The immune 
system distinguishes two groups of foreign
substances.
One group consists of antigens that are freely
circulating in the body.
These include molecules, viruses, and foreign
cells.
A second group consists of self cells that
display aberrant MHC proteins.
Aberrant MHC proteins can originate from antigens
that have been engulfed and broken down (exogenous
antigens) or from virus-infected and tumor
cells that are 
actively synthesizing foreign proteins (endogenous
antigens).
Depending on the kind of foreign invasion,
two different immune responses occur:
The humoral response (or antibody-mediated
response) involves B cells that recognize
antigens or pathogens that are circulating
in the lymph or blood (�humor� is a medieval
term for body fluid).
The response follows this chain of events:
Antigens bind to B cells.
Interleukins or helper T cells costimulate
B cells.
In most cases, both an antigen and a costimulator
are required to activate 
a 
B cell 
and initiate B cell proliferation.
B cells proliferate and produce plasma cells.
The plasma cells bear antibodies with the
identical antigen specificity as the 
antigen receptors of the activated B cells.
The antibodies are released and circulate
through the body, binding to antigens.
B cells produce memory cells.
Memory cells provide future immunity.
The cell-mediated response involves mostly
T cells and responds to any cell that displays
aberrant MHC markers, including cells invaded
by pathogens, tumor cells, or transplanted
cells.
The following chain of events describes this
immune response:
Self cells or APCs displaying foreign antigens
bind to 
T cells.
Interleukins (secreted by APCs or helper T
cells) costimulate activation of T cells.
If MHC-I and endogenous antigens are displayed
on the plasma membrane, T 
cells proliferate, producing cytotoxic T cells.
Cytotoxic T 
cells destroy cells displaying the antigens.
If MHC-II and exogenous 
antigens are displayed on the plasma membrane,
T cells proliferate, producing helper T cells.
Helper T cells release interleukins (and other
cytokines), which stimulate B cells to produce
antibodies that bind to the antigens and stimulate
nonspecific agents (NK and macrophages) to
destroy the antigens.
