T lymphocytes in the CD4+ lineage carry out a wide range of partially overlapping functions during immune responses. CD4+ T cells are critically involved in recruiting and activating other immune cells, dampening ongoing immune responses, and maintaining immunologic memory. When dysregulated, these cells can affect the severity of pathogenic infection, chronic inflammation, allergy, and autoimmunity.
CD4+ T cells develop in the thymus and differentiate into subsets of more specialized T lymphocytes. These subsets express characteristic combinations of transcription factors, cell surface proteins, and secreted molecules. Some cells exhibit phenotypic and functional plasticity by shifting from one subset to another. In addition, the recent description of CD4+ T cells at intermediate stages of differentiation may blur the current subset classification.