B-Cell Receptors (BCR), IgM & IgD as Receptors, Capping, Clonal Selection Theory, T-Cell Independent vs Dependent Activation

Primary response

First exposure to antigen; slow; IgM made first, then IgG.

Secondary response

Faster and stronger reaction upon re-exposure; mostly IgG with higher affinity.

Plasma cell

Short-lived cell that secretes large amounts of antibodies.

Memory B cell

Long-lived cell that provides rapid, strong antibody production during later exposures.

Isotype switching

Process where a B cell changes its antibody class (IgM → IgG/IgA/IgE) without changing antigen specificity.

Affinity maturation

Improvement of antibody binding strength through mutation and selection of better B cells.

IgM role

First antibody produced during primary immune response.

IgG role

Main antibody in secondary response; high affinity; long-term protection.

Booster vaccine purpose

Activates memory B cells to rapidly produce high-affinity IgG.

Measles immune amnesia

Measles destroys memory B/T cells, causing loss of previous immunity.

B-cell receptor (BCR)

Membrane-bound antibody (IgM or IgD) used to detect antigen.

IgM on B cell

BCR present on naive B cells; same antigen specificity as IgD.

IgD on B cell

Co-expressed with IgM on naive B cells; same antigen specificity.

Capping

Clustering of BCRs after antigen binding, triggering activation.

Clonal selection

 Only the B cell that matches an antigen is activated and multiplied.

T-independent antigen

Antigen with repeating epitopes that activates B cells without helper T cells.

T-dependent antigen

Antigen requiring helper T cells; leads to memory formation and class switching.

Igα / Igβ

Signaling proteins attached to the BCR that transmit activation signals inside the cell.

Repeating epitopes

Identical antigen patterns that cross-link many BCRs to trigger T-independent activation.

Naive B cell

B cell that has never encountered antigen; displays IgM and IgD.