The third line of defense is part of the specific immune response and it can either be cell mediated or humoural. This type of immunity is long lasting due to the production of memory cells. This is shown through the diagram below; with innate immunity having resulted from genetic factors that are there from birth such as intact skin, mucous membranes and phagocytes which are all part of the second line of defense. Acquired immunity refers to immunity that, instead of being born with, the person must either experience the disease and make their own memory B and T cells or, in the case of pathogens that can not be cured by human antibodies, or fast-acting pathogens that will kill the person before the B and T memory cells are even made, the person must be given ready made antibodies. These can be through artificial antibodies from other sources (snake anti-venom) or natural, maternal antibodies (such as those found in breast milk). Acquired immunity can also be acquired through active process through the use of own antibodies. Humans, when they encounter a disease will create antibodies in case said pathogen invades again and attempts to invade the body. For this to occur, the person must encounter the pathogen, defeat it, create memory T and B cells and then they will have relative immunity from the specific pathogen for life.
T and B cells are made in the bone marrow. B cells are matured in the bone marrow and the T cells leave the bone marrow soon after creation and mature in the thymus. Before the aforementioned cells leave their ‘home’ they are given the ability to distinguish between self and non self through being able to identify markers on a cell. Antigens are chemicals that are specific to ‘self’ and therefore are used to signal to the B and T cells to distinguish between self and non-self; if the T cells see the cell as non-self they will stimulate a specific antibody for the specific antigen. Humans have two groups of MHC markers. Group one is found on all cells except red blood cells. Group two is found only on t cells, B cells and some macrophages. These markers are used to distinguish between self and non self. If seen as non-self the B and T cells will set off a response. The exact way in which T and B cells work and are formed will be explored in a latter tab.