Human CD2 is also referred as LFA-2 (leukocyte function-associated antigen-2), which belongs to the immunoglobulin supergene family. CD2 is a monomeric transmembrane glycoprotein with a weight of 45–50 kDa and binds CD58, CD48, or CD59. It consists of two extracellular Ig-like domains (one V-like and one C-like) and a proline-rich cytoplasmic tail. CD2 is commonly expressed on most mature peripheral T cells, NK-cells, and a subset of thymic B-cells during human thymocyte development. CD2 is crucial in T-cell activation, T- or NK-mediated cytolysis, apoptosis in activated peripheral T-cells, and the production of cytokines by T-cells. CD2 is an excellent pan-T-cell marker. The low expression level of CD2 is thought to associated with some of the T-lymphoid malignancies, particularly peripheral T-cell lymphomas. CD4 (Cluster of differentiation 4) is a glycoprotein discovered on the surface of immunocytes including T helper cells, macrophages, monocytes, and dendritic cells. It was found in the 1970s and was first known as leu-3 or T4 before being named CD4. The protein of it in humans is encoded through the CD4 gene. It is cell surface receptors, and like other markers, CD4 also belongs to the immunoglobulin superfamily being a member. There are four immunoglobulin domains (D1, D2, D3, D4) in CD4 exposed on the extracellular surface, of which D1 and D3 resemble immunoglobulin variable (IgV) domains, D2 and D4 resemble immunoglobulin constant (IgC) domains. CD4 generally works as a co-receptor of T cell receptor (TCR). Furthermore, it can offer assistance for TCR in signal communicating with antigen-presenting cells.