（1）CD45RA-代表什么？为什么是“-”，而不是“+”？

答：CD45分子亦称白细胞共同抗原（leukocyte common antigen）包括一组膜糖蛋白，只表达于不成熟和成熟白细胞，可包括T和B细胞、胸腺细胞、单核-巨噬细胞以及中性粒细胞。CD45分子的异构体（isofoums）常限定在某些T细胞表面表达，故称之为CD45R。

未受抗原刺激的天然T细胞可表达CD45RA，而记忆T细胞可表达CD45RO。另外，CD45R分子胞浆区含有内源性酪氨酸磷酸酶活性，它可能对各种活化途径具有重要调节作用。

CD45存在于除红细胞和浆细胞以外的所有血细胞。 它在T，B细胞上的不同作用是因为不同的CD45RA, CD45RB, CD45RC, CD45RAB, CD45RAC, CD45RBC, CD45R0, CD45R (ABC) 这些Isoform导致的 这里又段文献，描述的很详细了 CD45RA is located on naive T cells and CD45R0 is located on memory T cells. CD45 is also highly glycosylated. CD45R is the longest protein and migrates at 200 kDa when isolated from T cells. B cells also express CD45R with heavier glycosylation, bringing the molecular weight to 220 kDa, hence the name B220; B cell isoform of 220 kDa. B220 * is not restricted to B cells and can also be expressed on activated T cells, on a subset of dendritic cells and other antigen presenting cells. Naive T lymphocytes express large CD45 isoforms and are usually positive for CD45RA. Activated and memory T lymphocytes express the shortest CD45 isoform, CD45R0, which lacks RA, RB and RC exons. This shortest isoform facilitates T cell activation. The cytoplasmic domain of CD45 is one of the largest known and it has an intrinsic phosphatase activity that removes an inhibitory phosphate group on a tyrosine kinase called Lck (in T cells) or Lyn/Fyn/Lck (in B cells) and activates it.

CD45R又称为IL-2R，是活化T细胞的一种标志（另外一种活化标志CD69）。因此，CD45R只表达于T细胞表面，不出现在B细胞表面。