Plasma membrane – must be breached if a pathogen is to enter a host cell. Some pathogens bind to specific receptors on the outside of a cell and enter via endocytosis, some use phagocytosis (a host defence mechanism) to enter, and some fuse with the plasma membrane.
Cytoplasm – cytoplasmic structures are used by pathogens, particularly viruses
Cytoskeleton – used by pathogens to move about the host cell and from one host cell to another; the cytoskeleton also play an important role in host defence (phagocytosis).
Cilia – pathogens attach to them in the initial stages of infection; the also host uses them to remove microorganisms.
Ribosomes – viruses take over host cell ribosomes to make their protein components; bacterial ribosomes can be a target for antibiotics because they are slightly different to eukaryotic ones.
ER – used to synthesize and assemble new viral particles; also play a role in the adaptive immune response.
Lysosomes – play a major role in destroying pathogens.
Proteasomes – degrade pathogen proteins and enable them to be recognized by the host defences.
Nucleus – viruses take over the host nucleus to reproduce their nucleic acids and therefore replicate.
Bacterial quorum sensing causes certain genes to be expressed only when there is a sufficient population density. Salmonella bacteria do not secrete intestinal toxin until there are sufficient numbers present in the host. The suggested explanation for this is probably that the bacteria have evolved to delay the production of toxins as a means of hiding from the host defences, which would have no trouble in dealing with small numbers of pathogens.