Dear Yubin Kim,

Red clumps in a lymphoblastoid cell line culture are likely due to the presence of dead or dying cells, possibly due to contamination or culture stress.

Are these red clumps accompanied by a change in media color (yellowing)? Which is often a sign of cell death and potential contamination. 

Is the culture under stress? That includes overcrowding or nutrient depletion, which can also contribute to cell death and formation of these clumps.

Regards,

Malcolm Nobre