B-cells produce antibodies (a subset of adaptive immunity called humoral immunity). The usual sequence involves B-cell activation via interleukins from helper T-lymphocytes, which cause the B-cell to undergo mitosis, creating numerous clones that will differentiate into plasma cells (rapid antibody producers) or memory B-cells.

Antibodies (or immunoglobulins) are produced by B-leukocytes and plasma cells, and are a key part of humoral (having to do with bodily fluids) immunity. As such, they can be found in several of the fluids circulating or exiting the body. Some types of immunoglobulins can even cross the placenta or be secreted in breast milk to pass immunity from a mother to her child.

All of the given answer options describe mechanisms for immunoglobulin function.

Neutralization occurs when the antibodies simply cover the biologically active portion of the pathogen, rendering it harmless. Complement fixation refers to the antibodies binding to the pathogen and facilitating the activation of the complement system (a series of plasma proteins that activate other immune processes). Precipitation is when antibodies link the antigens on many pathogens together, creating an insoluble clump ready for removal.