Lloyd J. Old identified the first cell surface antigens that could differentiate among different cell types.

Lloyd John Old was an American medical researcher, and one of the founders of the field of cancer immunology. When Old began his career in 1958, tumor immunology was in its infancy. Today, cancer immunotherapies are a significant advance in cancer therapy.

The cell membrane is a biological membrane that separates and protects the interior of a cell from the outside environment. The cell membrane is a lipid bilayer, usually consisting of phospholipids and glycolipids; eukaryotes and some prokaryotes typically have sterols interspersed between them as well, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that attach to the surface of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose.

In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.

A cell type is a classification used to identify cells that share morphological or phenotypical features. A multicellular organism may contain cells of a number of widely differing and specialized cell types, such as muscle cells and skin cells, that differ both in appearance and function yet have identical genomic sequences. Cells may have the same genotype, but belong to different cell types due to the differential regulation of the genes they contain. Classification of a specific cell type is often done through the use of microscopy. Recent developments in single cell RNA sequencing facilitated classification of cell types based on shared gene expression patterns. This has led to the discovery of many new cell types in e.g. mouse grey matter, hippocampus, dorsal root ganglion and spinal cord.