to be involved in tissue remodeling and matrix deposition after inflammation has been downregulated [21]. A third sub-class of M2 activation has been observed following exposure to immune complexes and stimulation of TLR. This class is termed ‘M2b’ or Type II [21,50]. Of these three classes, M2b macrophages are the least understood. Interestingly, they more closely resemble M1 macrophages, owing to the lack of any M2 specific markers, such as Arg1, YM1, or FIZZ1. However, they do express the typical IL-10High, IL-12Low M2 cytokine profile [9]. Moreover, they have higher levels of MHCII and CD86, suggesting that they retain their ability to stimulate T cells [50]. Interestingly, it appears that when M2b macrophages stimulate T cells they are biased towards a Th2 response [51]. Being able to induce Th2 T cells suggests that M2b might be a potential regulator or initiator of the M2 response in general. One additional type of M2 activated cell, the so-called tumor-associated macrophage, has recently been recognized [52]. Although tumor-associated macrophages are an area of intense research [53], they are beyond the scope of this review. An important consideration regarding M2 phenotypes is that these states were typically elucidated in vitro following exposure to one
