DCs
Have several PRRs (direct binding, NLR, CLR, TLR, RLR) and Fc and complement (indirect binding)
receptors. Due to signal 2 (CD80/86 on APC with CD28 on T cell), DCs are the only cells that can
activate T cells. also, DCs are the only cells that can cross-present antigen.
Macrophages
Recognize self vs non-self with their macrophage receptors. These recognize the cell-surface
carbohydrates of bacterial cells but not those of human cells. Macrophages induce inflammation.
Have several PRRs (direct binding, NLR, CLR, TLR, RLR) and Fc and complement (indirect binding)
receptors.
NK cells
Recognize self vs non-self using their Natural killer cell receptors. These recognize changes at the
surface of human cells that are caused by a viral infection.
Innate immune response
Phagocytosis and signalling (TLRs).
Macrophage can bind a bacterium with phagocytic receptor → synthesis of inflammatory cytokines
→ engulfment → phagosome with bacterium → phagolysosome degrades bacterium.
Complement (= aanvulling: it complements immune cells)
Specific complement factors (e.g. C3b, CR1: synthesized by the liver and present in serum,
lymph and extracellular fluids) bind to bacteria to further stimulate phagocytosis.
There are 3 pathways of complement system activation (all leading to C3 activation):
1. The alternative pathway Antibody-independent pathway first to act
Spontaneous hydrolysis of C3 into C3 (H2O). Together with other factor this leads to
the formation of fluid phase C3 convertase which converts C3 into C3a and C3b.
2. The lectin pathway Lectin-dependent pathway second to act
Microbe has Mannose on surface. Mannose-binding-Lectin can bind to this (PAMP
recognition). Eventually C3 convertase is formed: converts C3 into C3a and C3b.
3. Classical pathway Antibody-dependent pathway third to act
Antibodies bind to pathogen → C1 (complement) activated → C3 convertase
formation: converts C3 into C3a and C3b.
Complement activation leads to:
C3a – Anaphylatoxins
− Ensures inflammation:
, o It recruits leukocytes (neutrophils, mast cells and basophils)
o It increases the vascular permeability
C3b – Opsonization → enhances:
− Phagocytosis (C3b – CR1 binding (opsonization))
− Lysis of microbes (bacteria) via the MAC formation
Three phases of immunity:
1. Innate immunity (0-4 hours)
Recognition by preformed nonspecific or broadly specific effector cells
2. Early induced innate response (4-96 hours)
Recognition of PAMPs, activation of complement and phagocytes (macr. & DCs) & inflammation
3. Adaptive immune response (>96 hours)
Recognition by naïve B and T cells, clonal expansion and differentiation to effector cells
Adaptive immune response
Immature DC takes up antigen in the peripheral tissue. The DC becomes activated and migrates to
the LN via the afferent lymphatic vessel, whilst processing the antigen. In the LN, the mature DC
presents antigen to the adaptive immune system. T cells move to the peripheral tissue via the
efferent lymphatic vessel.
Differences immature vs mature DCs:
Immature DC Mature DC
Good at phagocytosis Good at processing/presenting antigen
Low level of MHC II High level of MHC II
Low level of lysosomal protein High level of lysosomal protein
Low level of co-stimulatory molecules High level of co-stimulatory molecules
Low level of migration (markers) High level of migration (markers)
Increase in inflammatory cytokines
B cells
Upon antigen recognition, B plasma cells produce antibodies/immunoglobulins. Effector molecule =
antigen receptor. B cells need help from CD4+ TFH cells to become activated (or to form memory B
cells). B cells phagocytose pathogens, degrade them, present them to the Tfh cell leading to cytokine
production from Tfh cell to B cell. Also: CD40L (Tfh) – CD40 (B) leads to activation of the B cell:
− Isotype switch
− Increased antibody affinity
− Memory B cell formation
Plasma B cell formation: 2 waves of plasma B cell formation at different sites of the LN
, 1. B and Tfh cells expand in the medulla. Plasma cells produce IgM (always the first antibody).
