Immunoglobulin isotypes

Tfh cytokines influence which Ab isotype made by selected B cell.

IgM
Functions
 * Primary isotype during primary infections
 * Blood
 * Pentameric; large is size - not efficient at entering tissues
 * Can be transported by poly Ig receptor
 * C' activation
 * Opsonization

IgG

 * Main Ab in blood
 * Smaller than IgM → can enter tissue

Functions
 * Taken up by pinocytosis from blood
 * In endosome (acidic conditions), binds FcRn
 * Vesicles fuses with basolateral membrane
 * Neutral pH of extracellular environment → release of IgG from FcRn
 * C' activation
 * Opsonization
 * Neutralization
 * Toxins
 * Transplacental transfer

IgA
Functions
 * Dimeric
 * Blood and body secretions
 * Maternal milk
 * Transport to tissues via poly Ig receptor
 * Neutralization
 * Toxins
 * Mucosal infections