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T cells are a diverse and important group of lymphocytes that mature and undergo a positive and negative selection processes in the thymus. These cells play a vital role in both components of active immunity, including cell-mediated and to some extent humoral immunity. There are several types of T cells; the most common and well-known are the CD4+ T cells (helper T cells) and CD8+ T Cells (cytotoxic T cells, or killer T cells). T cells cannot recognize soluble, free antigens. T cells can only recognize protein-based, receptor-bound antigens. This recognition occurs via the use of the MHC (also known as HLA) 1 and 2 receptors, which along with the TCRs (T-cell receptors) bind the antigen in question and form a complex that allows the T cell to recognize the antigen. CD4+ T cells recognize MHC 2 bound antigens, while CD8+ T cells recognize MHC 1 bound antigens. Both CD4+ T cells and CD8+ T cells have the TCR (and the co-receptor CD3), but (as evidenced by their name) their other co-receptors vary. CD4+ T cells have CD4, whereas CD8+ T cells have CD8 as an added co-receptor. T cells are identified via flow cytometry for their CD markers as opposed to EM or light microscopy.