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What evidence must be considered when determining whether or not a trait is an adaptation? (Site 1)

An adaptation is a trait that "evolved" for a particular condition that allows the animal to survive. Something is adaptive if it is a useful trait but not necessarily one that occurred in response to a particular selection pressure. Basically a trait is an adaptation if it is the product of selection, and adaptive if it is beneficial in the present time.

Explanation:

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.