B Cells and Antibodies Understanding the Immune System B Cells and Antibodies Each B cell is programmed to make one specific antibody. For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces antibody that zeros in on a bacterium that causes pneumonia. Every plasma cell is essentially a factory for producing antibody. A given antibody matches an antigen much as a key matches a lock. To some degree, however, the antibody interlocks with the antigen and thereby marks it for destruction.
Antibodies belong to a family of large molecules known as immunoglobulins .
Immunoglobulins are proteins, made up of chains of polypeptides, strings of the basic units known as amino acids. The sections that make up the tips of the Y's arms vary greatly from one antibody to another, creating a pocket uniquely shaped to enfold a specific antigen. The stem of the Y serves to link the antibody to other participants in the immune defenses. IgM, which usually combines in star-shaped clusters, tends to remain in the bloodstream, where it is very effective in killing bacteria. IgD is almost exclusively found inserted into the membranes of B cells, where it somehow regulates the cell's activation.