Memory cells

Memory is an important aspect of adaptive immunity.

During the early stages of infection, effector cells outnumber memory cells. As the infection subsides, memory cells outnumber effector cells.

Secondary response
During secondary responses to the same antigen, memory cells are preferentially activated over naive cells for they...
 * Outnumber naive cells
 * Do not need constant rescue from apoptosis
 * Easier to activate
 * Have undergone affinity maturation, thus have higher affinity BCR for pathogen (B cell)
 * Don't express inhibitory receptor FcγRIIBI (B cell)
 * When Ag-Ab complexes bind to this receptor, initiates apoptosis in naive B cell
 * Ensures immune response targeted to produce high affinity Abs by memory cells
 * Express higher levels of MHC II and co-stimulatory molecules (B cell)
 * Do not need co-stimulation for activation (T cell)

Distinguishing different types of T cells
More challenging. Combinations of cell surface markers are used to identify memory T cells

CD45 is a tyrosine phosphatase that dephosphorylates Lck. It is found in all hematopoietic cells, except erythrocytes and plasma cells.