How the Immune System Works by Sompayrac Lauren;

Author:Sompayrac, Lauren;
Language: eng
Format: epub
ISBN: 9781119542209
Publisher: John Wiley & Sons, Inc.
Published: 2019-01-31T17:00:00+00:00

When blood enters from the splenic artery, it is diverted out to the marginal sinuses from which it percolates through the body of the spleen before it is collected into the splenic vein. As they ride along with the blood, naive B cells and T cells are temporarily retained in different areas – T cells in a region called the periarteriolar lymphocyte sheath (PALS) that surrounds the central arteriole, and B cells in the region between the PALS and the marginal sinuses.

Of course, since the spleen has no lymphatics to transport dendritic cells from the tissues, you might ask, “Where do the antigen presenting cells in the spleen come from?” The answer is that the marginal sinuses, where the blood first enters the spleen, is home to “resident” dendritic cells. These cells take up antigens from invaders in the blood and use them to prepare a class II MHC display. Resident dendritic cells also can be infected by pathogens in the blood, and can use their class I MHC molecules to display these antigens. Once activated, resident dendritic cells travel to the PALS where T cells have gathered. So although the dendritic cells which present antigens to T cells in the spleen are travelers, their journey is relatively short compared with that of their cousins which travel to lymph nodes from a battle being waged out in the tissues. Helper T cells that have been activated by APCs in the PALS then move into the lymphoid follicles of the spleen to give help to B cells. And you know the rest of this story!

Some of the most dangerous blood-borne pathogens such as Streptococccus pneumoniae and Haemophilus influenzae surround themselves with a polysaccharide capsule. Helper T cells can only be activated by protein antigens, so these bacteria, with their carbohydrate cloaks, are “invisible” to helper T cells. Now if B cells in the spleen could not be activated and make antibodies to protect against these dangerous invaders, we’d be in trouble. Fortunately, the spleen is one of the main places in the body where B cells can be activated without the assistance of Th cells. These “helpless” B cells, called marginal zone B cells, are stationed out in the marginal sinuses where they come in contact with blood as it enters the spleen. And because these marginal zone B cells do not have to wait for T cells to be activated, they can respond quickly before encapsulated bacteria have a chance to multiply to dangerous levels. The importance of this Th-independent B cell activation is underscored by the fact that humans who have lost their spleen (e.g., due to injury) are in danger from serious infections by encapsulated bacteria.

How these marginal zone B cells are activated without T cell help is still a mystery. It likely has to do with the fact that bacterial capsules are composed of many repeating carbohydrate molecules, so there are many epitopes close together to cluster a ton of BCRs. T cell-independent activation also probably depends

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