Philippa "Pippa" Marrack, FRS (born 28 June 1945) is an English immunologist and academic, based in the United States, best known for her research and discoveries pertaining to T cells.

Marrack notes that the longest she lived in one place during her early years was in Cambridge where she began pursuing her undergraduate degree.

[4] Marrack met her lifelong partner and current husband, John W. Kappler, as a postdoctoral fellow at the University of California, San Diego.

[5] In 1983 while working in the labs at the National Jewish Health in Denver, Colorado, Philippa Marrack and her husband and research partner, John Kappler, discovered and isolated the T cell receptor, together with Ellis Reinherz and James Allison.

She learned that destructive cells that fail to be destroyed can cause autoimmune diseases like AIDS, diabetes, Multiple sclerosis, and lupus.

This foundational work on immunological tolerance by Marrack and Kappler led to their later discovery in 1990 of superantigens: powerful toxins that stimulate a large amounts of T-cell proliferation and can cause devastating immune response and violent symptoms such as those seen in toxic shock syndrome or food poisoning.

[5][10] Marrack's current research projects focus on why certain autoimmune diseases, like lupus or Multiple sclerosis, are more prevalent in women than in men.

[11] Her pioneering and revolutionary work isolating the T-cell receptor and describing how T cells protect against infection, drive autoimmune and allergic diseases, and play a possible role in rejection of cancers, has contributed greatly to the current understanding of vaccines, HIV, and immune disorders in the medical field.