12th Zoology

Acquired Immunity

Acquired Immunity (Specific immunity)

Acquired immunity, also known as adaptive or specific immunity, is
capable of recognizing and selectively eliminating specific microorganisms.
Acquired immunity is found only in verterbrates. It supplements the
protection provided by innate/natural immunity. It is generated in response
to an exposure or encounter to the microorganisms in question. Specific
defence mechanisms require several days to be activated, following the
failure of non-specific defence mechanisms.

Unique features of the Adaptive immunity

(i) Specificity : It is the ability to distinguish differences among various
foreign molecules.
(ii) Diversity : It can recognize a vast variety of foreign molecules.
(iii) Discrimination between Self and Non-self : It is able to recognize and
respond to molecules that are foreign (non-self) to the body. At the same
time, it can avoid response to those molecules that are present within the
body (self antigens) of the given animal. (Acquired immunity)

(iv) Memory : When the immune system encounters a specific foreign agent,
e.g., microbe, for the first time, it generates an immune response and
eliminates the invader. The immune system retains the memory of this
encounter for a prolonged interval. As a result, a second encounter with the
same microbe evokes a heightened immune response.

Specific immunity employs two major groups of cells

(a) lymphocytes, and (b) antigen presenting cells. A healthy individual
possesses about a trillion of lymphocytes. The lymphocytes are of two types
viz., T- lymphocytes or T-cells and B – lymphocytes or B – cells. Both
the types of lymphocytes, as well as the other cells of the immune response,
are produced in bone marrow.

The process of their production is called haematopoiesis. Some immature lymphocytes, destined to become thymocytes, migrate via blood to the thymus, where they mature and differentiate as T – cells. The B- cells, on the other hand, mature in the bone marrow itself.

The B and T cells, together, generate two types of specific immunity, viz., (a) cell-mediated and (b) antibody-mediated or humoral immunity respectively. (Acquired immunity)

(a) Cell-mediated Immunity (CMI)

Cell-mediated immunity is the responsibility of a subgroup of T cells,
called cytotoxic T lymphocytes (CTLs). An activated cytotoxic lymphocyte
is specific to a target cell, which has been infected, and kill the target cell by
a variety of mechanisms. This prevents the completion of life cycle of the
pathogen and its growth, since it depends on an intact host cell to do that.
Cell-mediated immunity is also involved in killing of cancer cells. (Acquired immunity)

(b) Antibody-mediated Immunity / Humoral Immunity

Antibody mediated or humoural immunity involves the synthesis of
specific antibody molecules called immunoglobulins by the B-lymphtocytes.
Each antigen has many different antigenic determinants, each of which
matches a specific antibody and binds to it. The B cells, direct the antibodymediated immunity. The antibody molecules (Igs) may be bound to a cell
membrane in the form of receptors or they may remain free.

The free antibodies have three main functions viz.,

1. agglutination of particulate matter, including bacteria and viruses,

2. opsonisation or coating over bacteria to facilitate recognition and phagocytosis by the phagocytes and

3. neutralization of toxins released by bacteria. (Acquired immunity)

Adaptive immunity may be active or passive. Active immunity is
due to the immune response generated in the individual in question by a
pathogen or vaccine, whereas passive immunity is conferred by transfer
of immune products, like antibodies, etc., from an individual into a nonimmune
individual. (Acquired immunity)

Activation of Adaptive Immunity

Every antigen is processed by antigen presenting cells(APC), like
macrophages, B lymphocytes and dentric cells. The processed antigen is
presented on the surface of these cells.

A subgroup of T cells called T helper cells, specifically interacts with the presented antigen and becomes activated.

The activated T helper cells then activate B cells, and a subgroup
of T cells called cytotoxic lymphocytes (CTLs), in a specific manner. The
activated B and cytotoxic lymphocytes proliferate to produce clones. All the
cells of a clone can recognize the same antigen and eliminate it. (Acquired immunity)

Related Topics in Zoology:

Bio Zoology All Important Topics

  1. Microbiology Introduction and History of Medical Microbiology

  2. Pasteur, Koch, Lister

  3. Structure of Viruses

  4. Viral genetics

  5. Virus Culture

  6. Viral Diseases

  7. Bacteria Structure Culture

  8. Bacterial Genetics

  9. Bacterial Diseases

  10. Protozoan microbiology

  11. Pathogenecity of Microorganisms

  12. Antimicrobial Resistance

  13. Antibiotics and Chemotherapy

  14. AIDS – HIV

Unit 3 – Immunology Topic List Zoology


  2. Acquired Immunity

  3. Structure and Functions of the Immune System

  4. Peripheral Lymphoid Organs

  5. Secondary Lymphoid Organs

  6. Antigenic determinants and epitopes

  7. Antibodies – Immunoglobulins

  8. Transplantation immunology

  9. Genetic basis of organ transplants

  10. Immune system disorders

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