Autoimmune diseases develop when the immune system mistakenly attacks healthy parts of the body. Despite much research, we don’t understand how the immune system makes this ‘mistake’. Consequently, autoimmune diseases remain poorly treated. Autoimmune diseases share many immunological features which point towards a central role of a type of immune cell, called a CD4+ (aka helper) T cell. A CD4+ T cell’s job is to ‘manage’ the immune response by guiding the function of many other immune cells. Usually, CD4+ T cells guide our immune systems to protect us from infectious agents and cancer. However, when an autoimmune disease develops it is also the CD4+ T cells that lead the development of the autoimmune disease. Despite the key role of the CD4+ T cells in guiding immune responses, it has been very difficult to measure them. Broadly speaking, each CD4+ T cell ‘sees’ a different target (which we call an antigen) and can only drive an immune response once they have ‘seen’ that antigen. Since we’re interested in autoimmune diseases, we want to measure only the CD4+ T cells that recognize antigens from healthy tissues (called ‘autoantigens’). We face two challenges: (i) CD4+ T cells that recognize autoantigens are scarce in the blood; and (ii) it’s not always clear which autoantigens are ‘seen’ the by CD4+ T cells that cause an autoimmune disease. While researchers have developed some tests which can detect autoantigen-specific CD4+ T cells, the tests are complex, technically demanding and require large volumes of blood. This makes them unsuitable for routine clinical use. For this reason, current tests focus on measuring antibodies that bind to autoantigens (aka autoantibodies). While autoantibody tests are useful, they do not give any insights into the underlying CD4+ T-cell responses driving autoimmune diseases. We have set out to address this capability gap. We have developed a simple blood test, focusing for the moment on type 1 diabetes (T1D). This test is sensitive enough to detect the very rare autoantigen specific CD4+ T cells. Importantly, it has many advantages, it is: simple to perform, requires only a few milliliters of blood, logistically robust, and gives a measure of the number and activity of autoantigen-specific CD4+ T cells. Working with blood samples from people with, and without, T1D we have been able to optimize and validate this blood test. We can measure CD4+ T-cell responses to proinsulin, an important target in T1D. Importantly, this blood test can distinguish between people with and without T1D. Our current goal is to use our new blood test to analyze autoantigen specific CD4+ T-cell responses in three autoimmune diseases: multiple sclerosis (MS), systemic lupus erythematosus (SLE) and T1D. We aim to test a panel of autoantigens implicated in MS and SLE to determine which antigens provoke a CD4+ T-cell responses in people with these diseases. In T1D we will build on our work to examine responses to autoantigens other than proinsulin. Upon completion of this project, we will deliver a blood test that can measure autoantigen-specific CD4+ Tcell responses in MS, SLE and T1D. This will meet a long unmet need, enabling routine measurement in the clinic of autoantigen specific CD4+ T-cell responses in people with autoimmune diseases. This will allow, for the first time, the analysis of the autoimmune response which underlies the disease process. This will guide physicians in their diagnosis and care of people suffering from autoimmune diseases and be an invaluable tool for scientists seeking to understand autoimmune disease and develop safe and effective therapies to prevent or reverse these diseases.