The document summarizes disorders of the immune system. It describes how immunity has both beneficial and harmful effects. The immune system is divided into innate and adaptive immunity. Innate immunity provides non-specific first line defense while adaptive immunity involves antigen-specific responses by antibodies and T cells. Disorders of the immune system include immunodeficiencies from deficient immunity, hypersensitivity reactions from excessive responses, and autoimmune diseases where the immune system attacks self-tissues.

1. DISORDERS OF
IMMUNE SYSTEM

2.  Immunity and immunopathology are proverbial
two edges of‘double-edged sword’ i.e. it is a
defense mechanism but it can be injurious to
the human body in a variety of ways.
 Broadly speaking, immunity or body defense
mecha nism is divided into 2 types,
 natural (innate) and specifi c (adaptive),

3.  Natural or innate immunity is non-specifi c and is
considered as the fi rst line of defense without
antigenic specifi city.
 It has 2 major components:
 a) Humoral: comprised by complement.
 b) Cellular: consists of neutrophils, macrophages, and
natural killer (NK) cells.
 Specifi c or adaptive immunity is specifi c and is
characterised by antigenic specifi city.
 It too has 2 main components:
 a) Humoral: consisting of antibodies formed by B cells.
 b) Cellular: mediated by T cells.

4.  Th e major functions of immune system are
as under:
 i) Recognition of self from non-self
 ii) Mounting a specifi c response against non-
self
 iii) Memory of what was earlier recognised as
non-self
 iv) Antibody formation
 v) Cell-mediated reactions

5.  DISORDERS OF IMMUNE SYSTEM:
 1.Immunodeficiency disorders are
characterised by deficient or absent cellular
and/or humoral immune functions.
 2. Hypersensitivity reactions are characterised
by hyperfunction or inappropriate response of
the immune system and cover the various mecha
nisms of immunologic tissue injury.
 3. Autoimmune diseases occur when the
immune system fails to recognise ‘self’ from ‘non-
self’. A growing number of autoimmune and
collagen diseases are included in this group.

7. ORGANS AND CELLS OF IMMUNE
SYSTEM
 a) Primary lymphoid organs:
 i) Th ymus
 ii) Bone marrow
 b) Secondary lymphoid organs:
 i) Lymph nodes
 ii) Spleen
 iii) MALT (Mucosa-Associated Lymphoid Tissue
located in the respiratory tract and GIT).

8.  Depending upon the rapidity, duration and type of the immune
response, there are 4 types of hypersensitivity reactions
grouped into two categories:
 immediate (typeI, II, and III) and
 delayed type (type IV).
 Type I (or anaphylactic) hypersensitivity is a state of
rapidly developing or anaphylactic type of immune response
to an antigen (i.e. allergen) to which the individual is
previously sensitised.
 The reaction appears within 15-30 minutes of exposure to
antigen.
 It is mediated by humoral antibodies of IgE type for which
mast cells and basophils play a key role.

9.  Type II or cytotoxic reaction is a reaction by humoral
antibodies that attack cell surface antigens on the specific
cells and tissues and cause lysis of target cells.
 Type II reaction is tissue-specifi c and occurs after antibodies
(IgG or IgM) bind to tissue specifi c antigens, most often on
blood cells.
 Type III reactions result from deposition of antigen-
antibody complexes on tissues, which is followed by activation
of the complement system and inflammatory reaction and
tissue injury.
 Antigen in type II reaction is not tissue specific.
 Type IV or delayed hypersensitivity is tissue injury by T
cell-mediated immune response without formation of
antibodies.
 The reaction occurs about 24 hours (delayed) after exposure
to antigen and the response is prolonged.

11. AUTOIMMUNE DISEASES
 Autoimmunity is a state in which the body’s immune system
fails to distinguish between ‘self’ and ‘non-self’ and reacts by
formation of autoantibodies against one’s own tissue antigens.
 Depending upon the type of autoantibody formation, the
autoimmune diseases are broadly classifi ed into 2 groups:
 1. Organ specifi c (Localised) diseases In these, the
autoantibodies formed react specifically against an organ or target
tissue component and cause its chronic inflammatory destruction.
 The tissues affected are endocrine glands (e.g.thyroid, pancreatic
islets of Langerhans, adrenal cortex),alimentary tract, blood cells
and various other tissues and organs.
 2. Organ non-specifi c (Systemic) diseases These are diseases
in which a number of autoantibodies are formed which react with
antigens in many tissues and thus cause systemic lesions.
 The examples of this group are various systemic collagen diseases.

12. IMMUNODEFICIENCY
DISORDERS
 Traditionally, immunodeficiency diseases are
classified into 2 types:
 A. Primary immunodeficiencies are usually the
result of genetic or developmental abnormality
of the immune system.
 B. Secondary immunodeficiencies arise from
acquired suppression of the immune system.
 The most important example being acquired
immunodeficiency syndrome (AIDS).

13. AIDS
 It is caused by retrovirus, HIV-1 or HIV-2, the former
being much more common etiologic agent in most parts
of the world.
 The routes of spread of infection are: sexual (both homo-
and heterosexual), via blood route and by use of
contaminated blood products, perinatal transmission to
the new born from infected mothers, needle stick injuries,
and rarely from other body fluids
 Mechanism of acquiring disease is by selective tropism
of HIV for CD4 molecule located on CD4+ helper T cells,
causes a progressive reduction in the CD4 cell count.

14.  As per CDC, HIV is defined by clinical features
and by CD4+ T cell count <200/μl.
 AIDS involves multiple systems and affects
almost all organs. The main pathologic lesions
in full-blown case are due to opportunistic
infections, secondary tumours and CNS
manifestations.
 Diagnosis of AIDS is made by tests to establish
infection (antibody testing by ELISA, confi
rmation by Western blot), and to detect defect
in immunity (low CD4 cell counts and reversal
of CD4+:CD8+ cells).

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