Changes

===Antigen Recognition===

===Antigen Recognition===

"Antigen" refers to the parts of a foreign organism recognisable by the adaptive immune system.  Typically, these are structural proteins, such as the spike proteins of viruses.  Antigens can be huge, and are more often identified by '''epitopes''', or smaller fragments of the folded proteins.  As such, a single antigen can be recognised by multiple antigen receptors.  [[Immunoglobulins|Antibody]] has evolved to recognise a dizzying array of antigen epitopes.  Antigen can be picked up by [[Lymphocytes|lymphocytes]] in the lymph tissues ([[Lymphocytes#T Cells|T cells]] and [[Lymphocytes#B Cells|B cells]]) or the blood stream ([[Lymphocytes#T Cells|T cells]] only).

"Antigen" refers to the parts of a foreign organism recognisable by the adaptive immune system.  Typically, these are structural proteins, such as the spike proteins of viruses.  Antigens can be huge, and are more often identified by '''epitopes''', or smaller fragments of the folded proteins.  As such, a single antigen can be recognised by multiple antigen receptors.  [[Immunoglobulins|Antibody]] has evolved to recognise a dizzying array of antigen epitopes.  Antigen can be picked up by [[Lymphocytes - Introduction|lymphocytes]] in the lymph tissues ([[Lymphocytes#T Cells|T cells]] and [[Lymphocytes#B Cells|B cells]]) or the blood stream ([[Lymphocytes#T Cells|T cells]] only).

[[Lymphocytes#T Cells|T cell]] receptors ('''TCR''') recognise '''antigen fragments''' (that is digestion products) on the '''surface of cells''', whereas [[Lymphocytes#B Cells|B cell]] receptors ('''BCR''') bind '''whole antigen''' in the '''extracellular fluid'''.  [[Lymphocytes#T Cells|T cells]] only "see" antigen when it is presented by [[Major Histocompatability Complexes|'''MHC''']] (Major Histocompatibility Complex) on the cell surface.  Antigen digestion and presentation is one of the major functions of the [[T cell differentiation#Antigen Presentation by Dendritic Cells|'''dendritic cells''']] (circulating [[Monocytes|monocytes]]) and '''[[Macrophages|macrophages]]'''.  These are referred to as '''Antigen-Presenting Cells''' (APCs).   

[[Lymphocytes#T Cells|T cell]] receptors ('''TCR''') recognise '''antigen fragments''' (that is digestion products) on the '''surface of cells''', whereas [[Lymphocytes#B Cells|B cell]] receptors ('''BCR''') bind '''whole antigen''' in the '''extracellular fluid'''.  [[Lymphocytes#T Cells|T cells]] only "see" antigen when it is presented by [[Major Histocompatability Complexes|'''MHC''']] (Major Histocompatibility Complex) on the cell surface.  Antigen digestion and presentation is one of the major functions of the [[T cell differentiation#Antigen Presentation by Dendritic Cells|'''dendritic cells''']] (circulating [[Monocytes|monocytes]]) and '''[[Macrophages|macrophages]]'''.  These are referred to as '''Antigen-Presenting Cells''' (APCs).