B_T_2015_02_

types of acquired immunity

• Acquired immunity is of two types – natural or
active and artificial or passive.

• Active immunity is the resistance induced after
contact with foreign antigens, e.g., after an attack
of small pox.

• Main advantage of active immunity is that resistance
is long term and its major disadvantage is its slow
onset, especially the primary response.

• Active immunity is produced by clonal selection and
expansion.

• Clonal selection leads to the eventual production
of :

• A pool of antibody-secreting plasma cells


• A pool of “memory” cells
  • Passive immunity occurs when antibodies produced
  artificially are injected into a person to counteract
  antigens such as snake venom, rabies, tetanus toxin
  and Salmonella infection etc.
  • The yellowish fluid, colostrum secreted by a lactating
  mother during the initial days of lactation has
  abundant antibodies (IgA) to protect the infant.
  • The foetus also receives some antibodies from its
  mother, through the placenta during pregnancy.
  • It has the advantage of providing immediate relief
  due to prompt availability of large amount of
  antibodies.

Active immunity Passive immunity

1. It is produced due to contact with pathogen or its
   antigen.

It is produced due to antibodies obtained from outside.

2. Immunity is not immediate. A time lapse occurs for its
   development.

Immunity develops immediately.

3. It lasts for sufficiently long period, maybe lifelong. It lasts for a few days.


4. Side effects are very few. At times, the body reacts to the introduction of antisera.
   It is called serum sickness.

Table : Differences between active immunity and passive immunity

Components of acquired immunity

•    Acquired immunity has two components – humoral

and cell mediated immunity.

1. Humoral immune response or antibody
   mediated immunity
   • Antibody-mediated (or humoral) immunity is
   associated with the appearance of antibodies, secreted
   by B-lymphocytes, in the extracellular fluids such as
   plasma, lymph and external secretions.
   Antigen enters body

Flow chart:Summary of humoral immunity

Other B-cells

“remember”the antigen

Memory cells cause the

production of new plasma cells

when antigen again enters body

Some B-cells become

plasma cells

Plasma cells produce

antibodies

Antibody reacts

with antigen

•    It defends the body against viruses, bacteria with

polysaccharide capsules and toxins that enter the

body fluids (blood and lymph).

2. Cell-mediated immune response or cell
   mediated immunity
   • Cell-mediated immunity is mediated by cells of the
   T-lymphocyte with antigen-specific receptors on their
   surfaces.
   • It defends the body against viruses, fungi and some
   bacteria which have entered the host’s cell.