The document discusses immunity and the immune system. It defines immunity as the body's ability to resist harmful organisms and toxins. The immune system is composed of specialized cells that fight pathogens like bacteria, viruses, fungi and protozoa. The immune system produces antibodies and cells that can deactivate pathogens. Immunity can be active, acquired after infection or vaccination, or passive, acquired from external sources like breast milk or gamma globulin injections. The document also describes innate immunity, the body's first line of defense, and acquired immunity, the second line of defense involving lymphocytes and lymphoid tissues.

1. IMMUNITY
Prior Concepts: 5th semester do read about topics like leucocytes, White blood cells, neutrophils,
basophils, eosinophilsetc
IMMUNITY: The ability of human body to resist almost all kinds of organisms and toxins that tends to
damage the tissues and organs is called immunity.
Immunity is the body’s ability to fight off harmful micro-organism –PATHOGENS – that invade
it.
Fungi, Protozoans, Bacteria and viruses are all potential pathogens.
IMMUNE SYSTEM: It refers to a system composed of specialized cells that fight against disease
producing bacteria and toxins.
The immune system produces antibiotics or cells that can deactivate pathogens.
The immune system includes all parts of the body that help in the recognition and destruction of
foreign materials. White blood cells, phagocytes and lymphocytes, bone marrow, lymph nodes,
tonsils, thymus and your spleen are all part of the immune system.
TYPES OF IMMUNITY
According to ability of body to develop immunity, it is classified into two types:
Active immunity
Passive immunity
Active immunity is further divided into two types with respect to body’s ability to develop specific or
non-specific immunity.
Immunity
Passive immunity Active immunity
Acquired immunityInnate immunity
Antibody mediated immunity Cell mediated immunity
ACTIVE IMMUNITY: occurs when one makes his / her own antibodies. This type of immunity is long
term.
Getting the disease: if you get an infectious disease (like chicken pox), often times that stimulates
the production of MEMORY CELLS which are then stored to prevent the infection in the future.
VACCINATION: is an injection of a weakened form of the actual antigen that causes the disease.
The injection is too weak to make you sick but your B-lymphocytes will recognize the antigen

2. and react as if it were the “real thing”. Thus, you produce MEMORY CELLS for long term
immunity.
PASSIVE IMMUNITY: occurs the antibodies come from some other source. This type of immunity is
short term.
Breast milk: milk form a mother’s breast contains antibodies. The body is acquiring passive
immunity. These antibodies will only last several weeks.
Gamma globulin: A gamma globulin shot is pure an injection of antibodies to provide temporary
immunity. You might receive a gamma globulin shot if you travel outside of the country.
INNATE IMMUNITY (non-specific): the natural resistance of the body to various bacteria, toxins and
other foreign agents without any specific immune process is called innate immunity. Body’s first line
defenses(physical and chemical barriers)
Innate immunity can be obtained by:
Cellular components: phagocytosis of bacteria and other invaders by white blood cells and tissue
macrophage system.
Non-cellular components:
 Destruction by the acid secretions of stomach and by the digestive enzymes of
organisms swallowed into stomach.
 Resistance of skin to invasion by organisms.
 Presence in the blood of certain chemical compounds that attach to foreign
organisms or toxins and destroy them e.g. lysozymes, basic polypeptide,
complement complex.
 Other examples include tears, saliva, mucous, sweat etc.
ACQUIRED IMMUNITY (specific): human body has ability to develop extremely powerful specific
immunity against invading agents and this is called the acquired immunity.
TYPES OF ACQUIRED IMMUNITY:
a- HUMORAL (ANTIBODY-MEDIATED) IMMUNITY: in this type of immunity, body develops
circulating antibodies which are globulin molecules and are capable of attacking the invading
agents. This type of immunity is composed of specific antibodies synthesized by plasma cells
derived from B-lymphocytes.
b- CELL-MEDIATED IMMUNITY: in this type of immunity body develops large number of
lymphocytes which are specifically activated against foreign agent. These activated or sensitized
lymphocytes have the ability to attach to a foreign agent and to destroy it. This type immunity is
composed of sensitized T-lymphocytes.
Acquired immunity is the second line defense of the body. If a pathogen is able to get past the body’s first
line of defense and an infection starts, the body can rely on its second line of defense. The results in what
is called an inflammatory response which cause:
Redness and heat: due to capillary dilation resulting in increased blood flow.
Swelling: due to passage of plasma from the blood stream into the damaged tissue.
Pain: due mainly to tissue destruction and to a lesser extent swelling.

