Cells of the adaptive immune system
- Positive selection ensures MHC restriction by testing the ability of MHCI and MHCII to distinguish between self and nonself proteins. In order to pass the positive selection process, cells must be capable of binding only self-MHC molecules. If these cells bind nonself molecules instead of self-MHC molecules, they fail the positive selection process and are eliminated by apoptosis.
- Negative selection tests for self tolerance. Negative selection tests the binding capabilities of CD4 and CD8 specifically. The ideal example of self tolerance is when a T cell will only bind to self-MHC molecules presenting a foreign antigen. If a T cell binds, via CD4 or CD8, a self-MHC molecule that isn’t presenting an antigen, or a self-MHC molecule that presenting a self-antigen, it will fail negative selection and be eliminated by apoptosis.
- Helper T cells express CD4, and help with the activation of T cells, B cells, and other immune cells.
- Cytotoxic T cells express CD8, and are responsible for removing pathogens and infected host cells.
- T regulatory cells express CD4 and another receptor, called CD25. T regulatory cells help distinguish between self and nonself molecules, and by doing so, reduce the risk of autoimmune diseases.
Humoral vs. Cell Mediated Immunity
Innate Immunity vs. Adaptive Immunity: A summary
|Attribute||Innate Immunity||Adaptive Immunity|
|Response Time||Fast: minutes or hours||Slow: days|
|Specificity||Only specific for molecules and molecular patterns associated with general pathogens or foreign particles||Highly specific! Can discriminate between pathogen vs. non-pathogen structures, and miniscule differences in molecular structures|
|Major Cell Types||Macrophages, Neutrophils, Natural Killer Cells, Dendritic Cells, Basophils, Eosinophils||T cells, B cells, and other antigen presenting cells|
|Key Components||Antimicrobial peptides and proteins, such as toxic granules||Antibodies|
|Self vs. Nonself Discrimination||Innate immunity is based on self vs. nonself discrimination, so it has to be perfect||Not as good as the innate immune system, but still pretty good at determining which is which. Problems in self vs. nonself discrimination result in autoimmune diseases|
|Immunological Memory||None||Memory used can lead to faster response to recurrent or subsequent infections|
|Diversity and Customization||Limited: Receptors used are standard and only recognize antigen patterns. No new receptors are made to adapt the immune response||Highly diverse: can be customized by genetic recombination to recognize epitopes and antigenic determinants.|