Methods of immune evasion and subversion

Pathogens have been mutating and sharing resistance genes for millions of years. As such they have developed some very effective methods to evade, subvert and destroy the immune response. Here are a few examples:

Polysaccharide capsule

• A capsule of sugars can be used by extracellular bacteria to cover up the components of the outer membrane or wall. Elements such as lipopolysaccharide are easy targets for immune detection. Covering them can buy the pathogen more time to replicate.

Hemolysin production

• Some intracellular bacteria can produce enzymes known as hemolysins to break out of the phagosome once the bacterium has been ingested. Escaping into the cytosol means the bacterium can divide without being detected, using the host cell as camouflage.

Antioxidant defense

• The production of antioxidant enzymes such as catalase and superoxide allows some bacteria to defend themselves against the production of degradative reactive oxygen species (ROS) by immune cells. Chemicals such as superoxide and hydrogen peroxide can damage pathogenic membranes and proteins. These enzymes allow pathogens to detoxify ROS into less harmful chemicals such as water and oxygen.

Camouflage with self-proteins (mimicry)

• Some organisms can express elements that would be recognized as 'self' by the immune system to mask their surface antigen. Some bacteria use fibrin, a structural self molecule found in scar tissue to mask surface peptidoglycan.

Antigenic variation

• Pathogens can make small changes to the surface presentation of particular components so that any humoral immune response, such as previously generated antibodies, won't recognize the antigen any more. such alterations happen randomly and can help individual members of a colony to survive and proliferate.

Secretion of degradative enzymes

• Some bacteria carry cellular weapons in the form of degradative proteins that can destroy immune cell-secreted antimicrobial peptide defenses or degrade the complement signaling molecules themselves.