There are barriers to prevent the entry of pathogens into the body:
protective covering- the skin covers the body surface which many pathogens find difficult to penetrate through.
epithelia covered in mucus- the epithelial layers produce mucus. In the lungs the pathogen stick to the mucus and is then transported by the cilia, up the trachea and then swallowed into the stomach.
HCL in the stomach- this acid has a very low PH so the enzymes of the pathogens denature causing the organism to die.
Phagocytosis- the white blood cells which carry out phagocytosis are called phagocytes. chemicals from pathogens attract the phagocyte which moves toward the pathogen.
the phagocyte binds with the pathogen by attaching to its surface
the phagocyte engulfs the pathogen which forms a vesicle, phagosome
lysosomes from the phagocyte move toward the phagosome and fuses with it
enzymes from the lysosomes break the pathogen down by hydrolysis
the soluble products are adsorbed in the cytoplasm go the phagocyte