The immune system is a system that provides an organism with a defense against infection. In
higher organisms it is afforded by the presence of circulating antibodies and leukocytes (white blood cells). Antibodies
are manufactured specifically to deal with the antigens associated with different diseases as they are encountered. White blood
cells attack and destroy foreign particles in the blood and other tissues.
The "soldiers" of the immune system are white blood cells, including T-lymphocytes and B-lymphocytes, which originate in the bone marrow from hematopoietic stem cells.
Every day the body comes into contact with many organisms such as bacteria, viruses, and parasites. Unopposed, these organisms have the potential to cause serious infections, such as pneumonia or AIDS. When a healthy individual is infected, the body responds by activating a variety of immune cells.
Initially, invading bacteria or viruses are engulfed by an antigen presenting cell (APC), and their component proteins (antigens) are cut into pieces and displayed on the cell's surface. Pieces of the foreign protein (antigen) bind to the major histocompatibility complex (MHC) proteins, also known as human leukocyte antigen (HLA) molecules, on the surface of the APCs. This complex, formed by a foreign protein and an MHC protein, then binds to a T cell receptor on the surface of another type of immune cell, the CD4 helper T cell. They are so named because they "help" immune responses proceed and have a protein called CD4 on their surface. This complex enables these T cells to focus the immune response to a specific invading organism.
The antigen-specific CD4 helper T cells divide and multiply while secreting substances called cytokines, which cause inflammation and help activate other immune cells. The particular cytokines secreted by the CD4 helper T cells act on cells known as the CD8 "cytotoxic" T cells (because they can kill the cells that are infected by the invading organism and have the CD8 protein on their surface). The helper T cells can also activate antigen-specific B cells to produce antibodies, which can neutralize and help eliminate bacteria and viruses from the body.
Some of the antigen-specific T and B cells that are activated to rid the body of infectious organisms become long-lived "memory" cells. Memory cells have the capacity to act quickly when confronted with the same infectious organism at later times. It is the memory cells that cause us to become "immune" from later reinfections with the same organism.