B_T_2015_02_

t-lymphocytes or t-cells

• When triggered by antigens, T-lymphocytes produce
a clone of T-cells such as :

• Cytotoxic T-cells or Killer T-cells : These cells
  directly attack and destroy antigens by secreting
  a protein perforin.


• Helper T-cells : These cells are numerous and
  stimulate the B-cells to produce antibodies.
  They help in overall regulation of immunity by
  forming a series of protein mediators called
  lymphokines.


• Suppressor T-cells : These keep a check on
  the entire immune system from attacking the
  body’s own cells. Some of these cells become
  memory cells.


• Memory T-cells : These are sensitized T-cells
  which retain memory of antigen specificity for

future, which is often lifelong.

B-lymphocytes or B-cells

•    B-cells mature in the bone marrow itself, and produce

specialised proteins called antibodies. They generate

antibody mediated or humoral immunity.

•    B-cells give rise to the following cells :

• Plasma cells (Effector B-cells) : The antigen
  specific T-cells stimulate specific B-lymphocytes to
  multiply rapidly, forming clone of plasma cells.


• In the presence of antigen, the plasma cells form
  antibodies at the rate of 2000 molecules per cell
  per second, a total of some 20 trillion per day.


• Memory B-cells : Some of the activated B-cells
  do not differentiate into plasma cells, but rather
  remain dormant as memory B-cells, until activated
  once again by a renewed attack of the same
  antigen.
  Antigen presenting cells (ApCs)
  • APCs are specialized cells which include macrophages
  (monocytes as blood macrophages and histiocytes as
  tissue macrophages), B-lymphocytes and dendritic cells
  (e.g., Langerhans cells of epidermis of skin).
  • APCs are distinguished by the following two
  properties:


• they express class II MHC molecules on the
  membrane.


• they are able to deliver a co-stimulatory signal
  that is necessary for helper T-cell activation.

Attribute Innate immunity Adaptive immunity

Response time Minutes/hours Days

Specificity Not specific for molecules or molecular

patterns associated with pathogens.

Highly specific; discriminates even minor differences in

molecular structure; details of microbial or non-microbial

structure recognized with high specificity.

Diversity A limited number of germ line-

encoded receptors.

Highly diverse; a very large number of receptors arising

from genetic recombination of receptor genes

Memory responses None Persistent memory, with faster response or greater

magnitude on subsequent infection.

Self/non-self

discrimination

Perfect; no microbe-specific patterns

in host.

Good; occasional failures of self/non-self discrimination

result in autoimmune disease.

Soluble components

of blood or tissue

fluids

Many antimicrobial peptides and

proteins.

Antibodies

Major cell types Phagocytes (monocytes, macrophages,

neutrophils), natural killer (NK) cells,

dendritic cells.

T-cells, B-cells, antigen-presenting cells.

Table : Innate and adaptive immunity