Allergy Cases: B Lymphocytes - Case-based Curriculum of Allergy and Immunology

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

B-cell development

Hematopoiesis. Image source: Wikipedia.

B-Cell Activation by Antigen (Ag)

Activation of B cells. Image source: Wikipedia.

The activation of B cells occurs when 2 Ig receptors are bridged (cross-linked) by antigen (Ag). This process transmits a signal to the cytoplasm via 2 molecules (Igα and Igβ). All these structures together form the B-cell antigen receptor (BCR) complex.

The T cells regulate B-cell proliferation and differentiation which takes 3-7 days. Thereafter, some activated B cells (called centroblasts) migrate deep into the lymphoid follicles and proliferate rapidly, forming the dark zone of the germinal center. B cells also give rise to a generation of plasma cells and memory B cells. Those memory B cell enables the host to mount a rapid and stronger secondary immune response with a subsequent encounter with the same antigen.

Germinal Centers. This video describes the formation and function of germinal centers. This video is from: Janeway's Immunobiology, 7th Edition, Murphy, Travers, & Walport. Source: Garland Science.

Lymphocyte Trafficking. This video describes how lymphocytes travel through the blood stream and into a lymph node. This video is from: Janeway's Immunobiology, 7th Edition, Murphy, Travers, & Walport. Source: Garland Science.

B-cells are the only cell types that synthesize antibody molecules.

The earliest B cell that produces immunoglobulins is the pre-B lymphocyte. An adult produces 2-3 grams of immunoglobulin every day

Light chains κ and λ are encoded on diffrent chromosomes - κ is encoded on chromosome 2 and λ is encoded on chromosome 22. An immunoglobulin molecule has either κκ (60%) or λλ (40%) but never one of each. An individual B lymphocyte will produce only κ or λ chains but never both.

CDs on Different Cells

CD25 and CD3 are found on all T cells. CD25 is a receptor for IL-2.

CD19 and CD21 are found on all B cells.

CD16 are found on natural killer (NK) and NK T cells. CD16 is a receptor for IgG.

IL7 activates naive T cells. BAF activates naive B cells.

Mnemonics

T cells have CD 3, 4 and 8 (double, 4 x 2)
B cells have CD 19, 20 and 40 (double 20 x 2)

Stem cell CD34+
Plasma cell CD 38+

Surface Markers

L
Ligands on
Lymphocytes

CD40L on T helpers (CD4+)
FAS-L on CTL 9CD8+)

CD40 on B cells
FAS ("death signal") on cells to be killed by CTLs

Gene Recombination. This video shows how gene recombination affects immunoglobulins. This video is from: Janeway's Immunobiology, 7th Edition Murphy, Travers, & Walport. Source: Garland Science.

B cell activation

Complement receptor CD21 (CR2) activates BCR if the antigen is opsonized by C3b component of the complement.

CR2-CD19-CD81 complex is expressed on the surface of B lymphocytes.

When CD21 (CR2) interacts with C3d, the complex is brought into the BCR. CD19 has an ITAM that is phosphorylated, thus recruiting Lyn to enhance phosphorylation.

References