# B-Cells --- A **B-cells** (which you can remember because they mature in the "b"one marrow) are one of the [[lymphocytes]] of the [[adaptive immune system]]. They are responsible for **humoral** [[immunity]] or the production of [[antibodies]] that fight antigens. B-cells go around the [[lymphatic system|lymphatic]] and [[cardiovascular system|circulatory]] systems and sample any molecules that they find. If they find something they fine-tune their immune response to make their B-cell receptors "fit" the antigen better, for a stronger response. They can use two methods to accomplish this, antibody [[class switching]] where they use different types of antibodies over the course of a response and [[affinity maturation]] where some B-cells go to a lymph node and iterate changes to their receptors for a super tight fit. B-cells are born in the [[bone marrow]], where a rigorous [[b-cell development]] takes place to train them up. When they leave the bone marrow they are considered *"naïve" B-cells*. When they first encounter an antigen they become *"effector" B-cells*. B-cells have various properties that make them effective antigen-finding machines. - A **B-cell receptor**, or BCR, is basically a membrane bound [[antibodies|antibody]] that the cell can use to bind to [[antigen#Epitopes|epitopes]]. Unlike [[T-cells#T-Cell Receptor|T-cell receptors]], BCRs do not need to be presented an antigen, and can just scoop them up from the environment. - B-cells have [[toll-like receptor|toll-like receptors]]. - B-cells have both classes [[MHC molecule]] and [[MHC molecule]] molecules. The MHC I is used as a regular self-protein presentation. If their BCR binds to an epitope, it takes that molecule in, chops it up, and presents it on their MHC II molecule. - The **CD40 molecule** is a very important molecule for activating B-cells. When the B-cell wants to confirm it has an antigen with a [[helper T-cells|effector T-cell]], and that T-cell is already on alert because it has also spotted the antigen, it has produced a [[helper T-cells#Effecter Helper T-Cells CD40 Ligand|CD40 ligand]] that can bind with the B-cell's CD40 molecule. ![[exogenous antigen concept map.png]] ### T-Dependent vs T-Independent Antigens If, in their patrolling a B-cell finds something it thinks is an epitope it goes on alert. If the epitope is particularly large, the B-cell can activate all on it's own. These antigens are called **T-independent antigens**. (The reason an antigen is T-independent is if it large enough to engage several of the B-cell's [[#B-Cell Receptor|BCRs]] or [[toll-like receptor|toll-like receptors]] at the same time.) T-independent antigens are generally poor at producing [[memory cells]] because they do not produce [[cytokines]] which B-cells use as a trigger to produce them. For most smaller epitopes, it needs to double check with a [[helper T-cells]] before it can be truly activated—these antigens are called **T-dependent antigens**. If the B-cell binds with a T-dependent antigen it then travels to a [[lymphatic system#Secondary Lymphoid Organs|secondary lymphoid organ]] like a lymph node to get confirmation from an [[helper T-cells|effector helper T-cell]]. The effector helper T-cell binds it's [[T-cells|T-cell receptor]] with the B-cell's antigen laden[[MHC molecule]] molecule. If the T-cell agrees it's dangerous, then it will produce a [[helper T-cells|CD40 ligand]] and release [[cytokines]]. If these two molecules are registered in their respective receptor molecules, the B-cell becomes activated. T-independent antigens are more likely to be made of lipids or carbohydrates (like virus capsules). T-dependent antigens are more likely to be made of protein. ## Problems with B-Cells - [[hypersensitivity|Hypersensitivity]] types I-III all have to do with B-cells making too much and/or the wrong type of antibodies. ___