B cell

From IKE

Basics

• The B stands for Bursa of Fabricius and not bone marrow
• Expression of B-cell receptor genes
• Membrane-bound immunoglobulins serve the receptor functions
• Some B cells will become memory cells after any given encounter
• The memory cells will then escalate to the next immunoglobulin type the next time they are exposed to the same antigen

B Cell receptors

• aka BCR
• B Cells recognize antigens in their natural form through the surface antigen receptors known as B cell receptors
• Each receptor consists of a recognition, signaling, and coreceptor unit

Recognition unit

• Essentially a membrane-bound immunoglobulin
• Has the same structure as immunoglobulins
• Recognizes structures known as antigenic determinants or epitope
• Recognition involves hydrogen bonding between CD3 of the immunoglobulin and the antigen epitope

Signaling unit

• Includes at least two types of chains: Igα and Igβ

Coreceptor unit

• A molecular complex formed by CR2 (binds complement components), CD19 (binds intracellular signaling molecules), and TAPA-1 (function unknown) molecules

Light and heavy chain construction highlighting Ig diversity generation

Development

• The construction of the heavy and light chains always happens sequentially (Heavy, then light).
• This is done so that the molecule ends up with only one specificity
• This is called allelic exclusion -- making sure that only one specificity is expressed
• Also see Janeway Figures 7.1, 7.3, 7.5, 7.14, 7.18, 7.25, 7.26, 7.37, 7.40, 7.45

Heavy Chain

• First part of cell development
• Joins V_H - D_H - J_Hand then tests to see if the strands check out.
• Once joined, V_HD_HJ_H is then joined to C_μ, which is then also tested.

Light Chain

• Once the heavy chain is validated, the light chain is formed
• Joins a V_lightκ to J_lightκ, then to L_κ
• Once the κ light chain is constructed, then the heavy λ chain will be constructed
• This is why there is always more variation in κ light chains than in λ light chains

• There is always a number of cells that express antibodies against self -- a screw-up in development

• Somatic hypermutation - aims to increase the affinity of immunoglobulins.
• Called somatic because it is not in the germ line (e.g. does not occur during development)

• If a B cell does not see an antigen in three days, then the B cell dies
• If it does see an antigen, then clonal selection begins

Resources

• Wikipedia:B cell