Introduction

Cells of the immune system have a large number of different glycoprotein receptors on their surfaces, with a wide range of biological functions. Most membrane glycoproteins are, however, constructed from a limited set of protein structural units, which are recognisable at the amino acid sequence level. The three-dimensional structure of many of these domains or modules is now known. For leukocytes a systematic approach to naming the surface molecules has been adopted and numbers have been assigned to surface proteins based upon the binding of groups of antibodies. These groups of antibodies have been used to define antigens associated with cell differentiation and have been termed clusters of differentiation or CDs. So far 247 CDs have been defined and a further 40 cell-surface proteins have been described in detail (Barclay et al., 1993, 1997; Campbell, 1998). A CD guide may be found on the worldwide web at http://www.ncbi.nlm.nih.gov/prow.

Modules and domains

The idea that most cell surface molecules are constructed from a limited repertoire of building blocks is now well established. For example, a domain is a spatially distinct structural unit that folds independently; the sequence need not be contiguous. Modules are a subset of domains that are contiguous in sequence and that are repeatedly used in functionally diverse proteins. They have identifiable amino acid patterns that can be described by a ‘consensus’ sequence. In the case of extracellular modules, compatible phases at the exon/intron boundaries are often observed. A repeat is a module unit that does not occur as a single copy and several repeats are needed to form a superstructure. An example is the leucine-rich repeat that occurs in CD42.