5.3 T lymphocytes and cell mediated immunity

The ability for organisms to resist infection by protecting against disease-causing microorganisms or their toxins that invade their bodies

Antigens are parts of an organisms or substances that are recognised as non self/foreign by the immune system and stimulates an immune response. They are usually proteins that are part of the cell surface membrane or the cell walls of invading cells eg microorganisms/ abnormal body cells like cancer cells. The presence of an antigen triggers the production of antibodies

Lymphocytes: They are produced by stem cells in bone marrow . They react to specific antigens . They are slower in action at first but they provide long term immunity

. B lymphocytes mature in the BONE marrow . T lymphocytes are made in bone marrow but mature in the THYMUS

. B cells are associated with humoural immunity, which is immunity involving antibodies that are present in bodily fluids, or ‘humour’ such as blood plasma . T cells are associated with cell-mediated immunity, which is immunity involving cells

. Phagocytes that have engulfed and hydrolysed a pathogen, then present some of the pathogens antigens on their own cell membrane . Body cells invaded by a virus, then present some of the viral antigens on their own cell surface membrane . Transplanted cells from individuals of the same species have different antigens on their cell surface membrane . Cancer cells are different from normal body cells, so they present antigens on their cell surface membranes

Lymphocytes respond to an organisms own cells that have been infected by non self material from a different species eg a virus They also respond to cells from other individuals of the same species because these are genetically different so have different antigens on their cell surface membrane from the antigens on an organisms own cells. T lymphocytes have to distinguish between these

Cells that display foreign antigens on their surface are antigen presenting cells because they can present antigens of other cells on their own cell surface membrane

T cells / lymphocytes only respond to cells that have antigens presented on them so it’s also called the cellular response The receptors on T cells respond to a single antigen, and each T cell responds to a different antigen

. Pathogens invade body cells or are taken in by phagocytosis . The phagocyte presents antigens from the pathogen on its cell surface membrane . Receptors on a specific T helper cells fit exactly onto these antigens . This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells

There are 4 different options . They develop into memory cells that enable a rapid response to future infections by the same pathogen . Stimulate phagocytes to engulf pathogens by phagocytosis . Stimulate B cells to divide and secrete their antibody . Activate cytotoxic T cells

. Kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin . Perforin makes holes in the cell surface membrane, so the cell membrane becomes freely permeable to all substances, and the cell dies as a result.

. Viruses because they replicate inside of living cells . So if living cells die, the viruses can’t multiply as much