To make a probe requires a purified DNA sequence which is complementary to the gene of interest.  Since (1) mRNA can be transcribed which is complementary to the target gene, (2) is plentiful in lysed red blood cells,  and (3) most contain between 100-200 adenine residues at the 3' end called poly(A) tails, the above technique utilizing oligo(dT) primers and reverse transcriptase can be used to produce cDNA probes.