B Cells and Antibody Notes

B Cells

• Lymphocytes that develop in bone marrow.

• Express unique antigen receptors (BCR or secreted antibody).

• Plasma cells are activated B cells which secrete antibodies.

• Memory B cells provide immunological memory.

B Cell Receptor (BCR)

• Each B cell surface has ~100,000 BCRs (mainly IgM / IgD antibodies).

• BCR binds antigen, activating the B cell.

• BCR is membrane-anchored via a transmembrane domain (TM); secreted antibodies lack a TM.

Functions of Antibodies

1. Neutralization

2. Opsonization (enhances phagocytosis)

3. Complement activation

Antibody Binding

• Antibodies bind native antigens without needing MHC processing.

• Epitope: Antibody binding site on an antigen.

Classes of Immunoglobulin (Antibody)

• IgG (monomer):

  • Most abundant in blood.

  • Opsonizes/neutralizes and targets viruses/bacteria.

  • Only Ig class to cross the placenta, providing 'passive immunity'.

• IgA (dimer):

  • Present in secretions (tears, saliva, mucus, breast milk).

  • Defends mucous membranes, especially the gut.

  • Provides 'passive immunity' to nursing infants via breast milk.

  • Targets viruses/bacteria.

• IgM (pentamer):

  • First Ig class produced after initial antigen exposure.

  • Expressed on naive B cells.

  • Effective in activating complement.

  • Targets extracellular bacteria.

  • Acts as antigen receptor (BCR).

• IgE (monomer):

  • Low concentrations in blood.

  • Immunity to multicellular parasites.

  • Involved in allergic reactions, activating mast cells.

• IgD (monomer):

  • Expressed on naive B cells.

  • Acts as antigen receptor (BCR) together with IgM; specific function largely unknown.

Memory Responses

• Antigen + T cell stimulation of B cells leads to plasma cell formation.

• Some stimulated B cells form memory cells.

Memory Cells

• Persist for years in blood and lymphatic tissue.

• Express antibody as BCR, but do not secrete it.

• Respond rapidly to antigen encounter and become plasma cells.

Primary Immune Responses

• Takes 7-14 days to produce sufficient antibody to eliminate the pathogen.

• Relatively low antibody amount produced, mainly IgM.

Secondary Immune Responses

• Basis of vaccination success, relies on memory B cells.

• Fast: 2-3 days to produce sufficient antibody to eliminate pathogen.

• Mainly IgG, with class switching to IgA and IgE (low levels).