题目

    I just mailed the chicken and the egg, each in its own separate packaging, and kept careful track of when each shipment was sent from a post office in Cambridge, Massachusetts, and when it later arrived at its intended destination in New York City.    In mailing the chicken, I was careful to stick to the restrictions described in the American Postal Service’s Domestic Mail Manual 57. This version of the Manual states that:    “Adult chickens must be sent by Express Mail. The containers used must pass the standards in International Safe Transit Association Test Procedure 1A; be 用A、B、C三个纯种品系的小鼠进行皮肤移植实验。移植时，在某小鼠背部切除部分皮肤，将其他个体的皮肤移植上去，实验过程如下图所示。请据图回答下列问题。 (1) A2的皮肤片移植到A1上，皮肤片存活，这是因为它们属于________。 (2) Bl的皮肤片移植到A2上，移植后14天，皮肤片结痂脱落。这表明它们________，因此，A2对B1皮肤片产生了________反应。这种反应属于细胞免疫，主要是由________细胞引起的。被生物细胞识别为异物的物质，称为________。 (3) 如果B2和Cl的皮肤片同时移植到曾移植过Bl皮肤片的A2上，移植后，B2皮肤片6天脱落，而Cl皮肤片14天脱落，产生这两种现象的原因分别是________。