Changes

They are also known as leukocytes. There are five main types of leukocytes, and these types are produced from immature cells in the [[lymphatic system]] in response to the body's needs. For example, infection with bacteria will cause more [[macrophage]]s to be produced and fewer NK cells, in response to "alarm signals" called [[chemokine]]s. Macrophages then exit the bloodstream, migrate towards the site of infection, and attempt to neutralize the invading pathogens. Others begin producing antibodies or recruit other systems to help.  

They are also known as leukocytes. There are five main types of leukocytes, and these types are produced from immature cells in the [[lymphatic system]] in response to the body's needs. For example, infection with bacteria will cause more [[macrophage]]s to be produced and fewer NK cells, in response to "alarm signals" called [[chemokine]]s. Macrophages then exit the bloodstream, migrate towards the site of infection, and attempt to neutralize the invading pathogens. Others begin producing antibodies or recruit other systems to help.  

The immune system of all [[vertebrates]] has [[memory]], which in this context means that it keeps a record of invaders it has previously encountered. Immune memory is not completely understood, but involves the cells that contributed most to fighting off the infection being stored away in the bone marrow. This system is why you will not usually get [[chickenpox]] if you have already had it, and is also why vaccines work (the vaccine looks similar to a pathogen such as [[polio]], so the immune system will "remember" it and then be ready if it should ever encounter the real thing).

The immune system of all [[vertebrates]] has [[memory]], which in this context means that it keeps a record of invaders it has previously encountered. Immune memory is not completely understood, but involves the cells that contributed most to fighting off the infection being stored away in the bone marrow. This system is why you will not usually get [[chickenpox]] if you have already had it, and is also why vaccines work (the vaccine contains a tiny, harmless bit of a pathogen such as [[polio]], so the immune system will "remember" it and then be ready if it should ever encounter the real thing).