What is a word for "atom or molecule"?
The classic catch-all term is molecular entity, often abbreviated to just entity. There's a Wikipedia page for it, which references the IUPAC Gold Book entry. Quoting from the latter:


  Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable entity.
  
  Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities. Note that the name of a compound may refer to the respective molecular entity or to the chemical species, e.g., methane, may mean a single molecule of $\ce{CH4}$ (molecular entity) or a molar amount, specified or not (chemical species), participating in a reaction. The degree of precision
  necessary to describe a molecular entity depends on the context. For example, 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.
  
  Source: PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1142.


As you can see, the definition is exceptionally general. It also specifically includes atoms. Usage of the term is widespread, even if it seems a bit exotic when first encountered.

Another option which can work is chemical species (often abbreviated to just species), which is even mentioned in the definition above. Again, there is a Wikipedia page. IUPAC actually has two definitions for chemical species, but the relevant definition is the following:


  An ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment. The term is applied equally to a set of chemically identical atomic or molecular structural units in a solid array.
  
  For example, two conformational isomers may be interconverted sufficiently slowly to be detectable by separate NMR spectra and hence to be considered to be separate chemical species on a time scale governed by the radiofrequency of the spectrometer
  used. On the other hand, in a slow chemical reaction the same mixture of conformers may behave as a single chemical species, i.e. there is virtually complete equilibrium population of the total set of molecular energy levels belonging to the two conformers. Except where the context requires otherwise, the term is taken to refer to a set of molecular entities containing isotopes in their natural abundance. The wording of the definition given in the first paragraph is intended to embrace both cases such as graphite, sodium chloride or a surface oxide, where the basic structural units may not be capable of isolated existence, as well as those cases where they are. In common chemical usage generic and specific chemical names (such as radical or hydroxide ion) or chemical formulae refer either to a chemical species or to a molecular entity.
  
  See also: chemical species (of an element)
  
  Sources:
  PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1096.
  PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2202.


In this case, there is a notion of an ensemble (a collection/multiple) of whatever is being referred to. This is why "chemical specimen" (singular) is never used. 

In short, a (chemical) species is composed of a collection of (molecular) entities. Both are umbrella terms covering virtually everything, and therefore can be used in almost all contexts. Perhaps in your case, (molecular) entity is the preferred term, as it emphasizes the differences in identity of what interacts with the cells.