The pathogenic organisms, which survive and cross the non-specific defense barriers, are engaged by the ‘second line of defense’, called the immune system.

The elements of this system pre-exist the invasion of the foreign agent, but the reaction itself is generated in response to a foreign agent called an antigen. It has the following properties: (1) relatively slow initiation, (2) ability to distinguish between self and non-self (3) high specificity and (4) memory. Due to the high specificity of this mechanism, it is also known as the specific host defense.

This defense mechanism is not present right from birth, but rather is learnt or acquired from experiences (encounters with pathogenic organisms or antigens). Acquired immunity can be active or passive.

Active Acquired Immunity:

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The immune reaction generated in response to the administered purified antigens pathogenic organisms or pathogenic organisms themselves is known as active act ‘ immunity. The generation of immunity by the administration of antigens is known active acquired immunity /immunization.

A long lasting immunity is generated immunization method. For example, agents causing diphtheria, whooping cough, r pox, mumps and measles induce long term immunity to the respective diseases, antigens administered must be inactivated such that they induce antibody synthesis o and not the disease.

This type of immune response is executed by two methods: hub and cellular. The former is conferred by soluble antibodies present in the serum other body fluids. Cellular immunity is conferred by a type of lymphocyte (T-lymph and other accessory cells, which process and display antigens for their true identify consequently leading to their destruction. Humoral and cellular responses coope

Passive Acquired Immunity:

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Immunity conferred by blood cells or serum of an actively immunized individual is know as passive acquired immunity. The developing human fetus is maternal antibodies, which cross the placental barrier into the fetal circulation. Anti test serum (ATS) contains antibodies raised in a horse confers passive immunity against tee bacteria or their antigens. The new born baby is immunized by the antibodies present the colostrums (first milk of the lactating mother).