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Leaving Cert Biology Revision: The Human Defense Systems

Updated: Jun 12

The Human Defense Systems

An organism that causes disease is called a pathogen.

The universal defense mechanism does not target any one disease; rather, it combats all pathogens.

The Unique Defense System:

  • operates against only one particular type of pathogen.

  • is called the immune system, which acts by generating antibodies and by killing infected cells.

General Defense System

The skin, blood clotting, lysozyme (in tears), chemicals in sebum, mucous membranes, cilia, stomach acid, and lactic acid in the vagina are the two main components of the general defense system.

System of Defense in General

White blood cells that cause inflammation, phagocytosis, and defense proteins like complement and interferons.

Specific Defense System

  • Monocytes and lymphocytes are types of white blood cells. Present in lymphatic tissue

  • They both respond to antigens.

T cells

  • Attack the body's antigen-displaying cells.

  • Make anti-bodies.

Antigens and Antibodies

A foreign molecule known as an antigen causes the body to produce more antibodies. It is found on a bacterium, virus, or fungus' coat. A protein that lymphocytes create in response to an antigen is known as an antibody.

Every kind of antibody is extremely specific to a single antigen, just like enzymes. The pathogen invaders are encircled by the antibodies, which then trigger the complement system to explode.

Duration of Immunity

Certain lymphocytes can remain memory cells for many years, enabling long-term immunity.

Induced Immunity

Induced immunity is the ability to resist disease.

(by producing antibodies) caused by specific antigens.

There are two types of Immunity Active and Passive;

Active immunity means that a person makes his/her antibodies.

This provides long-term immunity in two ways:

  • when pathogens naturally enter the body cold or flu.

  • when antigens are artificially placed in the body due to vaccination.

Passive immunity occurs when foreign antibodies are introduced into the body. This provides short-term immunity in two ways:

  • a child naturally gets antibodies from the placenta or mother’s milk.

  • getting an injection of foreign antibodies.

Higher Level Types of Lymphocytes

B lymphocytes, or B cells, develop in the bone marrow. The thymus gland is where T-cells, also known as T lymphocytes, mature.


  • recognize an antigen

  • produce antibodies that control mostly bacterial infections (and some viral infections.


All four T cell types can identify distinct antigens:

  • B cells are stimulated by helper T cells to generate antibodies.

  • Cytotoxic killer T cells release perforin, which causes aberrant body cells to rupture. 

  • Immune responses are inhibited by suppressor T cells.

  • Long-lasting memory T cells can later develop immunity to the same antigen.

How B-Cells work

  • B cells are a type of white blood cell that helps fight infections.

  • When germs enter the body, B cells recognize them as invaders.

  • B cells produce proteins called antibodies that target and attach to the germs.

  • Antibodies help other immune cells find and destroy the invaders.

  • B cells remember the germs they've encountered for faster response in future infections.

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