Page 167 - 84 human physiolofy part-1

The Immune System

lymphocytes are found in lymphoid tissues, where they are more likely to encounter invaders. By one
estimate,the adult body contains a trillion lymphocytes at any one time.

Dendritic Cells

Dendritic cells activate lymphocytes. They are antigen-presenting cells characterized by long, thin
processes that resemble neuronal dendrites. Dendritic cells are found in the skin called Langerhans
cells and also in various organs. When dendritic cells recognize and capture antigens, they migrate to
secondary lymphoid tissues, where they present the antigens to lymphocytes.

Defenses Against Infection

Innate Defense – first line of defense

Physical and chemical barriers are the body's first line of defense.

Physical or Mechanical barriers

• Skin

One of the bodies first line of defenses against bacteria and other harmful organisms is the skin.
Our skin is a barrier which stops infection from entering the body. Millions of microorganisms live
harmlessly on the skin and in the air around us. Sebaceous glands in the skin produce sweat and sebum,
which, combined help to protect the skin. Both substances contain antiseptic properties, of lysozome is
the main property. Although our skin is a good defense, it isn’t perfect. The skin itself can also become
infected by bacteria, viruses, fungi or tiny parasites. Some examples of these are: boils, impetigo;
ringworm, athletes foot; cold sore, wart, verucca; and scabies.

• Mucus membranes

Another very important first line of defense is our mucus membranes. The mucous membranes (or
mucosae; singular: mucosa) line various body cavities that are exposed to the external environment and
internal organs. It is at several places continuous with skin: at the nostrils, the lips, the ears, the genital
area, and the anus. The nose and mouth serve as passageways for air going to and from the lungs. As
we inhale and exhale, the mucus membranes that line these passageways warm and humidify the air. It
has been said that there is more bacteria contained in a human mouth than the the sum of all the people
that have ever lived on the earth. Mucus membranes serve different functions, however, their more
important job is to secrete mucus that traps bacteria and other foreign debris that irritates the lining of
the respiratory tract. This mucus is produced and stored in the sinuses by other mucus membranes. We
get congested when there is excessive fluid in the sinus cavities. This is a result of an increase in mucus
secretions, as well as an increase in the amount of fluids that passes across the blood vessels of the
mucus membranes that line the nose and sinus. There are also many chemicals, such as pesticides and
anthrax that are absorbed through the skin. All mucous membranes are ciliated. Cilia are thin, tail-like
projections extending approximately 5–10 micrometers outwards from the cell body. Their main
function is to move things across their surface.

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