2. Second expansion takes place in the follicle. Germinal centre formation: affinity maturation
and isotype switching. Cytokines regulate which isotype is generated (IgG, IgA, etc).
Antibodies
How antibodies are made: Fuse mouse B cells immunized with antigen with myeloma cells. Grow
these cells in drug-containing medium (only the hybrid cells live, single B or myeloma cells die). Select
for antigen-specific hybridoma and clone these selected hybridoma cells.
IgA1-2, IgD, IgE, IgG1-4, IgM. Heavy chain is Greek letter. The Fc tail and Fab arms can move and
rotate to bind (reflexive; Especially IgG3). Antibodies link the pathogen to immune cells. Fc tail
determines the effector function.
IgA, IgM and IgG protect the internal organs. They are transported to the organs via Fc receptors
(FcnR).
IgG: is endocytosed out of the blood by endothelial cells towards the extracellular space. The pH
level regulates the protection of proteolysis in the vesicle (acidic pH) and the dissociation of IgG from
the FcR to the extracellular space (basic pH). FcR is recycled.
IgG3 is very likely to be cleaved proteolytically
(also has shortest half-life: 7 compared to 21)
It also has the highest capacity to activate
complement (+++). IgG1 as well (++).
IgG1 and IgG3 mostly respond to protein
antigens, IgG2 mostly to carbohydrate
antigens, and IgG4 mostly to allergens (++).
, IgA: protects the mucosa. It keeps the microbiome in the lumen. Epithelial cells transport IgA from
the lamina propria to the lumen. Here, the Fc receptor is cleaved and where IgA is now bound to the
mucus (slijm) through the secretory piece of the receptor: IgA dimer + secretory component).
IgE: protects against parasitic infections.
1) By activating mast cells to release their granules
Inactive mast cells have preformed granules with histamine / other inflammatory mediators.
Multivalent antigen cross-links IgE antibodies bound at the mast cell surface causing a
release of those granule contents.
2) By activating eosinophils and basophils
Their FceRI binds IgE that is binding the antigen on the parasite. As a result, the
eosinophil/basophil secretes mediators that expel the parasite from the body.
Functions of antibodies:
✓ Neutralization = direct effect, blocking receptor/cytokine/growth factor IgG, IgA
✓ Opsonization = FcR mediated effector functions (NK, macr., neutr.) IgG1, IgG3
✓ Activation of complement (Complement-Dependent Cytotoxicity) IgM, IgG1, IgG3
Also:
✓ Transport across placenta (IgG1, IgG3, IgG4)
✓ Diffusion into extravascular sites (IgG, IgA monomer)
✓ Tool for research (detection of cells labeled with fluorescent Ab using flow cytometry)
IgM+ is a B cell marker.
Techniques that use antibodies
Flow cytometry, ELISA, western blot, immunohistochemistry, immunofluorescence, etc.
Antibodies as a drug
To block cytokines (to suppress auto-immunity), to block a receptor (to reduce tissue damage; e.g.
Neutrophil FcaRI), to remove B cells (that produce auto-antibodies) (Ab targets CD20 on B cell).
As drugs for cancer
Blocking growth factors (problem = redundancy: other growth factors after a while),
checkpoint inhibitors (block PD-L1 on tumor cell and/or PD-1 on T cell or other cell), or
antibodies targeting the tumor (markers) directly.
Upper part humanized = blue.
Ways to improve antibody efficiency
1. Change the isotype (e.g. IgG → IgA)
2. Glycosylation can improve FcR affinity and thus the effector function
3. A tumor-specific antibody can be coupled to A) a toxin or B) a radionuclide
➢ This Ab binds to the tumor cell where A) the toxin is internalized and kills the cell or
B) the radiation kills the tumor cell and neighbouring cells.
