Hm, this question already appeared before. Please use the search function to find it 😉

http://en.wikipedia.org/wiki/Immune_system

Wikipedia never fails!

read an immunology book.

Actually, lymphocytes B and T are the main intervenients of the specific immunity.
While non-specific immunity destroys pathogenic agents and prevent the generalization of an infection, this one acts specifically directed and with great efficiency against strange elements. Specific immunity is also called acquired immunity.
Every component capable of stimulating the specific immunity is called an antigen. Antigens may happen as free molecules or as molecular structures in cell surfaces. Almost all of them are molecular structures present in the surface of invasive organisms or produced by them such as bacterial toxins.
Lymphocytes B and T have their origin in the red bone marrow and are preceded by lymphoblasts. These develop in the bone marrow or migrate to the thymus.
Cells that differentiate in the thymus are transformed in lymphocytes T (thymus), while B (bone marrow) cells continue their differentiation in the bone marrow.
In the thymus, lymphocytes acquire, by hormonal action, specific antigenic – receptors, which will permit them to recognize antigens later. B cells acquire receptors in the bone marrow.
After lymphocytes acquire those receptors they are immune competent, that is, capable of an immune response. Then, they migrate to blood or also to secondary lymphocyte organs such as spleen, lymph nodes and tonsils (where an immunity response develops)
Specific immunity is divided in two main groups. Humoral immunity (or immunity mediated by antibodies) and Cell-mediated immunity.
Lymphocytes B are the effectors of Humoral Immunity.
Every lymphocyte B with them same receptors, capable of recognizing the same antigens are from the same clone. There is a great diversity of lymphocytes B which makes our body able to recognize an almost infinite number of antigens.
There are 3 main phases in Humoral immunity:
Clone selection, clone proliferation of the activated lymphocyte and B lymphocyte differentiation.
When an antigen enters the organism, it will stimulate lymphocytes B that are specific for that antigen. We say then that the clone is activated. This particular clone will face multiple mitotic divisions originating clones with the same genetic information. Some of those originated B lymphocytes will then differentiate in plasmocytes (or plasma cells) which can produce antibodies due to its developed endoplasmic reticulum. Those other originated B lymphocytes will constitute the memory B lymphocytes. These are long life lymphocytes, which remain inactive but ready to act rapidly if antigen enters the organism again. These permit a second more effective response to an antigen contact.
The antibody is not capable of directly destroying the invasive organisms but to mark those strange elements to be destroyed by further methods: precipitation, agglutination, direct intensification of Phagocytosis and neutralization.
Cell-mediated immunity is mediated by lymphocytes T. These are only capable of recognizing antigens presented in the surface of cells of our own body. Therefore we’re able of recognizing our own antigens (self) and the antigens of strange elements (non self) when presented by specific cells (presentation cells). When, for instance, a macrophage destroys a virus or bacteria, some peptide fragments are formed. Those antigen fragments will connect to some surface elements and are exhibited and presented to lymphocytes T. These are efficient against transplanted organs, transplanted skin, cancer cells, multicellular parasites, fungus and virus/bacteria infected cells.
When lymphocytes T connect to those exhibited antigens they’ll be activated. A clone proliferation (divisions) starts to produce more lymphocytes T specific to those antigens. The activation of lymphocytes T will make them to produce and liberate chemical substances and produce memory T lymphocytes. There are several types of produced lymphocytes T. Some that by liberating chemical mediators control other immunity defenses (TH lymphocytes, from help), some that kill cells that present those antigens like bacteria/virus infected ones and cancer cells (TC lymphocytes) and some that will moderate or suppress the immunity response (TS lymphocytes) when the infection is being controlled. Those long life memory T lymphocytes will remain inactive until the same antigen invades the organism.
Just want to give the idea that the humoral immunity and the cell mediated immunity are not independent. The antibodies produced in a humoral response can intensify or diminish the action of T lymphocytes and T cells can also influence the production of antibodies by plasma cells.

Janeway is a good choice, right? 🙂

From your paragraph it seems that you refers "specific immunity" to "specific immune response" which could be humoral or cell-mediated immunities. If this is so, the more proper term is "immunogen" rather then "antigen". This because antigen can be self or nonself, which sel-antigen won’t generate immune response but immuno-tolerance. However, this "immunogen" term is not popular, we are familiar with "antigen". If the discussion refers to immune response, we can use the term "antigen". I think this is important to provide a basic understanding for new students in Immuno(bio)logy Class because in my experience they always mix up antigen for immunogen, even antigen for pathogen 😉

Great point.

While an antigen is, but not only, a component capable of stimulating an immune response, the reverse is true.
That is, every component capable of stimulating an immune response is called an antigen.