The ability of the organisms to resist almost all types of toxins that damage the tissues and organs is called Immunity. The study of resistance towards disease is called Immunology. Protection against infectious disease is immune response which is concerned with the reaction of the body against foreign antigen.


Immunity against infectious diseases is of two important types viz.

1. Innate Immunity and 2. Acquired Immunity.

1. Innate immunity

Innate or native immunity is the resistance to infections which an individual possesses by virtue of his genetic and constitutional make up. It is not affected by prior contact with microorganisms or immunisation. The various organs in human body like epithelial surface, blood and tissues are naturally adapted for exhibiting immunity. Natural defence in the body is also mediated by

i. Microphages - Polymorpho - nuclear leucocytes and

ii. Macrophages - Histiocytes, reticulo endothelial cells and monocytes.

2. Acquired Immunity

The immunity that is acquired by an individual is known as Acquired immunity. Acquired immunity is of two types, namely

a. Active immunity b. passive immunity 


a. Active immunity:

If the resistance is developed as a result of an antigenic stimulus, it is called Active Immunity. For example a person who has recovered from an attack of measles develops Natural immunity.

b. Passive immunity:

If the resistance is transmitted to a recipient in a readymade form, it is known as passive immunity.

The acquired immunity is also of two types namely

i. Humoral immunity or B cell immunity ii. Cell mediated immunity or T cell immunity.

i. Humoral Immunity:

The term humor refers to plasma and lymph. The humoral immune system defends mostly against bacteria and viruses that enter the body fluids. The humoral immunity is antibody mediated.

ii.Cell mediated Immunity:

The cell mediated immune system is carried out by highly specialsed cells, which work against pathogens, including fungi and protista that have invaded host cells. The system also reacts against transplants. It is thought to be important in protecting the body from its own cells if they become cancerous.