The third line of defense of immunity: specific immunity. Mainly by the immune organs (thymus, lymph nodes and spleen, etc.) and immune cells (lymphocytes), lymphatic B-cell responsible for humoral immunity; lymphoid T cells responsible for cellular immunity. (Cellular immunity often end up also need to humoral immune to the aftermath), the third line of defense is gradually built up by the body after birth acquired defensive function, the feature is generated after the birth, only works against a particular pathogen or foreign body thus called specific immunity (also known as acquired immune).
The specificity of the acquired immune system, is a specific immune mechanism against an antigen generated by immune lymphocytes (plasma cells) secrete antibodies, only play on the same antigen immune function. There is no variation or other antigens.
1st and 2nd line of defense, like the body of the anti-virus software; 3 line of defense designed to kill the software like a virus / trojan.
Only three lines of defense at the same time, a complete, well-play the immune function, our health can be more fully guaranteed.