Each individual organism has to protect itself against a large variety of infectious microbial agents, such as bacteria, fungi and parasites to prevent pathological damage and death. In vertebrates, defense mechanisms against foreign substances, antigens, have evolved in the immune system, which has two functional divisions: the 'innate' immune system and the 'adaptive' immune system. The 'innate' immune system is aspecific and acts as a first line of defense, mediated by cells from thc myeloid lineage and soluble factors like complement and lysozyme. The main function of the 'innate' immune system is to avoid entering of micro or gall isms into the body and to clear it of killed pathogens. In contrast to the 'adaptive' immune system, repeated infection does not improve the resistance of the 'innate' immune system. If the first line of defense is defeated, the second line of defense, the 'adaptive' immune system, which is very specific and can develop memory to earlier accounted pathogens, is activated. The specific immune response is mediated by lymphocytes belonging to the B andlor T lineages (B and T cells). Both Band T cells express receptor molecules on their cell membrane, which specifically can bind antigens. The T ceH receptor (TCR) can only bind antigens if these are processed into small peptides, and presented by major histocompatibility complex (MHC) class I molecules on the surface of host cells and MHC class II molecules on the surface of antigen presenting cells. Intracellular antigens are processed into small peptides and presented on the surface by the MHC class I complex. Recognition of the MHC-class I-peptide complex by the TCR of cytotoxic T cells results in killing of the presenting host cells. MHC class II molecules present processed peptides, which are derived from external clldocytosed antigens. Recognition of MHC class II-peptide complexes by the TCR ofT helper (T H) cells results in the production of cytokines and stimulation of cells of the immune system ('cellular' immune response). The B cell receptor (BCR) binds to unprocessed antigens. Stimulation of the BCR results in a 'humoral' immune response, i.e. the secretion of soluble immunoglobulins (Ig)J with the same binding specificity as the BCR to the triggering antigen.

F.G. Grosveld (Frank)
Erasmus University Rotterdam
NWO, Praeventiefonds
Erasmus MC: University Medical Center Rotterdam

Maas, A. (1998, December 16). Studies on the function of Brutoll 's tyrosine kinase in B cell development. Erasmus University Rotterdam. Retrieved from http://hdl.handle.net/1765/17508