B cells

Here is an intuitive animation describing this process:

http://www.wwnorton.com/college/biology/mbio/animations/main.asp?chno=ch24a01

There are basically two types: T cell-dependent activation and T cell-independent activation.

In T cell-dependent activation:

Once a pathogen is ingested by an antigen-presenting cell such as a macrophage or dendritic cell, the pathogen's proteins are then digested to peptides and attached to a class II MHC protein. This complex is then moved to the outside of the cell membrane. The macrophage is now activated to deliver multiple signals to a specific T cell that recognizes the peptide presented. The T cell is then stimulated to produce autocrines, resulting in the proliferation and differentiation to effector and memory T cells. Helper T cells (i.e. CD4+ T cells) then activate specific B cells through a phenomenon known as an Immunological synapse. Activated B cells subsequently produce antibodies which assist in inhibiting pathogens until phagocytes (i.e. macrophages, neutrophils) or the complement system for example clears the host of the pathogen(s).

Most antigens are T-dependent, meaning T cell help is required for maximal antibody production. With a T-dependent antigen, the first signal comes from antigen cross linking the B cell receptor (BCR) and the second signal comes from co-stimulation provided by a T cell. T dependent antigens contain proteins that are presented on B cell Class II MHC to a special subtype of T cell called a Th2 cell. When a B cell processes and presents the same antigen to the primed Th cell, the T cell secretes cytokines that activate the B cell. These cytokines trigger B cell proliferation and differentiation into plasma cells. Isotype switching to IgG, IgA, and IgE and memory cell generation occur in response to T-dependent antigens. This isotype switching is known as Class Switch Recombination (CSR). Once this switch has occurred, that particular B cell will usually no longer make the earlier isotypes, IgM or IgD.