Exam 2 - Lecture 8 (Antibody Classes)

IgM

• Surface (BCR): monomer

• Secreted (Ab): pentamer

  • Largest molecular weight

• First Ab type generated in immune response

• 5-10% of serum Abs

• Half life: approx. 1.5 weeks

  • Sign of acute infection

IgM Functions

• Complement fixation

• Pentamer form

• Neutralization, opsonization

• Little/no memory response

  • IgG highly involved in secondary response

IgG

• Most abundant in serum

  • 70-75% of total antibodies

• Longest half-life

  • Approx. 3 weeks

• Often persists after infection cleared

• Four subclasses

  • IgG1, IgG2, IgG3, IgG4

IgG Functions

• Neutralization (toxins, viruses)

• Opsonization

• Passive immunization of fetus/newborn

• Complement fixation

• Diffusion into extravascular sites

• Antibody-dependent cell-mediated cytotoxicity

IgG1 & IgG3 esp. good at:

• Opsonization

• Activating complement

• Crossing placenta

• ADCC w/NK cells

IgA

• 10-15% total serum Ab

• Mucosal linings

• Secretions (e.g., breast milk)

• Monomer or dimer

• Two subclasses: IgA1 & IgA2

IgA Functions

• Neutralization

• Passive immunization of newborn

• Some complement fixing activity

IgA2

more in mucosal secretions

IgA1

more in serum

IgE

• Lowest serum levels, normal conditions (0.0005% of total Abs)

• Participates in degranulation of mast cells, eosinophils, basophils

• Strong binding via Fc receptors

• Anti-parasite response

  • Allergic response

IgD

• Very low serum levels (0.001% of total Abs)

• Half-life: 3 days

• Few, if any, functions as Ab

• Expressed as BCR

  • Can bind Ag

  • Possible role in B cell activation, life span