please note: these are my definitions,not from a dictionary. You may want to compare to definitions at Medline's dictionary

Assay: test or experiment

-cyte: cell

in vitro: in an artificial environment such as a Petri dish

isolate (noun): a strain of microorganism isolated from an infected individual or cell.  Often used to refer to genetically distinct strains.

Leukopenia: low white blood cell count

lymph: immune system fluid.  Can be used as a prefix, meaning pertaining to the immune system, as in lymphocyte.

Lymphadenopathy: abnormally small lymph nodes, usually because of a lack or lymphocytes

Morphological: pertaining to the shape of an organism

Peripheral blood lymphocyte: immune cells found in the peripheral blood, as opposed to those found in organs such as the spleen

Reverse transcriptase: enzyme found in retroviruses that synthesizes DNA from an RNA template

Serum: fluid portion of blood, containing salts, free proteins and other molecules, but not whole cells

Tropism: in immunology or pathology, the target cell or tissue for an infectious agent.  Also as a suffix, as in lymphotropic.