CD4+T cells recognise peptides presented on MHC class II molecules, which are found on antigen presenting cells (APCs). As a whole, they play a major role in instigating and shaping adaptive immune responses.
Th1-polarised cells are responsible for control of intracellular pathogens such as viruses and some bacteria. IL-12 and IFN-γ are important cytokines involved in Th1 responses, and the intracellular transcription factors T-bet and STAT-4 are essential for Th1 cell differentiation and function. Th2 polarised cells are important in the defence against large extracellular organisms such as helminths, utilising cytokines such as IL-4, IL-5 and IL-13, promoting eosinophilia, mastocytosis and goblet cell hyperplasia. Gata-3 and STAT-6 are essential for Th2 cell differentiation and function.
If the Th1/Th2 balance is disturbed there can be severe consequences. Asthma and allergy are Th2-driven and some autoimmune diseases, such as type 1 diabetes and multiple sclerosis are Th1-driven.
This is a recently discovered T helper cell subset, characterised by its production of IL-17. IL-23 promotes the expansion of these cells and Th17 cells have been linked to several inflammatory conditions such as arthritis and IBD.
Regulatory T cells are a subpopulation of cells that maintain homeostasis and tolerance within the immune system. Subsets include inducible Tregs, CD25+CD45RBlo Tregs etc.
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