Introduction to Avoiding Host Defense Mechanisms: Videos & Practice Problems

Pathogens have developed various strategies to evade host defenses, enabling them to cause disease. Key mechanisms include hiding within host cells, avoiding phagocytes through capsules and Fc receptors, and surviving inside phagocytes. They can also evade the complement system and antibodies using IgA proteases, antigenic variation, or mimicking host cells. Understanding these mechanisms is crucial for grasping pathogen behavior and developing effective treatments against infections.

Introduction to Avoiding Host Defense Mechanisms

In this video, we're going to begin our introduction to how pathogens avoid host defense mechanisms. Pathogens have evolved a wide variety of mechanisms that allow them to avoid the defense mechanisms of a host. Below, we have a map of our lesson on these mechanisms of avoiding host defenses. You'll notice the title of our map here is "Avoiding Host Defenses" and how pathogens can avoid host defenses and cause disease. Notice that there are a wide variety of mechanisms that we're going to talk about in more detail as we move forward in our course. Because this image represents a map, you can use it as a guide as we progress. We're going to cover the leftmost branches first.

We'll discuss how pathogens can hide within host cells to avoid host defenses. We'll also explore how they can avoid phagocytes in many different ways such as by preventing their encounter with phagocytes, by avoiding opsonization by phagocytes using capsules, and preventing opsonization by using Fc receptors. More details about this will be discussed as we advance. Then, we'll cover surviving inside of phagocytes to avoid the host defenses, avoiding the complement system, and avoiding antibodies through various methods like using IgA proteases, using antigenic variation, or mimicking host cells.

This is just a map of our lesson moving forward, and you can refer back to this lesson to help guide you and assist you in understanding where we are within the lesson. This concludes our brief introduction and I'll see you all in our next lesson video where we'll discuss more details about hiding within host cells. See you in that video.

All of the following are methods that pathogens use to avoid phagocytosis by immune cells except which of these answers?

• A. The pathogens avoid being bound by C3b proteins that trigger opsonization.
• B. The pathogens possess Fc receptors which bind and change the orientation of antibodies bound to them.
• C. The pathogens create IgA proteases which break down IgA antibodies before they can bind the pathogen.
• D. The pathogens create toxins which damage the phagocytic immune cells stopping phagocytosis.

Answer: (C)

Certain pathogens, like Staphylococcus aureus, hide within host cells to avoid being phagocytosed by immune cells. What other advantageous reasons would a pathogen have for hiding within host cells?

• A. Avoid being bound by antibodies.
• B. Avoid being bound by opsonization proteins like C3b.
• C. Avoid triggering the complement system.
• D. The pathogen can lie dormant until an opportune time to cause disease.
• E. All of the above.

Answer: (E)

Here’s what students ask on this topic:

How do pathogens hide within host cells to avoid immune detection?

Pathogens can hide within host cells to evade immune detection by entering and residing in cells where they are less accessible to immune defenses. This intracellular lifestyle allows them to avoid extracellular immune components like antibodies and complement proteins. Some pathogens, such as viruses, naturally replicate inside host cells. Others, like certain bacteria, have evolved mechanisms to invade and survive within host cells. For example, Mycobacterium tuberculosis can inhibit phagosome-lysosome fusion in macrophages, allowing it to survive and replicate within these immune cells. By hiding within host cells, pathogens can effectively evade the host's immune surveillance and establish persistent infections.

What are the mechanisms pathogens use to avoid phagocytosis?

Pathogens employ several strategies to avoid phagocytosis, including:

• Capsules: Many bacteria produce a polysaccharide capsule that inhibits phagocytosis by preventing recognition and binding by phagocytes.
• Fc Receptors: Some pathogens express Fc receptors that bind the Fc region of antibodies, preventing opsonization and subsequent phagocytosis.
• Preventing Encounter: Pathogens can secrete enzymes or toxins that kill phagocytes or inhibit their chemotaxis.
• Surviving Inside Phagocytes: Some pathogens can survive and replicate within phagocytes by inhibiting phagosome-lysosome fusion or resisting the toxic environment of the phagolysosome.

These mechanisms help pathogens evade the immune system and establish infections.

How do pathogens avoid the complement system?

Pathogens can avoid the complement system through various mechanisms:

• Capsules: Capsules can prevent complement activation by inhibiting the binding of complement proteins to the bacterial surface.
• Complement Regulatory Proteins: Some pathogens mimic host complement regulatory proteins, which inhibit complement activation.
• Proteases: Pathogens can produce proteases that degrade complement proteins, preventing their activation and function.

By evading the complement system, pathogens can avoid opsonization, inflammation, and cell lysis, enhancing their ability to cause disease.

What is antigenic variation and how does it help pathogens evade the immune system?

Antigenic variation is a mechanism by which pathogens alter their surface proteins to evade the host's immune response. By changing the antigens that are recognized by the immune system, pathogens can avoid detection and destruction by antibodies and immune cells. This variation can occur through genetic mutations, recombination, or gene conversion. For example, the malaria parasite Plasmodium falciparum frequently changes its surface proteins to avoid immune recognition. Antigenic variation allows pathogens to persist in the host, cause chronic infections, and evade vaccine-induced immunity.

How do IgA proteases help pathogens evade the immune system?

IgA proteases are enzymes produced by certain pathogens that specifically cleave IgA antibodies. IgA is a critical component of the mucosal immune system, providing the first line of defense against pathogens at mucosal surfaces. By producing IgA proteases, pathogens can degrade IgA antibodies, reducing their ability to neutralize and opsonize the pathogen. This allows the pathogen to colonize mucosal surfaces and evade the host's immune response. Examples of pathogens that produce IgA proteases include Neisseria gonorrhoeae and Haemophilus influenzae.