3. ANTIGEN: any invading agent like foreign proteins, organisms, or toxins that can produce an immune
response is called an antigen. OR a substance usually protein in nature which when introduced into the
tissue, stimulate antibody production.
ANTIBODY: (Immunoglobulin) the specific globulin protein formed in the blood plasma ass a reaction to
antigen is called antibody. Blood contain three types of globulin: alpha, beta and gamma. Antibodies are
gamma globulins.
The antibodies that circulate in the blood stream are called as immunoglobulins (Ig)
Each antibody is composed of two light chains and two heavy chains. Each chain has a constant and
variable portion.
1- Variable portion (Fab fragment) attaches specifically to a particular type of antigen. It
determines antigen binding specificity.
2- Constant portion (Fc fragment) has receptors for attachment to complement complex. It
determines physical properties.
TYPES OF IMMUNOGLOBULIN: (acronym: GAMED)
IgG:
It is the most abundant type of immunoglobulin present in serum.
It is main antibody in secondary response (while IgM in primary response)
It provides an important defense against bacteria and viruses.
It is the only antibody that can cross placenta.
It is the most abundant immunoglobulin in newborn.
It activates complement (IgM also)
It acts as opsonin and therefore enhances phagocytosis.
IgA:
It is the main immunoglobulin in secretions such as colostrum, saliva, tears and respiratory,
intestinal and genital tract secretions.
It prevents attachment of microorganism to mucus membrane.
It cannot cross placenta and does not activate complement.
IgM:
It is also known as monomeric IgM on B – cell surface and constitutes antigenic receptor.
It is the main immunoglobulin produced early in the primary response.
It provides defense against bacteria and viruses.
It can activate complement.
It does not cross placenta.

4. IgE:
It is found in allergies and parasitic reactions.
It mediates type I hypersensitivity reactions by causing release of mediators from mast cells and
basophils upon exposure to antigen.
It is the main host defense against helminth (worm) nfections such as ascaris, hook worm.
It cannot cross placenta and does not activate complement.
Its concentration in serum is very low but rises in allergy and helminth infections.
IgD:
It occurs in B – cell surface.
ROLE OF LYMPHOID TISSUE IN ACQUIRED IMMUNITY
Acquired immunity is the product of body’s lymphoid tissue. There are two types of lymphoid tissue.
a- Central lymphoid tissue
b- Peripheral lymphoid tissue
CENTRAL LYMPHOID TISSUE:
Thymus,
bone marrow
These are the tissues in which primitive lymphoid cells in the fetus are developed and get maturation.
PERIPHERAL LYMPHOID TISSUE:
Lymph node
Spleen
Tonsils
Gut associated lymphoid tissue
Peripheral blood
These are the tissues in which mature lymphocytes reside and respond to antigenic stimuli.
CELLS OF THE IMMUNE SYSTEM:
LYMPHOCYTES: are cells derived from lymphoid stem cells in the bone marrow and develop in the fetal
life. Lymphocytes may be classified on the basis of their site of development in the fetus.
a- T lymphocytes: develop in thymus of the fetus. Then in adult life in bone marrow.
b- B lymphocytes: develop in fetal liver or bone marrow.

5. T-LYMPHOCYTES:
They arise from the stem cells in the bone marrow. They are immature and are taken to the thymus for
maturation during fetal life. About 80-90% of peripheral blood lymphocytes are T-lymphocytes. Mature
T lymphocytes circulate in the blood and pass to peripheral lymphoid tissue. e.g.
a- Paracortical areas of lymph nodes
b- Periarteriolar lymphoid sheath in the white pulp of spleen.
After stimulation or activation by specific antigen, T lymphocytes transform into large actively dividing
cells known as transformed T- lymphocytes, which then divide to produce effector cells. These effector
T-lymphocytes are also called sensitized, cytotoxic or killer T-cells.
T lymphocytes can’t be activated by free antigens. They are activated by processed antigens presented
to them by macrophages, dendritic and Langerhan’s cells.
T CELL SURFACE MOLECULES:
T Cell Receptor (TCR): binds to Antigen presented by macrophages and Langerhan’s cells.
CD3 molecular complex: they don’t bind to antigens but transduct signals to T cells after it has
bound to antigen. (CD = Cell Differentiation)
CD4 molecules: are expressed on 60% of mature CD3 + T cells. During antigen presentation they
bind to class II MHC molecules on antigen presenting cells.
CD8 molecules: are expressed on 30% of mature CD3 + T cells. (So CD4/CD8 ratio is 2:1)During
antigen presentation they bind to class I MHC molecules on antigen presenting cells.
TYPES OF T LYMPHOCYTES:
1- Helper T cell (CD4+) subdivided into
a- Helper inducer T cells
b- Helper suppressor T cells
2- Suppressor T cells (CD8+)
3- Cytotoxic/Killer T cells (CD8+)
FUNCTIONS OF T LYMHOCYTES:
Cellular immune reaction: They are responsible for cell mediated immunity e.g. against foreign
histocompatibility antigens, virus infected cells and some tumor cells.
Regulatory function: They control the T & B cell mediated response through:
1- T-helper cells which help in the production of T & B cells.
2- T-suppressor cells which suppress T & B cell function.

