Immunology Basics - Immunological Computation: Theory and Applications - page 19

h is feature allows a maximum number of antigenically committed lympho-
cytes to encounter and interact with antigens within a relatively short period of
time to generate a specifi c immune response. Diff erent populations of lymphocytes
circulate at primary and secondary lymphoid organs and are carefully controlled to
ensure those appropriate B- and T-cell populations (naive, eff ector, and memory)
are recruited into diff erent locations. h is diff erential migration of lymphocyte
subpopulations at diff erent locations of the body is called tra cking or homing.
As lymphocytes recirculate, they tend to home at various secondary lymphoid
organs. Secondary lymphoid organs trap antigens and present them in pieces on the
surface of A PCs to be recognized by immune cells. h ese organs provide specialized
environments to support clonal expansion and diff erentiation of antigen-activated
lymphocytes into eff ector and memory cells. Interestingly, memory cells exhibit
selective homing to the type of tissue in which they fi rst encountered an antigen.
Presumably, this ensures that a particular memory cell will return to the location
where it is most likely to reencounter a subsequent antigenic challenge.
Experiments have shown that when a particular antigen is injected, antigen-
specifi c T cells disappear from circulation within 48 hours. h is suggests that
specifi c T cells encounter an antigen in peripheral lymph organs and cease recir-
culating within such a time period. h is process is closely regulated to guarantee
steady-state levels of each blood cell type. Cell division and diff erentiation of each
lineage is balanced by programed cell death known as apoptosis. Such a behavior
provides increased sophistication without centralized control.
1.5 RegulatoryMechanisms
Immune response mechanisms are self-regulatory by nature. h ere is no central organ
that controls immune functions. h e regulation of immune responses can be broadly
divided into two branches: humoral immunity, mediated by B cells and their products
and cellular immunity, mediated by T cells. Both branches follow a similar sequence
of defense steps: proliferation, activation, induction, diff erentiation and secretion,
attack, suppression, and memory; however, they do it in diff erent ways.
When an antigen enters the body, self-regulatory mechanisms determine (infl u-
enced by prior exposure to antigen) the branch of the immune system to be acti-
vated, the intensity of the response, and its duration. Specifi cally, regulation of both
humoral and cellular immunity is conducted by a population of T cells referred
to as either helper or suppressor cells, which either augment or suppress immune
responses. T cells regulate immune responses by releasing soluble molecules, col-
lectively referred to as cytokines to activate B cells. Subsequently, B cells follow
one of two pathways: they either diff erentiate into plasma cells, which are basically
antibody-secreting factories, or they give rise to GCs, specialized structures within
lymphoid organs, where they undergo somatic mutation (a process called a nity
maturation).