TABLE OF CONTENTS
Host-Parasite Relationship: Immune Evasion and Survival Mechanisms
Parasites have evolved several survival mechanisms to escape destruction by the host immune system. These adaptations help them survive, establish infection, obtain nutrition, and reproduce within the host. Different parasites use different immune evasion strategies such as antigenic variation, antigenic mimicry, cyst formation, and immunomodulation to persist successfully inside the host body.
Parasites must adopt one or more tactics or strategies to evade host defense mechanisms; otherwise, they would be destroyed.
Parasites commonly adopt the following strategies to survive successfully within the host environment:
Site of Predilection
The site of predilection may provide some protection against host defense mechanisms. The lumen of the intestine is one of the safest sites for parasite survival because IgA is not a highly potent molecule for destroying worms, and complement proteins and phagocytic cells are normally absent from the intestinal lumen.
Development of Cyst Wall
Some parasites, such as larval tapeworms, develop a cyst wall that provides protection against the host immune response.
Antigenic Mimicry or Disguise
The blood fluke Schistosoma adsorbs host antigens onto its surface; therefore, the host immune system is unable to recognize the fluke as foreign and instead identifies it as self.
Antigenic Variation
Trypanosomes frequently change their surface coat through antigenic variation, leading to polyclonal activation of B cells followed by immunosuppression.
Immunomodulation
Ticks have the ability to modulate the host immune response in several ways to facilitate successful feeding. When a tick bites, the host’s innate and acquired immune responses act against tick feeding, whereas the tick’s immunomodulatory strategies counteract the host immunity.
Generally, tick feeding induces pain and stimulates the host’s grooming activity, which is an important factor in dislodging ticks from the body. However, tick saliva contains molecules with anticoagulant (apyrase), anti-inflammatory (PGE2), anti-hemostatic, and immunosuppressive properties that enable ticks to feed successfully.
| Mechanism | Function | Example |
|---|---|---|
| Site of Predilection | Protection from host immunity | Intestinal helminths |
| Cyst Wall Formation | Protection against immune attack | Larval tapeworms |
| Antigenic Mimicry | Avoids immune recognition | Schistosoma |
| Antigenic Variation | Changes surface antigens | Trypanosomes |
| Immunomodulation | Suppresses host immunity | Ticks |
