Immunity

The study of the immune system and immune responses is called immunology.

The overall ability of the host to fight the disease-causing organisms conferred by the immune system is called immunity.

• Immunity is the overall ability of the host to fight disease-causing organisms through the immune system.
• It is also described as the body’s defence against diseases and against harmful foreign substances.
• The body is exposed daily to many infectious agents, but only a few exposures result in disease because the body can defend itself against most foreign agents.
• The concept of immunity is associated with Edward Jenner. He developed the cowpox vaccine for protection against smallpox.

• Harmful substances may enter the body through the skin.
• They may also enter through the mucous membranes of the eyes, nose, urinary tract, or genital tract.
• Entry may also occur through food or water intake and through the air inhaled.
• The body first tries to prevent entry of harmful agents, and if they enter, it fights them so that they cause no harm.

• The defence system works at two levels.
• The first is the local defence system, which acts as a barrier at possible entry points of germs.
• It includes protective mechanical barriers, expulsion of harmful agents, germ-killing secretions, and germ-fighting white blood cells.
• The second is the immune system, which acts after germs enter the body tissues.

• The immune system distinguishes self from non-self.
• It differentiates between the body’s own cells and invading foreign molecules or cells.
• Any foreign substance capable of stimulating an immune response is called an antigen.
• The protective chemicals produced in response to antigens are called antibodies.

• A stimulated lymphocyte produces many types of cells.
• Most become sensitised lymphocytes or effector cells that fight the specific antigen and die after some time.
• Some cells become memory cells.
• Memory cells may remain in the lymph node without activity for many years.
• If the same antigen invades again, memory cells become active and divide repeatedly to produce large numbers of lymphocytes of that type.
• This produces a faster, stronger response during the second invasion.
• This explains why some childhood diseases are not contracted a second time.

• Immunology studies the immune system, while immunity is the host's ability to fight disease-causing organisms.
• Body defence works at two levels: external local barriers that prevent entry, and the internal immune system that fights invading germs.
• The immune system can accurately distinguish between the body's own cells (self) and foreign invaders (non-self).
• Foreign substances that trigger an immune response are antigens, and the protective chemicals produced against them are antibodies.
• Stimulated lymphocytes divide into active effector cells (to fight the immediate infection) and dormant memory cells.
• Memory cells stay in the lymph nodes to mount a much faster and stronger immune response upon subsequent encounters with the same antigen.