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[ruben]

The scanning electron micrograph (below) shows a human macrophage (gray) approaching a chain of Streptococcus pyogenes (yellow). Riding atop the macrophage is a spherical lymphocyte. Both macrophages and lymphocytes can be found near an infection, and the interaction between these cells is important in eliminating infection. Below is an animation that illustrates the basic cell-cell interactions that lead to antibody production can be seen in the accompanying animation



When the macrophage eats bacteria, proteins (antigens) from the bacteria are broken down into short peptide chains and those peptides are then "displayed" on the macrophage surface attached to special molecules called MHC II (for Major Histocompatibility Complex Class II). Bacterial peptides are similarly processed and displayed on MHC II molecules on the surface of B lymphocytes.

Helper T Cell Stimulating B Cell
When a T lymphocyte "sees" the same peptide on the macrophage and on the B cell, the T cell stimulates the B cell to turn on antibody production.

Antibody Production
The stimulated B cell undergoes repeated cell divisions, enlargement and differentiation to form a clone of antibody secreting plasma cells. Hence. through specific antigen recognition of the invader, clonal expansion and B cell differentiation you acquire an effective number of plasma cells all secreting the same needed antibody. That antibody then binds to the bacteria making them easier to ingest by white cells. Antibody combined with a plasma component called "complement" may also kill the bacteria directly.

Source:
http://www.cellsalive.com/antibody.htm

[ruben]

Lymphocytes are responsible for immune responses. There are two main types of lymphocytes: B cells and T cells. The B cells make antibodies that attack bacteria and toxins while the T cells attack body cells themselves when they have been taken over by viruses or have become cancerous. Lymphocytes secrete products (lymphokines) that modulate the functional activities of many other types of cells and are often present at sites of chronic inflammation.

Source:
http://www.medterms.com/script/main/art.asp?articlekey=4220