Changes

==Introduction==

==Introduction==

Adaptive immunity is a defence system built on specific cellular targeting.  It takes time for the immune system to develop its weaponry (up to 96 hours after infection), but ultimately the adaptive response is far more effective because of its precision.

Adaptive immunity is a defence system built on specific cellular targeting.  It takes time for the immune system to develop its weaponry (up to 96 hours after infection), but ultimately the adaptive response is far more effective because of its precision.

[[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).   

Naive B-cells express [[Immunoglobulin D|IgD]] and [[Immunoglobulin M|IgM]] on their cell surfaces, which bind antigen as it is washed into lymph tissue with the afferent lymph fluid.  Antigen is presented to [[Lymphocytes#B Cells|B cells]] by [[B cell differentiation#Antigen Presentation by Follicular Dendritic Cells|'''follicular dendritic cells''' (FDCs)]], which are also classed as APCs.  [[B cell differentiation#Antigen Presentation by Follicular Dendritic Cells|FDCs]] can endocytose antigen directly from the afferent lymph or receive them from [[Lymphocytes#Helper CD4+|CD4+ T-cells]].

Naive B-cells express [[Immunoglobulin D|IgD]] and [[Immunoglobulin M|IgM]] on their cell surfaces, which bind antigen as it is washed into lymph tissue with the afferent lymph fluid.  Antigen is presented to [[Lymphocytes#B Cells|B cells]] by [[B cell differentiation#Antigen Presentation by Follicular Dendritic Cells|'''follicular dendritic cells''' (FDCs)]], which are also classed as APCs.  [[B cell differentiation#Antigen Presentation by Follicular Dendritic Cells|FDCs]] can endocytose antigen directly from the afferent lymph or receive them from [[Lymphocytes#Helper CD4+|CD4⁺ T-cells]].

===Cellular response: Proliferation and Differentiation===

===Cellular response: Proliferation and Differentiation===

Once [[Lymphocytes#T Cells|T cells]] recognise antigen presence in the tissues, they go into action.  Their first response is always to recruit help, which is accomplished by returning to the nearest [[Lymph Nodes - Anatomy & Physiology|lymph node]] to carry out clonal expansion.  Daughter [[Lymphocytes#T Cells|T cells]] are created with identical TCRs in order to recognise the identified antigen.  These daughter cells are then returned to the circulation via the efferent lymph.   

Once [[Lymphocytes#T Cells|T cells]] recognise antigen presence in the tissues, they go into action.  Their first response is always to recruit help, which is accomplished by returning to the nearest [[Lymph Nodes - Anatomy & Physiology|lymph node]] to carry out clonal expansion.  Daughter [[Lymphocytes#T Cells|T cells]] are created with identical TCRs in order to recognise the identified antigen.  These daughter cells are then returned to the circulation via the efferent lymph.   

[[Lymphocytes#T Cells|T cells]] can differentiate three different ways, based on their Cluster of Differentiation (CD) number.  All [[Lymphocytes#T Cells|T cells]] are CD3+, and naive circulating [[Lymphocytes#T Cells|T cells]] will differentiate upon interaction with antigen to become either [[Lymphocytes#Cytotoxic CD8+|'''CD8+ (cytotoxic)''']] or [[Lymphocytes#Helper CD4+|'''CD4+ (helper)''']] [[Lymphocytes#T Cells|T cells]]. [[[[Lymphocytes#Helper CD4+|CD4+ T-cells]] will initially become CD4-TH₀ cells, and must differentiate to [[T cell differentiation#TH1 Cells|TH₁]] or [[T cell differentiation#TH2 Cells|TH₂]] depending on the whim of the adaptive response.  TH₁ and TH₂ cells carry out different types of responses: TH₁ is responsible for enhancing the [[Macrophages|macrophage]] response, whereas TH₂ cells enhance the [[Lymphocytes#B Cells|B cell]] antibody production.  Typically, animals produce a balanced response of TH₁ and TH₂ cells, though this can lead to pathology, as can a skewed response, depending on the nature of the foreign organism.  For more on [[Lymphocytes#T Cells|T cell]] differentiation, see [[T cell differentiation|here]].

[[Lymphocytes#T Cells|T cells]] can differentiate three different ways, based on their Cluster of Differentiation (CD) number.  All [[Lymphocytes#T Cells|T cells]] are CD3⁺, and naive circulating [[Lymphocytes#T Cells|T cells]] will differentiate upon interaction with antigen to become either [[Lymphocytes#Cytotoxic CD8+|'''CD8⁺ (cytotoxic)''']] or [[Lymphocytes#Helper CD4+|'''CD4⁺ (helper)''']] [[Lymphocytes#T Cells|T cells]]. [[Lymphocytes#Helper CD4+|CD4⁺ T-cells]] will initially become CD4⁺-T_H0 cells, and must differentiate to [[T cell differentiation#TH1 Cells|T_H1]], [[T cell differentiation#TH2 Cells|T_H2]] or [[T cell differentiation#TH17 Cells|T_H17]] cells depending on the whim of the adaptive response (T_H17cells are a recent addition to this model, with research still being conducted into function).  T_H1,  T_H2 and T_H17 cells carry out different types of responses: T_H1 is responsible for enhancing the [[Macrophages|macrophage]] response, whereas T_H2 cells enhance the [[Lymphocytes#B Cells|B cell]] antibody production, and T_H17 cells enhance the innate immune response through increased granulocyte trafficking.  Typically, animals produce a balanced response of T_H cells, though this can lead to pathology, as can a skewed response, depending on the nature of the foreign organism.  For more on [[Lymphocytes#T Cells|T cell]] differentiation, see [[T cell differentiation|here]].

*''[[Lymphocytes#B Cells|B cell]] response''

*''[[Lymphocytes#B Cells|B cell]] response''

*[[T cell differentiation#Antigen Presentation by Dendritic Cells|Interdigitating Dendritic Cells]]

*[[T cell differentiation#Antigen Presentation by Dendritic Cells|Interdigitating Dendritic Cells]]

**Only IDCs can incite a primary response in naive T-cells

**Only IDCs can incite a primary response in naive T-cells

*[[Helper CD4+|CD4+ Tcells]]

*[[Helper CD4+|CD4⁺ Tcells]]

*[[B cells|B-cells]]

*[[B cells|B-cells]]

*Prof Dirk Werling DrMedVet PhD MRCVS

*Prof Dirk Werling DrMedVet PhD MRCVS

[[Category:Adaptive Immune System]]

[[Category:Adaptive Immune System]]