+Soulaf Kakel Hi Soulaf, apologies for the delay in responding. What we mean by this is when using secondary antibodies within mouse tissue, there can be cross species reactivity because the secondary antibodies may recognise the endogenous mouse Ig that is already present within the tissue and not just recognise the primary antibody attached to the antigen of interest. Therefore the labeled secondary antibody will give a signal even in the absence of antigen recognition and as a result produce inaccurate and false readings.

:-) many many thanks, now its clear enough... I highly appreciated your answer...