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Immune system. Brief review. By Jon Trister MD

The Immune System, brief review

 To defend as form invaders  body has two immune systems:

The first defense system is an innate immune system we are born with. This is a  first defense against any foreigners-microbes, viruses, funguses. This is antigen non-specific defense mechanisms which respond to invaders immediately or within several hours after exposure to every foreigner. Cells associated with innate immune system are Macrophages, Dendritic cells and  B-lymphocytes

 The second defense system is an adaptive immune system (acquired) immunity refers to antigen-specific defense and require  several days to become protective and are designed to react with and remove a specific antigen. Adaptive immunity is the immunity one develops throughout life. Cells associated with adaptive immune system are T- and B-Lymphocytes

One of the function of the innate  immune system is an APC ( antigen presentation cells)-is an identification, isolation  of the antigen-specific protein -epitope-actual portions or fragments of an antigen that react with receptors on B-lymphocytes and T-lymphocytes, as well as with free antibody molecules) and presentation it in the from of complex : MHC I/epitope  or MHC ii/epitope on the surface of the APC ( Macrophages, Dendritic cells, B-Lymphocytes).Epitope is a fragment of the antigen which will provide adaptive immune system a specific  information about invader ( antigen).Without ability to perform this function as an APC innate immune system will be unable to “educate” adaptive immune system about structure of the antigen.

Polysaccharides antigens( 3-4 sugar residuals)  usually have many epitopes but all of the same specificity.

Proteins antigens (5-15 amino acids )usually have many epitopes of different specificities.

Immune responses are directed against many different epitopes of many different antigens of the same microbe.

The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes and T-lymphocytes by means of epitope-specific receptor molecules having a shape complementary to that of the epitope.


                     MHC molecules

MHC-I molecules are made by all nucleated cells in the body

MHC-I presents epitopes to T8-lymphocytes; MHC-II presents epitopes to T4-lymphocytes.

MHC-I molecules are designed to enable the body to recognize infected cells and tumor cells and destroy them with cytotoxic T-lymphocytes or CTLs.

CTLs are effector defense cells derived from naïve T8-lymphocytes.

MHC-I molecules are made by all nucleated cells in the body; bind peptide epitopes typically from endogenous antigens; present MHC-I/peptide complexes to naive T8-lymphocytes and cytotoxic T-lymphocytes CTL.


MHC-II molecules are made by antigen-presenting cells or APCs, such as dendritic cells, macrophages, and B-lymphocytes; bind peptide epitopes typically from exogenous antigens; and present MHC-II/peptide complexes to naive T4-lymphocytes or effector T8-lymphocytes that have a complementary shaped T-cell receptor or TCR

Exogenous antigens enter antigen-presenting macrophages, dendritic cells, and B-lymphocytes through phagocytosis, and are engulfed and placed in a phagosome where protein antigens from the microbe are degraded by proteases into a series of peptides. These peptides are then attached to MHC-II molecules that are then put on the surface of the APC.