The specific response to infection.

The immune system is the specific response of the body to invasion by pathogens. It enables the bodyto recognise anything that is non-self and remove it efficiently. It has 4 key characterisitics - 

• It can distinguish self from non-self
• It is specific
• It is diverse
• t has immunological memory

There are 2 main types of lymphocyte involved in the immune system.

• B cells are made in the bone marrow, but are found everywhere in the body. They have membrane-bound globular receptor proteins on their cell surface membrane, which are identical to the antibodies they will later produce. All antibodies are known as immunoglobulins.
• T cells are also made in the bone marrow but mature in the thymus gland. The surface of each T cell displays thousands of identical T-cell receptors. There are 2 types of T cell, T killer cells and T helper cells. The T killer cells produce chemicals that desroy pathogens. The T helper cells are involved in the process which produces antiodies against the antigens on a particular pathogen.

The Humoral response.

In the T helper stage, when a pathogen enters the body, the non-specific response will rapidly ring it into contact with macrophages which will engulf the pathogen by phagocytosis. The vesicle containing the pathogen fuses with a lysosome and the enzymes in the lysosome break down the pathogen and separate off the antigens. These then move to the surface of the cell outer membrane. The macrophage with these antigens displayed is now known as an antigen-presenting cell. Next, CD4 receptors on the outer membrane of aa T helper cell enable it to bind to the specific antigen which triggers the T helper cell to reproduce and form a clone of…