Tumour necrosis factor (TNF)-α is a key mediator of inflammation in rheumatoid
arthritis, and its discovery led to the development of highly successful anti-TNF therapy.
Subsequently, other biologic drugs targeting immune pathways, namely interleukin-6 blockade, B
cell depletion, and T cell co-stimulation blockade, have been developed. Not all patients respond to
a biologic drug, leading to a knowledge gap between biologic therapies available and the confident
prediction of response. So far, genetic studies have failed to uncover clinically informative
biomarkers to predict response. Given that the targets of biologics are immune pathways,
immunological study has become all the more pertinent. Furthermore, advances in single-cell
technology have enabled the characterization of many leucocyte subsets. Studying the blood
immunophenotype may therefore, define biomarker profiles relevant to each individual patient’s
disease and treatment outcome. This review summarises our current understanding of how
immune biomarkers might be able to predict treatment response to biologic drugs.