The key role of the Immune System is to protect the organism from infectious diseases (infections by bacteria, virus, parasites) or from cancers (due to the proliferation of tumour cells).
It is a very complex system and is made up of a network of highly specialized cells (B cells, T cells, etc), tissues and organs (thymus, bone marrow, spleen, lymph nodes disseminated in the body) that work together to protect the body through various, sophisticated mechanisms.
It is also a combinatorial system with a large number of products typically with >1015 antibodies and >1012 immune system cell clones in an individual. These numbers dwarf the 106 non-immune-system products (including modified and alternative products) encoded by the human genome.
Its function depends on both the individual genetic composition and previous exposure to the foreign agents - the experience of the organism. When a specific pathogen agent interacts with the organism for a second time, the immune system reacts more strongly and quickly to the attack. This occurs because the immune system is able to remember the first attack.
The Immune System acts mainly by producing highly specialized proteins (antibodies or immunoglobulins IG, T cell receptors TR) and specialized cells (lymphocytes) that oppose and destroy foreign agents (antigens).
Important examples of such cells include the T and B lymphocytes: T lymphocytes are usually divided into two major subsets that are functionally different, helper T cell and cytotoxic T cell.
The major function of B lymphocytes is the production of antibodies which are specialized proteins that specifically recognize and bind to one particular foreign substance or antigen (at the surface of parasites, bacteria, viruses, or tumour cells).
The action mechanism of the Immune System is briefly explained in the following: when a native antigen is specifically recognized by a B cell (the part of the antigen which binds to the B cell is the B cell epitope), the B cell is stimulated to proliferate and produce circulating antibodies that also specifically bind to the same antigen epitope.
T cells possess, on their surface, T cell receptors (TR) which also recognize specifically antigens, but these antigens are peptides (resulting from the fragmentation of the antigenic protein) presented by the MHC (major histocompatibility complex) classes I and II (human leukocyte antigens HLA, in human) (the part of the peptide which binds to the T cell is the T cell epitope).
The peptides can derive from endogenous pathogens (originated from the inside of the cell) or from extra cellular pathogens (coming from outside the cell). In the first case, the fragmentation of the antigenic protein occurs in a degrading machinery known as proteasome. Some of these peptides are transported to the endoplasmic reticulum through a special transporter known as TAP protein (Transporter associated with antigen processing). Finally, the peptides are bound to the MHC class I and recognized by cytotoxic T cells (which express the CD8 protein). In the second case, the fragmentation occurs in a special cell compartment which is acidic and rich in proteases. The peptides generated in this way compete for binding to MHC class II. The peptides bound to the MHC class II are recognized by helper T cells (which express the CD4 protein).
There is a highly connected network of interactions between the specialized cells of the immune system and these interactions occur by cell-cell contacts (B cell-T helper cell) and by secretion of many activating and inhibiting factors. For example, the production of antibodies by the B cells is highly regulated by the T helper cell subset. These interactions are as many signals which allow cells to proliferate, to be controlled and to function more efficiently.