Why many recombinant protein have coupled Fc fragments? if performing any assay with these proteins I have to first block the Fcs?? can they have some inexpectable effect by their own??

There are several reasons why fusing rec. proteins to Fc fragments can be beneficial e.g., 1) Ease of purification via Fc fragment 2) Increased stability and 3) if rec. protein requires a dimer configuration, attachment to Fc fragment (a dimer) could be advantageous. There are also pharmacologic benefits. Could having an Fc fragment around affect the overall function of the attached protein? The answer is that it depends on what system you are evaluating to ask this question. Controls are going to be very crucial to address this issue.