The immune system is a collection of organs, vessels and processes that protect the body from infection. It is a specific response system, as opposed to the nonspecific responses above.
These do not depend on a previous exposure to foreign substances – they include the mechanical and non-specific barrier discussed above (eg skin, phagocytic cells, tears, mucous, natural killer (NK) cells). All the elements of natural immunity are present in all normal individuals prior to the first exposure to an infectious agent. The effectiveness of natural immunity can be mediated by factors such as age, nutrition and hormone levels. It does not recognise specific species of microorganisms or have a more intense response on subsequent exposure.
• based on specific response to antigens
• only becomes active after exposure to a foreign agent (called antigens if invoke a specific response)
• stimulated by a specific immunising event (infection or vaccination)
• usually becomes more intense on subsequent response
Humoral immunity (antibody immunity):
• immune response mediated by antibodies
• protects against bacteria and viruses
Cellular immunity (cell-mediated immunity):
• the immune response in which the targets are attacked directly by the immune system cells (eg T cells, macrophages)
• protects against viruses, mycobacteria and fungi