kupffer cells ⇒ liver
- Osteoclasts ⇒ bone
- Alveolar macrophages (“dust cells”) ⇒ lung
Macrophage
Morphology
- Larger and more irregularly shaped
- Pale with euchromatic nucleus by LM
- Evidence of phagocytic activity by EM
- More phagosomes
- More secondary lysosomes
- Highly active plasma membrane
- Many thin ‘pseudopods’ on cell surface
Macrophage
Activation
- Interferon-gamma is an important activator
- Activation enhances activity and changes morphology
Macrophage
Functions
-
Phagocytosis
- Has specialized applications that neutrophils are not good for
- Apoptotic bodies, tissue debris, nuclei from RBCs in bone marrow, old RBCs in spleen
- Has specialized applications that neutrophils are not good for
-
Antigen-presenting cells (APCs)
- Activates T helper cells
- Secretes products involved in many processes
- Walling off large particles or microorganisms
Macrophage
Secretions
Secretes products involved in:
- Inflammation ⇒ IL-1 & IL-6
- Fever ⇒ IL-1 is a pyrogen
- Hematopoiesis ⇒ GM-CSF
- Wound healing ⇒ TGF-α (stimulates fibroblasts)
- Normal turnover of ECM ⇒ proteases like elastase & collagenase
Macrophage
Containment
Walls off large particles or microorganisms:
- Occurs in chronic inflammation if macrophages unable to eliminate foreign material
- Enlarge and congregate in groups to form histiocyte epithelioid cells
- Fuse to form multinucleated giant cells
Lymphocyte
Overview
- Major cell type of the immune system
- At least 3 functionally different types:
- B cells
- T cells
- NK cells
- All arise from pluripotent hematopoietic stem cells in bone marrow
- T & B cells indistinguishable by LM/EM
- NK cells w/ distinctive morphology
T-cell & B-cell
Overview
- Size varies based on activation state
- Inactive ⇒ small lymphocyte
- Active ⇒ large lymphocyte
- Activation ⇒ blast transformation ⇒ clonal expansion
- Daughter cells mature into either:
- Effector cells ⇒ participate in ongoing immune response
- Memory cells ⇒ reserved until a subsequent immune response
Small Lymphocyte
Morphology
- ~90% of lymphocytes in blood
- ~ 60-80% of blood lymphocytes are T-cells
- Nucleus ⇒ round/kidney bean shaped and heterochromatic
- Very thin rim of basophilic cytoplasm ⇒ ‘sky-blue’ color
- Few small azurophilic granules by LM
- No specific granules by LM or EM
- Few organelles
Large Lymphocyte
Morphology
“Immunoblasts or Lymphoblasts”
- Develop from small lymphocytes that undergo blast transformation
- Up to 30 microns
- Nucleus ⇒ euchromatic w/ ‘smudged’ chromatin pattern
- Amount of cytoplasm increases
- Few small azurophilic granules
Humoral Immunity
Immunity that can be transferred passively from an immune donor to a non-immune recipient using serum alone.
(Live cells are not required)
B-Cells
- Important in humoral immunity ⇒ Ab-mediated
- Specific for a particular Ag
- Ag binds BCR ⇒ activation ⇒ blast transformation ⇒ clonal expansion & affinity maturation
Plasma Cells
Effector cells derived from activated B-cells.
- Secrete Ab
- Short-lived ⇒ 10-30 days
- Old plasma cells may become Russell body cells
- Contain large, distended RER cisternae ⇒ Russel bodies
- Full of Ab and stain very eosinophilic
- Contain large, distended RER cisternae ⇒ Russel bodies
Cell-Mediated Immunity
Immunity that can only be transferred from an immune donor to a non-immune recipient via live T-cells.
- Live cells required because:
- Living T-cell has to directly carry out some function
- T-cell needed to produce constant supply of short-lived mediators
- Important in defense against viral and fungal infections and tumor cells
T-Cells
- Essential for cell-mediated immunity
- Specific for a particular Ag
- Ag binds TCR ⇒ activation
- Many different types including:
- Cytotoxic T-cells
- Helper T-cells
- Regulatory (suppressor) T cells
Cytotoxic T-cells
- CD8+ surface marker
-
Kill target cells that express ‘foreign’ Ag
- Virus infected cells
- Tumor cells
- Genetically different cells e.g. tissue grafts
- Killing requires binding of T-cell to target cell
- Kills via several mechanisms including secretion of:
- Perforins
- Granzymes
Perforins
Proteins that aggregate to form pores in target cell membranes.
Granzymes
“Fragmentins”
Proteases that enter target cells through pores and induce apoptosis.
Helper T-Cells
“TH cells”
- CD4+ surface marker
- TCR recognizes Ag bound to MHC II of antigen-presenting cells
-
Main function to secrete cytokines
- Helps activate cytotoxic T cells ⇒ stimulates cell-mediated immunity
- Helps active B cells ⇒ stimulates humoral immunity
Regulatory T-cells
“Suppressor T-cells”
- Diverse group
-
Downregulates immune responses
- Maintains tolerance to self
- Helps prevent autoimmune diseases
NK Cells
- ~5-10% of lymphocytes in blood
- ~ 15 microns
- Moderate number of large azurophilic granules by LM
-
Able to kill some virus-infected cells and some tumor cells
- Requires close contact to targets
- Several mechanisms
- Perforins
- Granzymes
