The Oxford comma, or serial comma, is the term used for a comma placed after the penultimate item in a list of 3 or more items, before the conjunction “and” or “or”.

The Oxford comma provides clarity and precision to a sentence and helps to distinguish separate items in a list. See the example below:

Immunostaining was performed to detect infiltrating neutrophils, T-lymphocytes, and glial cells as tissue biomarkers of inflammation.

Without the Oxford comma after ‘T-lymphocytes’ (shown in red), confusion may result, leading readers to wonder if infiltrating neutrophils are T-lymphocytes and glial cells.

Let’s look at a similar sentence in which the Oxford comma is not appropriate.

Immunostaining was performed to examine the distribution of infiltrating lymphocytes, T-cells and B-cells.

In this case, T-cells and B-cells are subsets of lymphocytes, and thus including an Oxford comma here would mislead the reader to think that T-cells and B-cells were separate and distinct from lymphocytes.

Although its use is sometimes considered superfluous, we recommend using the Oxford comma to be as clear as possible in your writing and to prevent unnecessary confusion.

For more information on when to use the Oxford comma, please see our article Do you know how to use the Oxford Comma?