Biology Today — May 2017

2. Cellular immunity or cell-mediated immune system
   or T-cell immunity (CMIS)

• T lymphocytes or T-cells are produced in the bone marrow
and mature in thymus.

Types of T-cells

• Cytotoxic or killer T-cells : Directly attack and destroy
antigens by secreting a protein perforin.

• Suppressor T-cells : Keep a check on entire immune
system from attacking the body’s own cells.

• Helper T-cells : Stimulate the B-cells to produce antibodies.
They form protein mediators called lymphokines.

• Memory cells : Sensitised T-cells retain memory of
antigen specificity for future, sometimes lifelong. They proliferate and differentiate into all types of T-cells.

Immune Response

• The specific reactivity induced in a host by an antigenic stimulus is known as the immune response.

5 days 15 days 10 days 15 days

Total antibody

IgG

IgM

Decline

Plateau

Antibody

IgG

IgM

Lag period

Antibody concentration

in serum

Secondary immune response/

Booster response

Reaction of the body’s immune system

to any subsequent infection of the same

microbe antigen. It is more rapid, intense

response that lasts for a longer period.

Primary immune response

Reaction of body’s immune

system to the first attack of

microbe (antigen). It takes

longer time to establish

immunity and declines rapidly.

Fig.: Primary vs Secondary immune responses

Immune System

• The human immune system comprises of lymphoid organs, tissue cells and soluble molecules such as antibodies.

• Lymphoid organs are of two types : Primary lymphoid organs where B-and T-lymphocytes mature and acquire antigen-
specific receptors such as bone marrow and thymus and secondary lymphoid organs where mature lymphocytes undergo
proliferation and differentiation such as spleen, lymph nodes, tonsils, etc.

• Antigen Presenting Cells (APCs) - APCs are the cells that engulf antigens and present fragments to T-cells. E.g., marophages,
dendritic cells and B cells.

Antibodies

• Antibodies are immunoglobulins (Igs) which are produced in response to antigenic stimulation.

• All antibodies are immunoglobulins but all immunoglobulins are not antibodies.

• Antibodies are produced by B-lymphocytes and plasma cells. The mature plasma cells produce antibodies which direct antibody
mediated immunity.

Flow chart :Functioning of Cell-mediated Immunity

Other T-cells “remember” the

antigeni.e., memory cells.