The immune system protects the body from foreign invaders. It has two types of immunity: innate immunity which is present at birth, and adaptive immunity which develops after birth upon exposure to antigens. The immune system uses physical barriers, chemical barriers, white blood cells, and inflammation to fight pathogens. It has three lines of defense - phagocytic response, humoral response involving antibodies, and cellular response using T cells. The immune response involves recognition of antigens, proliferation of lymphocytes, production of antibodies or T cells, and these effectors destroying the pathogen.

1. The Immune System
The Immune System
*The immune system functions as the body’s defense mechanism against
invasion of foreign agent or organism
Anatomy of the immune system
*bone marrow.
*white blood cells (WBCs) .
*lymphoid tissues.
General types of immunity
*Natural (innate) present at birth
*acquired (adaptive). develops after birth
1-Natural mechanisms include
*physical barriers
*chemical barriers
*the action of WBCs
*Inflammatory responses.
A. Physical barriers include
*intact skin and mucous membranes
*cilia of the respiratory tract along with coughing and sneezing.
B. Chemical barriers, such as
* acidic gastric secretions, mucus, enzymes in tears and saliva, and
substances in sebaceous and sweat secretions, act in a nonspecific way to
destroy invading bacteria and fungi.
C. White Blood Cell Action ,
D. WBCs, or leukocytes, participate in both the natural and the
acquired immune responses.
1-Granular leukocytes, (neutrophils, eosinophils, and basophils)
fight invasion of foreign bodies or toxins by releasing cell mediators, such
as histamine, bradykinin, and prostaglandins, and engulfing the foreign
bodies or toxins.
- Neutrophils are the first cells to arrive at the site where inflammation
occur.
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2. The Immune System
Eosinophils and basophils increase in number during allergic reactions
and stress responses.
2- Nongranular leukocytes include
- monocytes or macrophages and lymphocytes.
-Monocytes also function as phagocytic cells, engulfing, ingesting, and
destroying greater numbers and quantities of foreign bodies or toxins than
granulocytes.
-Lymphocytes, consisting of B cells and T cells, play major roles in
humoral and cell-mediated immune responses.
About 60% to 70% of lymphocytes in the blood are ‘T’ cells, and about 10%
to 20% are B cells .
D. Inflammatory Response
*is a major function of the natural immune system elicited in response to
tissue injury or invading organisms.
*Chemical mediators assist this response by minimizing blood loss, walling
off the invading organism, activating phagocytes, and promoting formation
of fibrous scar tissue and regeneration of injured tissue.
Acquired Immunity include
*it acquired during life but not present at birth.
*usually develops as a result of prior exposure to an antigen through
immunization (vaccination) or by contracting a disease, both of which
generate a protective immune response.
Response to invasion
*When the body is invaded or attacked by bacteria, viruses, or other
pathogens, it has three means of defending itself :
*The phagocytic immune response
*The humoral or antibody immune response
*The cellular immune response
1-The phagocytic immune response
– is the first line of defense, it involves the WBCs (granulocytes and
macrophages), which have the ability to ingest foreign particles.
– These cells move to the point of attack, where they engulf and destroy
the invading agents.
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3. The Immune System
– Phagocytes also remove the body’s own dying or dead cells.
2-The humoral immune response
*is a second protective response (sometimes called the antibody response),
*begins with the B lymphocytes, which can transform themselves into
plasma cells that manufacture antibodies.
*These antibodies, highly specific proteins, are transported in the
bloodstream and attempt to disable the invaders.
3-The cellular immune response is the third mechanism of defense,
*also involves the T lymphocytes, which can turn into special cytotoxic (or
killer) T cells that can attack the pathogens themselves.
Role Of Antibodies
- Antibodies are large proteins called immunoglobulins because they are
found in the globulin fraction of the plasma proteins.
Types of Immunoglobulins. ( Ig)
1-IgG (75% of Total Immunoglobulin)
• Appears in serum and tissues (interstitial fluid)
• Assumes a major role in blood borne and tissue infections
• Activates the complement system
• Enhances phagocytosis
• Crosses the placenta
2-IgA (15% of Total Immunoglobulin)
• Appears in body fluids (blood, saliva, tears, breast milk, and pulmonary,
gastrointestinal, prostatic, and vaginal secretions)
• Protects against respiratory, gastrointestinal, and genitourinary
infections
• Prevents absorption of antigens from food
• Passes to neonate in breast milk for protection
3-IgM (10% of Total Immunoglobulin)
• Appears mostly in intravascular serum
• Appears as the first immunoglobulin produced in response to bacterial
and viral infections
• Activates the complement system
4-IgD (0.2% of Total Immunoglobulin)
• Appears in small amounts in serum
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4. The Immune System
• Possibly influences B-lymphocyte differentiation, but role is unclear
5-IgE (0.004% of Total Immunoglobulin)
• Appears in serum
• Takes part in allergic and some hypersensitivity reactions
• Combats parasitic infections
Role of Interferon
-Interferon have antiviral and antitumor properties. In addition to
responding to viral infection, they are produced by T lymphocytes, B
lymphocytes, and macrophages in response to antigens.
-Interferon are effective in treating tumors and acquired
immunodeficiency syndrome (AIDS) multiple sclerosis) and chronic
hepatitis).
There are four stages in an immune response
*Recognition
*proliferation
*response
*effector
1-Recognition Stage
*The body must first recognize invaders as foreign before it can react to them.
The body accomplishes recognition using lymph nodes and lymphocytes for
surveillance.
*When foreign materials enter the body, a circulating lymphocyte comes
into physical contact with the surfaces of these materials. Upon contact, the
lymphocyte, with the help of macrophages, either removes the antigen from
the surface or in some way picks up an imprint of its structure, which comes
into play with subsequent re-exposure to the antigen.
2-Proliferation Stage
*The circulating lymphocyte containing the antigenic message returns to the
nearest lymph node.
*In the node, the sensitized lymphocyte stimulates some of the resident
dormant ‘T’ and B lymphocytes to enlarge, divide, and proliferate.
*T lymphocytes differentiate into cytotoxic (or killer) ‘T’ cells, whereas B
lymphocytes produce and release antibodies.
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5. The Immune System
*Enlargement of the lymph nodes in the neck in conjunction with a sore
throat is one example of Proliferation
3- Response Stage
*In the response stage, the changed lymphocytes function either in a
humoral or a cellular fashion.
*the production of antibodies by the B lymphocytes in response to a specific
antigen begins the humoral response.
4- Effector Stage
*In the effectors stage, either the antibody of the humoral response or the
cytotoxic (killer) ‘T’ cell of the cellular response reaches and couples with the
antigen on the surface of the foreign invader.
*The coupling initiates a series of events that in most instances results in the
total destruction of the invading microbes
*or the complete neutralization of the toxin. ‘the events involve an interplay
of antibodies (humoral immunity), complement, and action by the cytotoxic
‘I’ cells (cellular immunity).
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