Introduction Course to Porcine Immunology

IMMUNOLOGICAL MECHANISMS AGAINST BACTERIA AND PARASITES.

Both the innate and adaptive immune responses have a large number of mechanisms to fight against bacterial and parasitic infections.


Immune response against bacteria


The most effective immune mechanisms against bacteria will vary depending on the characteristics of the bacterial surface and their pathogenic processes.

One useful classification, which is according to the surface of the bacteria could be:

GRAM-positive bacteria

GRAM-negative bacteria

Mycobacteria

Spirochetes

GRAM (-) bacteria

Aspect of GRAM (-) bacteria (Salmonella) in XLD culture medium

Staphylococcus aureus colonies

  Aspect of Staphylococcus aureus colonies (GRAM (+)) in agar-blood.

Tthe most effective mechanism for destroying the bacterial membrane of Gram-positive bacteria, is phagocytosis; while for Gram-negative, mycobacteria and spirochetes (due to their double lipid envelope), the most effective way is cytotoxicity, mediated by complement and cytotoxic cells.  

The effectiveness of the immune mechanisms will be different depending on the pathogenic mechanism of the infectious agent (direct effect or toxins). For example: bacteria that produce toxins are effectively attacked by antibodies, while in pathologies directly related to the bacteria, cytotoxicity mechanisms will be more effective. 


Main mechanisms of innate and adaptive responses against bacteria 


The innate immune response has several mechanisms, in addition to the mechanical-chemical barriers, that play a main role in the case of bacteria. They are: complement activation through the alternative pathway, especially for Gram-negative bacteria, phagocytosis not activated by Fc, release of certain cytokines (tumor necrosis factor, interleukin IL 1) and activation of NK cells.  

Phagocytosis

As part of the adaptive immune response, antibody production plays a main role based on several mechanisms: neutralization of bacterial toxins, mucosa protection (IgA) and activation of the complement by the classical pathway, which produces the destruction of bacteria or activating phagocytosis by the stimulation of the Fc receptor. Phagocytosis is one of the most effective mechanisms against bacteria, so the release of cytokines is of great importance in this kind of infections. 


Immune response against parasites

Parasitic diseases have an important economic relevance in the swine species  

MAIN PARASITIC DISEASES IN THE PIG

EXTERNAL PARASITES:
Sarcoptic scabies 
INTERNAL PARASITES:
VERMIN ENTERITIS : Ascaris spp., Strongyloides spp., Echinorhynchus spp.
TRICHINOSIS: Trichinella spiralis
TAENIASIS: Taenia spp., Cysticercus spp.
SARCOCYSTOSIS: Sarcocystis suihominis
More parasites and images (Dr. Colin Johnstone. School of Veterinary Medicine. University of Pennsylvania)

Due to the complexity of their structures however, the real immunological mechanisms acting against parasites are not well known. The main characteristics of the immune response against parasites can be summarized in the following points:

  1. Parasites have a large number of antigens due to their bigger size and structural complexity. 

  1. Infections in pigs and other species are usually sub-clinical and frequently become chronic. The parasite lives for long periods of time in the host, inducing a prolonged antigenic stimulation and activating a large number of immunological mechanisms (both humoral and cellular). Antibodies and cytotoxicity are the most relevant.
  1. Parasites develop different mechanism to evade immune surveillance or at least make it less effective. Among them: changes in their antigenic structure in the different stages of the parasitic cycle; diversity in surface antigens; intracellular infections not reflected in changes of the host cell membrane. 

Main mechanisms of innate and adaptive responses against parasites

Even though parasites are very antigenic, not all of the responses that they induce are effective. This is mainly due to the fact that they produce different antigens during the different stages of their cycles. The immunological fight against parasitic diseases is still very limited. 


The first line of action against parasites is induced by macrophages, neutrophils and cytokines of the innate response (IL 1, IL 6, IL 12 and TNF). 

We can conclude that the phagocytosis mechanism is related to the first stages of the parasitic infection. 
During the adaptive response, antibodies and cytotoxic mechanisms will be the most important ones. Antibodies will have a direct effect and will also activate phagocytosis and the classical pathway of the complement activation, as well as ADCC mechanisms. The activation of CD 8+ cells is very important as a defensive mechanism, as well as several cytokines related to inflammation, especially  during the blood stage. 

 
 

 

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