T-cells, B-cells

Note: both B-cells and T-cells are lymphocytes.

B-cells are formed in the bone marrow, and provide humoral immunity mediated by antibodies. They can recognise parts of antigens free in solution, by fitting them to the antibodies they carry on their surface. When a particular B-cell come into contact with an antigen which it fits, the B-cell swells and divides (through mitosis, or clonal selection)and the new activated B-cells (Plasma cells) secrete antibodies proteins that attack the invader. Once activated, a B-cell can pump out more than 10 million antibody molecules per hour. The antibodies neutralize or precipitate the destruction of the antigens by complement enzymes or scavenger cells. The B-cell can also produce different isotypes of the antibodies, who fit the same antigen but who defend the body in different ways. 

T-cells originate in the thymus and provide cell-mediated immunity, when infected cells become inflamed. While B-cells react to toxin molecules and to the outer surfaces of microbes, T-cells are able to discover the antigens of hidden inner pathogens. They spot peptide fragments of antigens on the surface of body cells and are able to bring the peptide to the cell surface where the T-cell can bind to it, through a molecule known as a major histocompatibility complex (MHC) protein. 

B-cells and T-cells do not act independently. Instead, highly regulated positive and negative feedback loops between B-cells and T-cells are a hallmark of the immune system. For example, T-cells can stimulate B-cells into an active state by the secretion of lymphokines, molecules that promote antibody formation. T-cells can also suppress antibody formation by releasing inhibitory lymphokines. In turn, B-cells attach antigens to MHC molecules and display them on the cell surface, in order for T-cells to respond to them. B-cells can also inhibit T-cells under experimental conditions.