Microbiology And Immunology Notes

Immunity

Innate immunity

The term ‘immunity’ is defined as the resistance offered by the host against microorganism(s) or any foreign substance(s). Immunity can be broadly classified into:

1. Innate immunity: present right from the birth
2. Acquired/Adaptive immunity: acquired during the course of the life.

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Factors Influencing Innate Immunity

• Depends on the Species, Race, Individual (genetic influence)
• Age, Hormonal influence and Nutrition.

Toll Like Receptors

They are so named because they are similar to Toll receptors present in the fruit fly- Drosophila,where it is the main receptor for induction of innate immunity by bind to particular MAMP molecules on microbial surfaces. They are of 13 types, out of which important ones are:

• TLR-2 binds to bacterial peptidoglycan
• TLR-3 binds to dsRNA of viruses
• TLR-4 binds to LPS of Gram-negative bacteria
• TLR-5 binds to flagella of bacteria
• TLR-7 & 8 bind to ssRNA of viruses
• TLR-9 binds to bacterial DNA.

Acute Phase Reactant Proteins (APRs)

They are the proteins synthesized by liver at steady concentration, but their synthesis either increases or decreases exponentially during acute inflammatory conditions.

Though liver is the primary site, APRs can also be synthesized by various other cells such as endothelial cells, fibroblasts, monocytes and adipocytes.

Positive APRs are the proteins whose levels increase during acute inflammation. Examples:

• Serum Amyloid A
• C- Reactive protein
• Complement proteins: Complement factors (C1–C9), factor B, D, and properdin
• Coagulation protein, e.g. fibrinogen, von Willebrand factor
• Proteinase inhibitors, e.g. α1 antitrypsin
• α1 acid glycoprotein
• Mannose binding protein
• Haptoglobin
• Metal binding proteins, e.g. ceruloplasmin

Negative APRs: They are the proteins whose levels are decreased during acute inflammation thus creating a negative feedback that stimulates the liver to produce positive APRs.

Examples of negative APRs include: albumin, transferrin, and antithrombin.

Role of APRs: They have a wide range of activities that contribute to the host defense:

• APRs have various antimicrobial and anti-inflammatory activities (e.g. complement)
• Metal binding proteins can chelate various metals such as iron, copper, etc. making them unavailable for the bacteria.

C-Reactive Protein (CRP)

CRP is an example of APR that rise in acute inflammatory conditions including bacterial infections. It belongs to beta globulin family.

• CRP is so named because it precipitates with C- carbohydrate (polysaccharide) antigen of Pneumococcus.
  However, it is not an antibody against the C-carbohydrate antigen of Pneumococcus; it is nonspecific, can be raised in any inflammatory conditions.
• Commonest marker of acute inflammation, used in most diagnostic laboratories.

CRP can be detected by

• Precipitation method using C carbohydrate antigen (obsolete, not in use now)
• Latex (passive) agglutination test using latex particles coated with anti-CRP antibodies.
• It is the most widely used method employed worldwide.
  • Detection limit of CRP by latex agglutination test 0.6 mg/dL.
• Highly sensitive CRP (hs-CRP) test: Minute quantities of CRP can be detected by various methods (e.g. nephelometry, enzyme immunoassays).
  This is useful in assessing the risk to cardiovascular diseases.

Acquired Or Adaptive Immunity

Acquired immunity is defined as the resistance against the infecting foreign substance that an individual acquires or adapts during the course of his life.

It is of two types: Active immunity and passive immunity.

• Active immunity can again be divided into:
• Primary immune response (which develops after first microbial exposure)
• Secondary immune response (which develops after subsequent microbial exposure).

Other Types Of Immunity

Local or mucosal immunity is the immune response that is active at the mucosal surfaces such as intestinal or respiratory or genitourinary mucosa:

• It is usually mediated by a type of IgA antibody called secretory IgA.
• Local immunity can only be induced by natural infection or by live vaccination, e.g. after OPV (but not by killed vaccines).

Herd Immunity

Herd immunity is defined as the overall immunity of a community (or herd) towards a pathogen:

• Herd immunity plays a vital role in preventing epidemic diseases. If the herd immunity is good, that means large population of the community are immune towards a pathogen.
  Hence, epidemics are less likely to occur and eradication of the disease may be possible.
• Elements that contribute to create a strong herd immunity are:
  • Occurrence of clinical and subclinical cases in the herd
  • Ongoing immunisation programme
  • Herd structure, i.e. type of population involved
  • Type of pathogen: Herd immunity may not be strong in a community against all the pathogens.

Adoptive Immunity

It is the process of transfer of CMI from one individual to other.

• It occurs following injection of immunologically competent T-lymphocytes known as Transfer factor.
• It is useful for treatment when the CMI is low, e.g. in lepromatous leprosy.