Host-Parasite Interactions Background Information

Hosts and parasites have co-evolved throughout life on Earth. The first hosts evolved in the presence of parasites, and thus, throughout their evolution, were continually developing new strategies to protect themselves from these parasites.

Each time a host defense strategy evolved, a parasite attack strategy to counter that defense co-evolved. Over time this process of co-evolution in host-parasite relationships has led to the development of a very complex system of attack and counter-attack.

An example of this complex system of attacks and counter-attacks can be seen below for the bacteria Streptococcus pneumoniae which causes pneumonia.

Host - Human

Parasite - S. pneumoniae

Mucus(light green) and normal flora bacteria (dark green) line the nose and throat to prevent colonization by invading bacteria (purple).

S. pneumoniae is sometimes able to colonize the nose and throat, and can remain their transiently as part of the normal flora. From there they can be aspirated into the lungs.

The lungs contain alveolar macrophages, cells which engulf and destroy invading microorganisms.

Some S. pneumoniae have a polysaccharide capsule (tan) surrounding their surface. This makes them slippery, and the alveolar macrophages can no longer engulf and destroy them. The bacteria can now multiply in the lung.

The immune system produces antibodies (blue) that specifically recognize and attach to the polysaccharide capsule of S. pneumoniae. The antibody acts as a tag and calls in macrophages and helps them engulf the bacteria.


There are two major types of host defenses — innate and acquired. This exercise deals with innate defense mechanisms only.

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