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B Cells and Antibodies

B cells are a type of white blood cell. They are similar to swallowing cells like macrophages but they are specific, meaning they can only attack one kind of intruder. B cells create antibodies during an immune response. 

Origin

Like T cells, B cells are lymphatic cells that are born from stem cells in the bone marrow.

Lymph nodes
The bone marrow can be found in the center of the bone, like the top of the arm bone above. The bone marrow is richly fed with capillaries and is very active producing new cells all throughout your life!

But unlike T cells, B cells stay in the bone marrow until they are mature.  Once mature, they travel through the body, moving in and out of the lymph and blood streams and collecting in the lymph nodes.

The most important aspect of a B cell is its receptor site - called an antibody.  Each B cell is born with a specific site on their membrane that can bind to only one kind of harmful particle. This receptor allows the B cell to recognize and identify one kind infectious foreign particle by binding to the specific protein makeup of the particle’s surface. But antibodies do a lot more, read on!

Antibody-mediated Immune Response

When a B cell finds a particle in the body that matches its unique receptor site, it attaches by its receptor site and digest it through a process similar to phagocytosis. It then displays the digested viral or bacterial pieces on its surface, attracting a Helper T cell. If the Helper T cell also has a specific binding site that matches the digested bits on the Bcell, then it knows that the digested particle is harmful! If it hadn’t already been alerted (and activated) by a macrophage of the same threat, it now becomes activated.

The activate Helper T cell, in turn, activates the B cell, and this interaction causes the B cell to divide.  After a few days, the young B cells will mature and differentiate into plasma cells and memory B cells.  Plasma cells use their machinery to produce antibodies

Xray
B cells become plasma cells to produce antibodies.

How Antibodies Work
Antibodies are “Y” –shaped protein structures made by T cells. They are made to specifically bind one kind of infecting virus or bacteria, like a lock and key (antibodies are specific, see below). Each type of virus or bacteria requires a different set of antibodies—antibodies made against the Influenza virus don’t have any effect on, let’s say, the Mononucleosis virus, although their symptoms are very similar!

Antibodies can travel through the blood and lymph to the site of infection and attach themselves to the designated foreign particle.  This renders the foreign particle immobile, so they can’t infect more cells or reproduce.  Antibodies also make the foreign particle more attractive to macrophages and other phagocytes, who quickly come and eat the immobile particles.  Antibodies can also travel to the intestines or our external mucous membranes to stop their pathogen before more of it enters the body!

Memory and a Swift Second Response!
The other set of cells produced during B cell division—memory B cells—continues to exist in the body long after the infection has been cleared.  This way, they retain the antibody that recognizes the specific harmful outsider. In other words, they have the bacteria or virus on file!  The next time it enters the body, the memory B cells will have all of the information needed to initiate a quick antibody response and stop a chronic infection before it occurs.  These B cells can stay circulating around your body for years and maybe forever!

Memory B cells are the reason we can have vaccines and, unfortunately, allergies!

Specificity - How do B cells recognize viruses or bacteria? 

Xray
Each B cell only makes antibodies for one kind of germ- they are specific..

Each piece of virus or bacteria that circulates around the body during infection also has its own genetic makeup.  This genetic makeup is expressed in the proteins that are displayed either on its surface or on the surface of an infected body cell.  These proteins are called antigens.

The whole goal of the B cell is to recognize the specific structure of these proteins, and manufacture antibodies that will “fit” their shape. Antibodies then are specific binding sites, because they only bind one kind of antigen. It’s like creating thousands of one type of key—per second—that will unlock one door.  It is estimated that there are 10 million different variations in protein structure that viruses and bacteria can express, so the body competes by producing 10 million B cells that each have a uniquely-shaped receptor!  Our B cells represent one way that our immune system has adapted over time, enabling it to acquire and remember information about viruses, bacteria, and other harmful pathogens that threaten our well-being.



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