Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy
histology 2nd stage
1
Objectives
1. Briefly describe the cellular and humoral components of the innate and
adaptive immune systems.
2. Describe the main differences between the innate and adaptive immune
responses.
3. Briefly describe the classes of classes of Antibodies
The immune system provides defense or immunity against infectious
agents ranging from viruses to multicellular parasites. Histologically this system
consists of a large, diverse population of leukocytes located within every tissue
of the body and lymphoid organs interconnected only by the blood and
lymphatic circulation.
Immunologists recognize two partially overlapping lines of defense against
invaders and/or other abnormal, potentially harmful cells: innate immunity and
adaptive immunity . The first of these is nonspecific, involves a wide variety of
effector mechanisms, and is evolutionarily older than the second type. Among
the cells mediating innate immunity are most of the granulocytes and other
leukocytes described in adaptive immunity aims at specific microbial invaders, is
mediated by lymphocytes and antigen‐presenting cells (APCs) and produces
memory cells that permit a similar, very rapid response if that specific microbe
appears again.
The lymphocytes and APCs for adaptive immunity are distributed
throughout the body in the blood, lymph, and epithelial and connective tissues,
lymphocytes are formed initially in primary lymphoid organs (the thymus and
bone marrow), but most lymphocyte activation and proliferation occur in
secondary lymphoid organs (the lymph nodes, the spleen, and diff use lymphoid
tissue found in the mucosa of the digestive system, including the tonsils, Peyer
patches, and appendix).
The immune cells located diffusely in the digestive, respiratory, or urogenital
mucosae comprise what is collectively known as mucosa‐associated lymphoid
tissue (MALT) , proliferating B lymphocytes in the secondary structures of MALT
are arranged in small spherical lymphoid nodules . The wide distribution of
immune system cells and the constant traffic of lymphocytes through the blood,
lymph, connective tissues, and secondary lymphoid structures provide the body
with an elaborate and efficient system of surveillance and defense.
Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy
histology 2nd stage
2
Innate and Adaptive immunity
The system of defenses termed innate immunity involves immediate,
nonspecific actions, including physical barriers such as the skin and mucous
membranes of the gastrointestinal, respiratory, and urogenital tracts that
prevent infections or penetration of the host body, Bacteria, fungi, and parasites
that manage to penetrate these barriers are quickly removed by neutrophils and
other leukocytes in the adjacent connective tissue.
Other leukocytes orchestrate the defenses at sites of penetration natural
killer ( NK ) cells destroy various unhealthy host cells, including those infected
with virus or bacteria, as well as certain potentially tumorigenic cells.
Leukocytes and specific cells of the tissue barriers also produce a wide variety
of antimicrobial chemicals that Adaptive immunity, acquired gradually by
exposure to microorganisms, is more specific, slower to respond, and an
evolutionarily more recent development than innate immunity. The adaptive
immune response involves B and T lymphocytes, which become activated against
specific invaders by being presented with specific molecules from those cells by
APCs, which are usually derived from monocytes.
Unlike innate immunity, adaptive immune responses are aimed at specific
microbial invaders and involve production of memory lymphocytes so that a
similar response can be mounted very rapidly if that invader ever appears again.
Antigens and Antibodies
A molecule that is recognized by cells of the adaptive immune system is
called an antigen and typically elicits a response from these cells, antigens may
consist of soluble molecules (such as proteins or polysaccharides) or molecules
that are still components of intact cells (bacteria, protozoa, or tumor cells),
immune cells recognize and react to small molecular domains of the antigen
known as antigenic determinants or epitopes.
Classes of Antibodies
Immunoglobulins of humans fall into five major classes, with their structural
features, abundance in plasma, major locations, and functions. The classes are
called immunoglobulin G (IgG), IgA, IgM, IgE, and IgD, and key aspects for each
include the following:
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IgG is the most abundant class representing 75% to 85% of the
immunoglobulin in blood. Production increases during immune responses
following infections, etc. Unlike the other classes of antibodies, IgG is highly
soluble, stable (half‐life >3 weeks), and crosses the placental barrier into the
Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy
histology 2nd stage
3
fetal circulation. This confers passive immunity against certain infections until
the newborn’s own adaptive immune system is acquired.
