Two types of white blood cell: Phagocytes and lymphocytes.
Barriers to entry
- Protective covering = skin covers body surface, most pathogens find it hard to penetrate this physical barrier.
- Epithelia covered in mucus = Mucus produced by epithelial layers is further defence against invasion. In lungs, pathogens stick to this mucus, transported away by cilia, up trachea and swallowed into stomach.
- Hydrochloric acid in stomach = low pH denatures enzymes of most pathogens, killing the organisms.
Large particles are far too big to cross cell-surface membrane by diffusion/active transport.