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The Immune System

There are different classes of antibodies, or immunoglobulins (Ig), such as IgA, IgG, IgE, and IgM.
They can attach to the surface of a microbe and make it more easily phagocytized by neutrophils,
monocytes and macrophages. Anything that simplifies phagocytosis is called an opsonin. The process
of antibodies attaching to invaders can be termed 'opsonization.' Some antibodies can bind and
inactivate certain poisons or toxins and are called antitoxins (tetanus immunizations stimulate your
body to produce antibodies against the tetanus toxin rather than against the bacteria that produces the
toxin). Still other antibodies can bind to the surface of microbes and prevent their attachment to the
body's cells (thus preventing viruses from entering host cells). Also, some of them can stimulate nine
proteins found in plasma, called complement.

Memory B cells

At the time of activation some of the clones become memory B cells. These cells are long lived
and have recorded the information about the foreign antigen so antibodies can be made more quickly,
and in greater amount, in case a second exposure should occur. Since the second response is much
stronger than the first and puts more antibodies into circulation, we often receive "booster shots" for
immunizations.

T Cells Attack Infected Cells

Defending the body against intracellular pathogens is the role of T lymphocytes, which carry out
cell-mediated immunity(CMI). Macrophages phagocytize invading microbes and present parts of the
microbe (antigens) to the T cell lymphocytes. The appropriate T cell is turned on or stimulated. The
activated T cell rapidly multiplies into a large homogenous group (clone) of cytotoxic T cells (Tc cells).

• (a) Attack organisms directly, Also kill infected cells

These cytotoxic T cells migrate to the site of infection (or disease) and produce chemicals which
directly kill the invader. Cytotoxic T cells release “perforin” that causes pores to form in the plasma
membrane of the target cell, resulting in lysis.

• (b) T cells develop in the thymus gland from immature precursor cells that migrate there
from the bone marrow.
• (c) Killer and helper T cells
• (d) Memory T Cells

A portion of these activated T cells become memory T cells (Tm). These cells record the
information about the foreign antigen so T cells can respond more quickly, and more strongly, if a
second exposure occurs. A portion of the T cells become T helper cells (TH) or T suppressor cells (Ts).
TH cell stimulate other T cells and B cells by releasing cytokines and other stimulatory chemicals. Ts
cells suppress the immune response. Experience has shown that cell mediated immunity is most useful
to the body by: Protecting against microbes which exist inside of our body's cells (intracellular bacteria
and intracellular viruses). Protecting against fungal infections. Protecting against protozoan parasites.
Protecting against cancer cells.

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