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IgA is present in almost all exocrine secretions, IgA is produced by plasma
cells in mucosae of the digestive, respiratory, and reproductive tracts, another
protein bound to this immunoglobulin, the secretory component, is released by
the epithelial cells as IgA undergoes transcytosis, the resulting structure is
relatively resistant to proteolysis and reacts with microorganisms in milk, saliva,
tears, and mucus coating the mucosae in which it is made.
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IgM constitutes 5% to 10% of blood immunoglobulin and IgM is mainly
produced in an initial response to an antigen. IgM bound to antigen is the most
effective antibody class in activating the complement system.
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IgE, usually a monomer, is much less abundant in the circulation and exists
bound receptors on the surface of mast cells and basophils. When this IgE
encounters the antigen that elicited its production, the antigen‐antibody complex
triggers the liberation of several biologically active substances, such as
histamine, heparin, and leukotrienes.
Antigen‐Presenting Cells
Most specialized antigen‐presenting cells (APCs) are part of mononuclear
phagocyte system, including all types of macrophages and specialized dendritic
cells in lymphoid organs. Features common to all APCs are an active endocytotic
system and expression of MHC class II molecules for presenting peptides of
exogenous antigens. Besides dendritic cells (not to be confused with cells of
nervous tissue) and all monocyte‐derived cells, “professional” APCs include the
epithelial reticular cells of the thymus and B lymphocytes. During inflammation
transient expression of MHC class II is induced by interferon‐γ in certain local
cells that can be considered “nonprofessional” APCs, including fibroblasts and
vascular endothelial cells.
Lymphocytes
Lymphocytes both regulate and carry out adaptive immunity. In adults stem cells
for all lymphocytes are located in the red bone marrow, but cells of the major
lymphoid lineages mature
and become functional in two different central or primary lymphoid organs. Cells
destined to become B lymphocytes remain and differentiate further in the bone
marrow. Progenitors of T lymphocytes move via the circulation into the
developing thymus. After maturation in these primary structures,
B and T cells circulate to the peripheral secondary lymphoid organs, which
include the MALT, the lymph nodes, and the spleen (see Figure 14–1).
Lymphocytes do not stay long in the lymphoid organs; they continuously
Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy
histology 2nd stage
4
recirculate through the body in connective tissues, blood, and lymph. Because of
the constant mobility of lymphocytes and APCs, the cellular locations and
microscopic details of lymphoid organs differ from one day to the next. However,
the relative percentages
of T and B lymphocytes in these compartments are relatively steady (Table 14–
3). Lymphoid tissue is usually reticular connective tissue filled with large
numbers of lymphocytes. It can be either diffuse within areas of loose connective
tissue or surrounded by capsules, forming discrete (secondary) lymphoid
organs. Because lymphocytes have prominent basophilic nuclei and very little
cytoplasm, lymphoid tissue packed with such cells usually stains dark blue . In all
secondary lymphoid tissue the lymphocytes are supported by a rich reticulin
fiber network of type III collagen . The fibers are produced by fibroblastic
reticular cells, which extend numerous processes along and around the fibers.
Besides lymphocytes and reticular cells, lymphoid tissue typically contains
various APCs and plasma cells. Although most lymphocytes are morphologically
indistinguishable in either the light or electron microscope, various surface
proteins (“cluster of differentiation” or CD markers) allow them to be
distinguished as B cells and subcategories of T cells by immunocytochemical
methods.
Key features of B and T lymphocytes also include the surface receptors
involved in activating their different responses to antigens. Receptors of B cells
are immunoglobulins that bind antigens directly; those on T cells react only with
antigen on MHC molecules and this requires the additional cell surface proteins
CD4 or CD8.