The "goal" of most vaccines is the production of antibodies by memory B cells and plasma cells to the pathogen's antigens. However, in order to have an effective response, it is optimum to have a T cell response as well. In fact, for most antigens, there must be T cell help to properly stimulate the B cell to respond. Two major categories of T cells are T-helper cells (also known as CD4 lymphocytes) and T-cytotoxic cells (also known as CD8 lymphocytes).
A very simplified scenario of the initial or primary immune response is as follows (please refer to the image below):
1. A portion of a microbe is recognized by TLRs or other receptors on the surface of certain innate immune cells — dendritic cells and macrophages. These cells, called antigen-presenting cells or APCs, degrade the pathogen into smaller parts, which will serve as antigens. Each antigen is combined with a molecule called MHC II and the complex is displayed on the surface of the APC.
2. A T-helper cell which has a receptor that recognizes (i.e., fits) that antigen will engage with that MHC/antigen complex; this process leads to activation of the T-helper cell which then begins to secrete chemical helpers called cytokines. (T cytotoxic or T killer cells can also recognize antigens possessed by host cells containing an intracellular pathogen, such as a virus.)
3. A B cell that is specific for one of the microbe's antigens will engage with an antigen that it fits or recognizes. Together, with stimulation by the cytokines, will start to divide and clone itself. Thus what started as one B cell that recognizes that antigen, now becomes many. These daughter cells differentiate into plasma cells that will begin to synthesize immunoglobulins that are specific for that same antigen. These immunoglobulins, called antibodies, are then secreted and will target the invading pathogenic microbe.
4. A few of the daughter cells will not secrete antibodies but instead will become memory B cells which will live a long time and will respond to that same microbe if it invades the body again. However, the memory cells become activated more quickly, so the response is much faster than the initial primary response. The response by the memory B cells is known as the secondary immune response and is the goal of vaccination.
