2.4. Blood sampling and separation of leukocytes
Blood samples (6 fish/group) were collected from the caudal vein 1, 2, 3 and
4 weeks after start of feeding. Heparin was used as an anticoagulant. Individual
fish were sampled only once to avoid the influence on the assays due to multiple
bleeding and handling stress on the fish.
Leukocytes for assay were separated from each blood sample by densitygradient centrifugation. One ml of histopaque 1.119 (Sigma) containing 100 µl
of Bacto haemagglutination buffer, pH 7.3 (Difco, USA) was dispensed into
siliconised tubes. One ml of a mixture of histopaque 1.077 and haemagglutination buffer was layered on the top. One ml of blood sample was then layered
carefully on the top of the gradient. Sample preparations were centrifuged at
700 G for 30 min at 4 °C. After centrifugation, plasma was collected and stored
at −20 °C for future analysis. Separated leukocytes were gently removed and
dispensed into siliconised tubes, containing phenol red free Hank's Balanced
Salt Solution (HBSS). Cells were then washed in HBSS and adjusted to 107
viable cells/ml.