Immunity

Question 1

Define Immunity.

Answer 1

The ability of human body to resist almost all types of organisms or toxins that tend to damage the tissues and organs.

The ability of the body to defend against invading agents that threaten our normal health is called immunity. the invader may be a living organism or a non living substance.

Question 2

Define Immunology.

Answer 2

Immunology is the study of physiological responses by which the body destroys or neutralizes the foreign matter (living or non-living) as well as cells of its own that have become altered.

Question 3

Classification of Immunity.

Answer 3

A) Innate or Nonadaptive immunity

• non specific defences
• relatively specific defense by natural killer cells.

B) Acquired or Adaptive Immunity
- Natural
› Active: usually through infections
a) cell mediated (T lymphocytes)
b) humoral (B lymphocytes)
› Passive: transfer of antibodies from mother
eg: IgG through placenta, IgA via breastfeeding

• Artificial
  › Active: Vaccination (antigens in vaccines are immunogenic not pathogenic, stimulate immune responses and produce memory cells)
  › Passive: IV injection of antibodies
  eg. Anti-D immunization of Rh negative mother.

Question 4

define Immunity.Discuss the role of lymphocytes in immunity.

Answer 4

Definition: Immunity is the ability of the body to resist infection (invasion) caused by foreign agents and to neutralize their harmful effects on the body. It is a 
function of lymphocytes. 

There are two types of Immunity:
A) Humoral Immunity: It is due to B lymphocytes. In late fetal life and in the newborn, these cells are processed in the liver and bone marrow (Bursa of
Fabricius in birds) to produce antibodies. Each B lymphocyte is specific for a particular antigen.

Response of B lymphocytes: Foreign antigen is first ingested and processed by the tissue macrophage and
presented to specific B lymphocyte which then proliferates to form (1) Memory B cells of that clone and (2)
Lymphoblasts which differentiate into plasmablasts which then multiply to form plasma cells. The plasma cells then produce immunoglobulins (antibodies) which attack foreign agent. Interleukin 1 produced by macrophages and interleukin 4,5 and 6 produced by helper T cells facilitate the growth of B lymphocytes and production of anti bodies. Immunoglobulins are of 5 types:
IgG, IgA, IgM,IgE and IgD.

• Each immunoglobulin molecule is Y shaped and made up of 2 heavy and 2 light chain.

Various ways in which antibodies react with antigen and destroy them are:
a) Direct Antigen – Antibody reactions producing:
(1) Agglutination of bacteria.
(2)Precipitation of bacterial toxins
(3)neutralization of toxic sites of organism.
(4)
lysis, that is disintegration of organism.
b) By activation of complement system:
It is a system of 11 enzymes present in plasma in inactive state(C1 to C9, B and D). Antigen-antibody reaction activates complement C1 which then brings about sequential activation of other complements. The activated complements then attack and destroy antigenic agents by mechanisms like
1. Opsonisation (C3b)
2. Lysis.
3. Agglutination.
4. Neutralization of viruses
5. Chemotaxis (C5).
6. Activation of mast cells.

CELLULAR IMMUNITY :
Is due to T lymphocytes and is affective against -chronic infections and foreign cells like tumors, grafts and virus infested cells . These T lymphocytes are processed in thymus in late fetal life and in new born. Each T lymphocyte is specific for particular antigen and contains, Surface receptor proteins (T cell markers).

steps of cellular immunity

1) antigen recognition, processing and presentation
2) activation and proliferation of t cells
3) elimination of invader

Activation of T cell:
When a foreign antigen is presented by a macrophage to its specific T cell, binding of antigen to the T cell causes its activation. It then proliferates and forms large number of T lymphocytes of that clone. There are 4 types of such activated T cells.
1. Helper T cells (T4 cells): these cells secrete various chemicals called Lymphokines like
(a) Interleukin 2 which
causes growth and proliferation of cytotoxic and suppressor T cells.
(b) Interleukins 4,5 and 6 : Stimulate
antibody production by B lymphocytes.
(c) Interferon: destroys viruses.
(d) CSF -GM: stimulates leucopoiesis
(e) Migration Inhibition Factor: inhibits migration of macrophages.
2. Suppressor T cells (T8 cells) : These cells suppress the function of helper and cytotoxic cells and therefore limit the immune response. They are also responsible for immune (self) tolerance. Helper T cells and suppressor T cells are called Regulatory T cells
3. Cytotoxic (Killer) T cells: Can directly attack and destroy foreign cells. They attach to foreign cell, release a protein called Perforin which makes a hole in foreign cell through which cytotoxic enzymes are released into the foreign cell causing its destruction.
4. Delayed type of Hypersensitivity T cell: are activated after few days or weeks and are responsible for delayed reactions like graft rejection.

Cytotoxic and DTH T cells are called as Effector T cells.

Applied Physiology: In Acquired Immuno Deficiency Syndrome (AIDS), HIV attacks Helper T cells and suppresses humoral as well as cellular immunity.

Question 5

types of immunoglobulin

Answer 5

Immunoglobulins (antibodies) are divided on the basis of differences in amino acid
sequences in the heavy chains into fi ve major classes:
1. IgM (μ chain). Large share of antibodies formed during primary response are of this type. They have ten binding sites which make them highly eff ective in protecting the body.
2. IgM (gamma chain). These are most common and are also called gamma globulins.
3. IgA (alpha chain). These are secreted by linings of gastrointestinal tract, respiratory tract, etc. Generally they act locally and do not enter into the circulation.
4. IgD (delta chain). Functions are not yet clear.
5. IgE (epsilon chain). They mediate allergic responses and participate in defence against multicellular parasites.

