Defense Mechanisms in Immunology

Study Notes

Skin is a physical barrier against pathogens and is acidic (pH 3-5) which hinders microbial growth.
Mucous membranes are found in conjunctiva, alimentary, respiratory, and urogenital tracts, and are lined with epithelial cells for protection.
Mucus traps foreign microorganisms and cilia in the respiratory tract propel mucus-entrapped microorganisms out of the body.
Pathogens can invade the body by penetrating mucous membranes, but they face various innate immune mechanisms.
Saliva, tears, and mucous secretions wash away invaders and contain antibacterial or antiviral substances.
Normal flora outcompete pathogens for attachment sites and nutrients on epithelial surfaces of mucosal tracts.
Influenza virus uses specific surface molecules to bind firmly to respiratory tract mucous membrane cells, preventing its expulsion by cilia.
The bacterium that causes gonorrhea has surface projections to bind to urogenital tract epithelial cells, allowing it to adhere to mucous membranes.

Body temperature inhibits pathogen growth, for example, chickens are resistant to anthrax due to their higher body temperature.
Acidity of stomach contents (low pH) kills most ingested microorganisms, a reason why newborns are susceptible to some diseases.
Innate immunity is aided by lysozyme, interferon, and complement.
Lysozyme, found in mucous secretions and tears, cleaves the peptidoglycan layer of bacterial cell walls.
Interferon, produced by virus-infected cells, induces an antiviral state in uninfected cells by binding to them.
Complement is a group of serum proteins that circulate in an inactive state and can be activated by various specific and nonspecific immune mechanisms.

Tissue damage triggers the release of vasoactive and chemotactic factors, increasing blood flow and capillary permeability, leading to inflammation.
Increased capillary permeability allows influx of fluid (exudate) and cells from the blood into the affected area.
The exudate contains serum proteins with antibacterial properties, and attracts phagocytes to engulf bacteria.

Both B and T lymphocytes recognize discrete sites on antigens called antigenic determinants, or epitopes, which are the immunologically active regions.
B cells can recognize an epitope alone, but T cells need the epitope to be associated with an MHC molecule on the surface of a self-cell (either an antigen-presenting cell or an altered self-cell).
Each branch of the immune system (humoral and cell-mediated) has a unique way of recognizing antigens.
The humoral branch (B cells) can recognize a diverse variety of epitopes.

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