Why can't antibodies cross the cell membrane?

Because they are so big, and also are not recognized by any cell receptor (proteins that the cell use to sense it's environment). read more

Antibodies are meant to recognize and neutralize antigens not to cross cell membranes . The variable regions on the Fab end of antibodies recognize different receptors on the cell membrane which triggers action inside the cell, for expressing certain molecular reactions inside the cell. read more

Some antibodies cannot and some are assisted by the cell membrane itself to pass through. This is true to placental circulation wherein there is selective permeability of the membrane in pinocytosis. WBC's however can pass through the membrane through the process called diapedesis. read more

Because the phospholipids are in constant motion and they act like a thin layer of oil jostling with each other. read more