Question 6

steps of humoral immunity

Answer 6

• presentation of antigen
• activation of b cells
• differentiation of b cells into plasma cells
• proliferation of b cells and antibody production
• killing of the invaders by antibodies that include activation of complement system
• formation of memory b cells and subsequent logical responses

Question 7

complement system activation

Answer 7

classical
- once antigen-antibody complex activates c1, activated c1 (c1a) activates c2 and c4.
- activated c2 and c4 (c4a and c2a) activates c5 which (c5a) in turn activates c6,c7,c8,c9
- these activated complement proteins kill microorganisms and facilitate inflammation by the following mechanism
- Opsonization. Activation of neutrophils and macrophages to engulf the
bacteria.
- Lysis. Destruction of bacteria by rupturing the cell membrane.
- Chemotaxis. Attraction of leucocytes to the site of antigen-antibody
reaction.
- Agglutination. By causing of clumping of foreign bodies like red blood
cells.
- Neutralization. Covering the toxic sites of antigenic products.
-
Activation of mast cells and basophils. Th is causes liberation of histamine.
Histamine causes dilatation of blood vessels, increases capillary perme-
ability therefore plasma proteins from blood enter the tissues and antigenic
products are inactivated.
- Functions of c3a, c4a and c5a- promote phagocytosis
- function of c3b - promote optimisation
- functions of c5b, c6,c7, c8 and c9- cause cytolysis of the microbes

alternative pathway

• also called properdin pathway as the key protein is properdin
• in this system the circulating protein called factor 1 recognises the polysaccharide unit present on the surface of the microorganisms
• the interaction of factor 1 with the polysaccharide on microbes triggers a reaction that activates c3 and c5
• once c3 is activated it (c3a) activates other complement proteins.

Question 8

allergy

Answer 8

allergy is a hyper reactive response of the body to an antigen which is usually tolerated by others

Question 9

write the role of cell mediated immunity

Answer 9

The term cell-mediated immunity refers to the specific immune responses that do not
involve antibodies. T-lymphocytes are responsible for cell-mediated immunity.

Activation of lymphocytes
The macrophages are present along with lymphocytes in almost all lymphoid tissues.
When bacteria, viruses, foreign protein or toxins enter the body, macrophages ingest
them by phagocytosis. Th e antigenic products of these organisms are then liberated by
macrophages. Th ese antigenic products activate T- and B-lymphocytes. On exposure
to antigen, T-lymphocytes of specifi c lymphoid tissue clone proliferate and release large
number of activated T cells. Activation of B-lymphocytes causes release of antibodies.
Th e principal diff erence is that instead of releasing antibodies whole activated T cells
are formed and released into the lymph and then enter into circulation through which
they are distributed throughout the body. Th ey also pass out through capillary walls
and enter in tissue fl uid present in tissue spaces from where they enter back into the
lymph, then to lymphoid tissue and once again in blood. Th us T-lymphocytes circulate
again and again throughout the body, sometimes lasting for months or years.
T-lymphocyte memory cells are also formed. When a clone of T-lymphocytes is
activated many of the newly formed lymphocytes are preserved in lymphoid tissue
to become additional T-lymphocytes of that specifi c clone. Th ese are memory cells
spread throughout the lymphoid tissues of the entire body. Th erefore, on subsequent
exposure to the same antigen, release of T cells occurs far more rapidly and much more
powerfully than in fi rst response.
Antigenic products from macrophage also cause activation of helper T cells which
in turn stimulate activation of T and B cells, by secreting interleukins 2, 3, 4, 5 and 6,
granulocyte-monocyte colony stimulating factor and interferon 4. Th ese secretions are
proteins called lymphokines. Th ese activate formation of cytotoxic cells and suppressor
T cells (especially interleukin 2). Suppressor T cells suppress the activity of cytotoxic T
cells and play an important role in preventing cytotoxic T cells from destroying body’s
own tissues along with invader organisms.
Action of cytotoxic T cells
1. The outer membrane of cytotoxic T cells contain some receptor proteins. Th ese bind
the antigens or organisms tightly with cytotoxic T cells. Th en these T cells enlarge and
release cytotoxic substances like the lysosomal enzymes. Th ese substances destroy the
antigen or invaded organisms. Like this each cytotoxic cell can destroy large number of
organisms one aft er another.
2. At times T cells also destroy body’s own cells containing the antigen. Th erefore,
they are also called killer cells. Th e receptor proteins on the surfaces of cytotoxic cells
cause them to bind tightly with the cells that contain their binding specifi c antigen.
Aft er binding the cytotoxic cell secretes hole-forming proteins called perforins. Th ese
proteins literally punch round holes in the membrane of the attacked cell. Th en fl uid
rapidly fl ows into the cell from interstitial space. In addition cytotoxic cells also release
cytotoxic substances directly into the attacked cell. Almost immediately the attacked
cell becomes greatly swollen and usually dissolves shortly thereaft er. Cytotoxic killer
cells can pull away from the victim cells aft er they have punched holes then move on to
kill many more such cells. Aft er destruction of invaders these killer cells persist in the
tissue for months.
Some T cells are especially lethal to body tissue cells that have been invaded by
viruses (many virus particles become entrapped in the membrane of these cells). Th e
antigen of viruses attracts the T cells. Th e cytotoxic T cells then kill the aff ected cells
along with viruses.
3. Cytotoxic or killer T cells also destroy cancer cells, transplanted heart or kidney
cells or other types of cells that are foreign to the person’s own body.