What is the purpose of identity proteins found on the surface of the cells of multicellular organisms (such as humans)?
A) They function to exchange substances between the cell and its environment via endocytosis and exocytosis
B) They function as markers to the immune system that they are part of the ‘self’, i.e., that they are not invaders
C) They control the cell cycle, i.e., whether a cell should replicate via mitosis or not
D) They enable the cell to have motility, i.e., to move