6. B LYMPHOCYTES:
They also arise from the stem cells in the bone marrow and are taken to some unknown part of the body
possibly the fetal liver, bone marrow, GIT mucosa for maturation. Mature B-lymphocytes are taken to
the peripheral lymphoid tissue e.g. lymph node (lymphoid follicles in superficial cortex),
spleen(lymphoid follicles in white pulp) and Peyer’s patches of GIT. About 10-20% of peripheral blood
lymphocytes are B- cells.
FUNCTIONS OF B LYMPHOCYTES:
After stimulation by an antigen B cells differentiate into plasma cells that secrete immunoglobulins
(antibodies).
Specific antigenic stimulation B lymphocytes  lymphoblast  plasmoblast plasma cells specific
antibodies directed against the antigen that caused antibody formation
NONPOLYMORPHIC MOLECULES ON B – LYMPHOCYTES: CD17 & CD20 are two non-polymorphic
molecules present on B cell surface. They are of practical value in classification of lymphoid molecules.
(CD = Cell Differentiation)
MACROPHAGES: are the large, mononuclear highly phagocytic cells, occurring in the walls of blood
vessels (adventitial cells) and in loose connective tissue (histocytes). They are usually fix but when
stimulated by inflammation they become mobile.
FUNCTIONS OF MACROPHAGES:
They present antigens to immunocompetent T cells. (they process and present antigens to CD$
+ T cells which recognize antigen by binding to class II MHC molecules on macrophages. Note: T
cells can’t recognize free unprocessed antigen)
In addition to phagocytosis, macrophages also produce cytokines/monokines:interleukin – 1 (IL-
1) which promotes the differentiation of both T &B lymphocytes and TNF-α(Tissue Necroting
Factor) is also produced which is a pro-inflammatory.
Macrophages produce CytokinesIL-1enhanced activity of helper T cells
Macrophages produce  TNF inflammatory response i.e., swelling, redness, pain and edema
They lyse tumor cells by secreting toxic metabolites and proteolytic enzymes thus play a role in
immunosurveillance.
They act as powerful effector cells in certain forms of cell-mediated immunity, such as the
delayed hypersensitivity reactions (type IV).
NATURAL KILLER CELLS: are large granular lymphocytes because they contain azurophilic cytoplasmic
granules. They are peripheral blood lymphocytes (10-15%). They differ from T and B cell function and

7. are also larger than B&T cells. They are lymphocytes in nature but do not pass through thymus for
maturation. NK cells are devoid of cell surface immunoglobulins. They do not have immunological
memory and don’t have T cell receptors. NK cells have a basic role in innate immunity and are
considered to provide first line of defense against tumors and virus infections.
Surface molecules on NK cells are
CD2
CD16 (Fc receptor for IgG)
CD56
FUNCTIONS OF NK CELLS:
THEY are capable of lysing a variety of tumor cells, virus-infected cells, and fungi without prior
sensitization hence called natural killer cells. NK cells destroy tumor cells by secreting cytotoxins similar
to T lymphocytes.
NK cells have Fc receptors for IgG, so they can lyse IgG- coated target cells. (ADCC: Antibody dependent,
cell mediated cytotoxicity).
INTERFERONS: ϒ – interferons are produced by NK cells that activate macrophages.
NK cells ϒ – interferonsMacrophage activation
Whereas, α andβinterferones are produced by virus infected cells that activate NK cells.
Virus in fected cells α,βinterferonsNK cell activation
DENDRITIC AND LANGERHAN’S CELLS:
Dendritic cells are found in lymphoid tissue whereas,Langerhan’s cells are located in epidermis. They
have:
1- Dendritic cytoplasmic processes
2- Class II MHC molecules on surface
FUNCTIONS:
These cells themselves are poorly phagocytic in nature. They process and present antigens to CD4 + T
cells which recognize antigen by binding to class II MHC molecules on dendritic and Langerhan’s cells.
Further see: difference between innate and acquired immunity, between passive and active immunity,
between T and B lymphocytes etc.