Types of Immunity

  1.         The immune response is mediated by two arms:

a.         Cell-mediated immunity: T lymphocytes (helper T cells and cytotoxic T cells)

b.            Humoral (antibody)-mediated immunity: (plasma cells)

2.            Common characteristics:

a.            Remarkable diversity (i.e. they can respond to millions of different antigens)

b.         Long memory (i.e. they can respond many years after the initial exposure because memory T cells and memory B cells are produced.

c.            Exquisite specificity (i.e. their actions are specifically directed against the antigen that initiated the response).

3.         Major functions of T and B cells:

4.         The combined effects of these cells and certain proteins (e.g. antibodies and complement) produce an inflammatory response.

5.            Natural (Innate) Immunity

a.            Resistance not acquired through contract with an antigen.

b.            Nonspecific and includes host defenses such as barriers to infectious agents (skin and mucous membranes), certain cells, (natural killer cells), certain proteins (complement cascade and interferons) and other processes such as phagocytosis and inflammation.

c.         Acute-phase response: an increase in the level of various plasma proteins such as C-reactive protein and mannose-binding protein which bind to bacteria surface and activate complement killing the bacteria.

6.            Acquired Immunity

a.         Occurs after exposure to an agent.

b.         Specific in action

c.            Mediated by antibody and by T lymphocytes

d.         Can be active or passive

7.            Passive Immunity

a.            Temporary protection against an organism and is acquired by receiving serum containing preformed antibodies from another person or animal.

b.         Occurs normally in the form of immunoglobulins passed through the placenta (IgG) or breast milk (IgA) from mother to child – important in the early days of life, when the child has reduced capacity to mount an active response.

c.            Advantage: protective abilities are present immediately.

d.            Disadvantages:

i.          short life-span of these antibodies

ii.            possible hypersensitivity reactions if globulins from another species are used.

8.         Active Immunity

a.            Protection based on exposure to the organism in the form of overt disease, subclinical infection or a vaccine.

b.         Has a slower onset but longer duration than passive immunity.

c.            Advantage: there is a rapid response of large amounts of antibody to an antigen that the immune system has previously encountered.

d.         Active immunity is mediated by:

i.            Immunoglobulins: protect against organisms by neutralization of toxins, lysis of bacteria in presence of complement, opsonization of bacteria to facilitate phagocytosis and interference with adherence of bacteria and viruses to cell surface.

ii.          T cells: mediate a variety of reactions including cytotoxic destruction of virus-infected cells and bacteria, activation of macrophages, and delayed hypersensitivity.