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immunoglobulins to protect the ocular surface from parasitic infection
( Table 2 ) . More than 400 proteins have been detected in this layer using
mass spectrometry (de Souza et al. 2006).
Table 2 Major components of the aqueous layer and their concentrations in the tear fl uid.
Component
Examples
Concentration
Proteins
Mucins
MUC1; MUC4; MUC16; MUC5AC
Antimicrobials
sIgA
1.93 mg/ml
IgG
4 µg/ml
Lysozyme
2.07 mg/ml
Lipocalin
1.55 mg/ml
Lactoferrin
1.65 mg/ml
Transferrin
1 µg/ml
Defensins
Cytokines
IL-1α
IL-1β
IL-6
IL-8
Albumine
-
0.04 mg/ml
Surfactant proteins
SP-A, SP-D
SP-B, SP-C
Trefoil Factors
TFF2, TFF3
Electrolytes
Na +
128.70 mmol/l
K +
17.00mmol/l
Ca 2+
0.32 mmol/l
Mg 2+
0.35 mmol/l
HCO 3-
12.40 mmol/l
Cl -
141.30 mmol/l
Mucins found in the aqueous layer are either secreted (MUC5AC)
or shed from the cell surface (MUC1, MUC4, MUC16) (Spurr-Michaud
et al. 2007). Antimicrobials present in the aqueous compartment include
immunoglobulins A and G (IgA, IgG), as well as cytokines such as IL-1α,
IL-1β, IL-6 and IL-8, which promote immunological reactions to infection
(Nakamura et al. 1998). The iron-binding proteins lipocalin, lactoferrin and
transferrin act bacteriostaticly by removing free iron that is necessary for
bacterial replication (Fluckinger et al. 2004,Fullard and Snyder 1990).
Other proteins include surfactant proteins SP-B and -C, which affect
spreading properties and the fi lm's stability (Bräuer et al. 2007a), and SP-A
and D, which are immune modulating peptides (Bräuer et al. 2007b). As
the tear fi lm contains oxygen to supply the cornea, reactive oxygen species
(ROS) have to be rendered harmless by enzymes like catalase, superoxide
dismutase and glutathione peroxidase (Crouch et al. 1991). Lacritin is a tear
protein that promotes basal tear secretion and stimulates corneal epithelial
cell growth (Samudre et al. 2011).
 
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