The document discusses the human immunological apparatus. It describes how the immune system is comprised of circulating T and B lymphocytes that develop from stem cells in primary lymphoid organs like the thymus and bone marrow. These cells then undergo antigen-specific differentiation in secondary lymphoid organs like lymph nodes and spleen. The immune response involves interactions between humoral, cellular, nonspecific, and specific mechanisms through a complex series of interactions between immune cells.

1. The Immunological
Apparatus
Dr. Mukhtar, Family Physician, Associate Dean

2. Introduction
• The immune system is comprised of various continuously
circulating cells (T and B lymphocytes, and antigen-
presenting cells present in various tissues).
• T and B cells develop from a common stem cell type, then
mature in the thymus (T cells) or the bone marrow (B
cells), which are called primary (or central) lymphoid
organs.

3. •An antigen-specific differentiation steps then
takes places within the specialized and highly
organized secondary (or peripheral) lymphoid
organs (lymph nodes, spleen, mucosa-associated
lymphoid tissues [MALT]).

5. •The antigen-specific activation of B and/or
T cells involves their interaction with other
cells in a contact-dependent manner and by
soluble factors.

6. • B cells bear antibodies on their surfaces (cell-bound B-cell
receptors).
• They secrete antibodies into the blood (soluble
antibodies) or onto mucosal surfaces once they have fully
matured into plasma cells.
• Antibodies recognize the three-dimensional structures of
complex, folded proteins, and hydrocarbons.

7. •Chemically, B-cell receptors are globulins
(“immunoglobulins”) and comprise a variety of
specific types.
•Despite the division of immunoglobulins into
classes and subclasses, they all share essentially
the same structure.

8. • T cells recognize peptides presented on the cell surface by
major histocompatibility (gene) complex (MHC) molecules.
• A T-cell response can only be initiated within organized
lymphoid organs.
• Naive T cells circulate through the blood, spleen, and
other lymphoid tissues, but cannot leave these
compartments to migrate through peripheral nonlymphoid
tissues and organs unless they are activated.

9. • Self antigens (autoantigens), presented in the thymus
and lympoid tissues by mobile lymphohematopoietic
cells induce T-cell destruction(so called negative
selection).
• Antigens that are expressed only in the periphery, that
is outside of the thymus and secondary lymphoid organs,
are ignored by T cells; potentially autoreactive T cells
are thus directed against such self antigens.

10. •T cells react to peptides that penetrate into the
organized lymphoid tissues.
•New antigens are first localized within few lymphoid
tissues before they can spread systemically.
•These must be present in lymphoid tissues for three
to five days in order to elicit an immune response.

11. • An immune response can be induced against a
previously ignored self antigen that does not normally
enter lymphoid tissues if its entry is induced for
instance, because of cell destruction resulting from
chronic peripheral infection.

12. • It is important to remember that induction of a small
number of T cells will not suffice to provide immune
protection against a pathogen.
• Such protection necessitates a certain minimum sum of
activated T cells.

13. The function of immunological apparatus
• The function of the immunological apparatus is based on a
complex series of interactions between:
humoral
cellular
specific, and
nonspecific mechanisms.
• This can be better understood by examining how the
individual components of the immune response function.

14. • The human immunological system can be conceived as a
widely distributed organ comprising approximately 1012
individual cells, mainly lymphocytes, with a total weight
of approximately 1 kg.

15. • Leukocytes arise from pluripotent stem cells in the
bone marrow, then differentiate further as two distinct
lineages:
Myeloid lineage
Lymphoid lineage

16. Myeloid lineage
• The myeloid lineage constitutes granulocytes and
monocytes, which perform important basic defence
functions as phagocytes (“scavenger cells”).

17. Lymphoid lineage
• The lymphoid lineage gives rise to the effector
cells of the specific immune response, T and B
lymphocytes.

18. • These cells are constantly being renewed (about
106 new lymphocytes are produced in every
minute) and destroyed in large numbers.

19. • T and B lymphocytes, while
morphologically similar, undergo distinct
maturation processes.
• The antigen-independent phase of
lymphocyte differentiation takes place in
the so-called primary lymphoid organs: T
lymphocytes mature in the thymus and B
lymphocytes in the bursa fabricicius (in
birds).

20. • Although mammals have no bursa, the term B
lymphocytes (or B cells) has been retained to
distinguish these cells, with their clearly distinct
functions and maturation in the bone marrow,
from T lymphocytes, which mature in the thymus

21. • B cells mature in the fetal liver as well as in fetal and
adult bone marrow.
• In addition to their divergent differentiation pathways,
T and B cells differ with respect to their functions,
receptors, and surface markers.

22. • T and B cells communicate with each other, and with other
cell types, by means of adhesion and accessory molecules
or in response to soluble factors, such as cytokines, which
bind to specific receptors and induce the activation of
intracellular signalling pathways.

23. • The antigen-dependent differentiation processes
which leads to T and B cell specialization, takes
place within the secondary lymphoid organs
where lymphocytes come into contact with
antigens.