Components of the Immune System

The function of the immune system is to recognize
and to eliminate invading foreign organisms.
The immune response must be fast,
specific, and aimed exclusively against foreign
molecules or cells harboring an infective agent,
but never against its own normal cells. The effector
molecules must be prepared for contact
with a variety of organisms and molecular
structures. With these requirements, an extremely
efficient, genetically complex immune
system has evolved. Its development and function
are regulated by genes of extraordinary diversity.

Lymphatic organs

The immune system consists of peripheral
blood lymphocytes and of the lymphatic organs
(lymphoid tissue). The primary lymphoid tissues
are the thymus and bone marrow. Secondary
lymphoid tissues are the lymph nodes in
various regions of the body, especially the nasopharynx,
axillae, groins, and intestines. The
spleen is considered a secondary lymphatic
organ.

Lymphocytes and the immune response

Lymphocytes carry the immune response. This
was shown by the following experiment. The
normal immune response of a mouse to an administered
antigen (foreign organism, foreign
molecular structure) was destroyed by a high
dose of X-irradiation, to which the immune system
is especially sensitive. The immune response
was then restored by lymphocytes of a
genetically identical (from an inbred strain)
unirradiated mouse, but not by any other cells.
Other cells are ineffective since only lymphocytes
produce an immune response.

T cells and B cells

Lymphocytes exist as one of two functionally
and morphologically different types, T lymphocytes
and B lymphocytes. T lymphocytes undergo
differentiation in the thymus during
embryonic and fetal development, thus the designation
T lymphocyte or T cell. B lymphocytes
differentiate in the bone marrow in mammals
and in the bursa of Fabricius in birds (thus the
designation B cells). A series of further differentiating
steps take place in the lymph nodes (T
cells) and in the spleen (B cells).

Cellular and humoral immune response

During the first phase of the immune response
induced by an antigen (e.g., a bacterium, virus,
fungus, or foreign protein), there is rapid proliferation
of B cells (humoral immune response).
The B cells mature to plasma cells,
which form free antibodies (immunoglobulins)
directed at the antigen. The antibodies bind
specifically to the antigens. The humoral immune
response is rapid, but it does not reach
foreign organisms that have invaded body cells.
They are the target of the cellular immune response.

Antibody molecules (basic structure)

The basic structural motif of an antibody
molecule (immunoglobulin) is a Y-shaped protein
composed of two heavy chains (H chains)
and two light chains (L chains). They are held together
at defined sites by disulfide bonds. L
chains and H chains contain regions with variable
and constant sequences of amino acids.

Antigen–antibody binding

A foreign molecule, the antigen, is recognized
and firmly bound to a specific region of the antibody
molecule, the antigen-binding site. Here,
the amino acid sequence of the antibody
molecule differs from one molecule to the next
in three hypervariable regions. The result is the
ability to bind a wide spectrum of different antigenic
molecules. The three-dimensional
structure of this region is known precisely, and
important details of the binding process are understood.