Clusters of differentiation (CD antigens) mediate many of the interactions between the immune system and antigens, between components of the immune system, and between the immune system and other physiological systems, such as the nervous system. Studies of immunity and diseases of the immune system have been greatly facilitated over the last 20 years by the HLDA Workshops which have used antibodies to characterise many of the molecules involved in immunological processes, and have provided a nomenclature system, the CD system, which is used universally. The recent completion of the latest HLDA Workshop, the 10th, provides an opportunity to reflect on what has been achieved, what remains to be achieved, and how best to progress the field in the current state of biology.
The first HLDA Workshop provided a solution to these issues by organising multi-laboratory, blind, comparative analyses of CD antigens. Statistical analysis of data from several laboratories covering a number of cell types revealed "clusters of differentiation" (named for the statistical procedure of cluster analysis, and the focus on leucocyte differentiation), and immunochemical data provided supporting data in some cases. Antibodies thought to be detecting the same molecule, and the molecule itself, were given a "CD" designation. It would be difficult to exagger-ate the importance of this process at the time – it brought order to a chaotic field.
First, HLDA established a nomenclature and validation process that achieved order and confidence in the use of Workshop-classified antibodies. Researchers wanting to buy an antibody against a particular cluster of differentiation are well advised to buy one that has been validated by the HLDA Workshops. There are numerous examples of antibodies submitted as being against a particular CD turning out to be wrongly assigned by the submitters, and an antibody should not be referred to as being CD## unless this has been validated by the HLDA Workshops..
Second, HLDA has provided a major forum for discussion and experimentation in the field of leucocyte molecules, and fostered a collaborative spirit of exchange of reagents and information.
Third, many of our major research, diagnostic and now therapeutic targets and antibodies derive from the HLDA Workshops – CD3 and CD20 provide important examples.
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