3.2.4 Cell recognition and the immune system (T lymphocytes and cell-mediated immunity)

An antigen is any part of an organism/substance that is recognised as non-self (foreign) by the immune system and stimulates an immune response. The presence of an antigen stimulates the production of antibodies.

As mentioned in 3.2.4 Defence mechanisms, immune responses such as phagocytosis are non-specific. Specific responses are slower but can provide longer lasting immunity. The type of response (humoral or cell-mediated) displayed depends on the white blood cell present. There are two types of lymphocyte:

• B lymphocytes (mature in the bone marrow, think B for Bone). These are associated with humoral immunity (immunity involving antibodies present in body fluids or humour such as blood plasma)
• T lymphocytes (mature in the thymus gland, thing T for Thymus). These are associated with cell-mediated immunity/cellular response (immunity involving body cells).

Cell mediated immunity/cellular response:

Invader cells have different antigens on their surface to antigens on self-cells. T lymphocytes can distinguish invader cells from normal cells because:

• Phagocytes that have engulfed and hydrolysed a pathogen (phagocytosis) present some of a pathogen's antigens on their own cell surface membrane
• Body cells invaded by a virus present viral antigens on their own cell surface membranes
• Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane
• Cancer cells present antigens on their cell surface membrane

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

T lymphocytes differ from B lymphocytes as T lymphocytes will ONLY respond to antigens that are present on a body cell (rather than within body fluids). The receptors on T each T cell responds to a single antigen. It follows that there is a vast number of T lymphocytes, each responding to one antigen. The 'method' of cell-mediated immunity/cellular response is:

• Pathogens are taken in by phagocytes
• The phagocyte places antigens from the pathogen on its cell-surface membrane
• Receptors on a specific helper T cell fit onto these antigens
• This attachment activated the T cells to divide rapidly by mitosis and form a clone of genetically identical cells (clonal selection)
• These cloned T cells can either:
• Develop into memory cells that enable a rapid secondary response (for if the pathogen invades the body once again)
• Stimulate phagocytes to engulf pathogens by phagocytosis
• Stimulate B cells to divide and secrete their antibody
• Activate cytotoxic T cells. Cytotoxic T cells kill abnormal body cells/infected body cells by producing the protein perforin that makes holes in the cell-surface membrane meaning the membrane becomes freely permeable to all substances and the cell dies.

NOTE: The action of T cells is most effective against viruses as viruses replicate inside other cells.

Okay so i'm not too sure where to slot this next bit in but it's in the spec so i'll just put it here:

The effect of antigen variability on disease and disease prevention. Antigenic variability means that the antigens on the surface of the pathogen are constantly changing so, every time you're infected, you're immune system will not have the memory cells with complimentary antibodies, so the above process will have to start all over again. This means there will not be a rapid secondary response to the pathogen ( as it is technically a different pathogen). Therefore, even if you were vaccinated against the 'old' pathogen, since it's antigens have changed you're vaccine will not prevent you against infection. The common cold is a good example here, everyone gets infected pretty frequently (well, I do anyway). This is because each time you get infected with a slightly different pathogen, so you don't have the built up immune responses to immediately fight against it.