The smallest unit of biological structure that meets the functional requirements of "living" is the

Answer;

-Cell

Explanation;

-The cell is the smallest structural and functional unit of living organisms, which can exist on its own. Therefore, it is sometimes called the building block of life. Cells make up the smallest level of a living organism such as human beings and other living things.

-The cellular level of an organism is where the metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental unit of life. Some organisms, such as bacteria or yeast, are unicellular; consisting only of a single cell, while others, for instance, mammalians, are multicellular.

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.