Which type of immunity is characterised by memory ?

Acquired immunity.

The correct answer is Option (3) –Acquired immunity .

The type of immunity that is characterized by memory is Acquired Immunity.

1. Innate Immunity: Innate immunity is the first line of defense against pathogens and is present from birth. It is a nonspecific defense mechanism that provides immediate protection against a wide range of pathogens. Innate immunity does not require prior exposure to a specific pathogen to be activated. It includes physical barriers (like the skin), chemical barriers (such as stomach acid), and cellular components like phagocytes (such as macrophages and neutrophils) and natural killer cells. Innate immunity recognizes general patterns shared by various pathogens rather than specific antigens.

2. Passive Immunity: Passive immunity involves the transfer of pre-formed antibodies or immune components from one individual to another. This transfer can occur naturally or artificially. Natural passive immunity occurs when a baby receives antibodies from the mother through the placenta during pregnancy or through breast milk after birth. Artificial passive immunity can be acquired through the administration of pre-formed antibodies, such as immunoglobulin injections or antivenom. Passive immunity provides immediate but temporary protection, as the transferred antibodies degrade over time and do not stimulate the immune system to produce its own antibodies.

3. Acquired Immunity: Acquired immunity, also known as adaptive or specific immunity, develops over time after exposure to specific pathogens, foreign substances, or through vaccination. It involves the activation of immune cells, particularly B and T lymphocytes, that specifically recognize and respond to particular antigens. Acquired immunity is characterized by immunological memory, where the immune system "remembers" specific pathogens upon re-exposure. This memory allows for a more rapid and effective response upon encountering the same pathogen, providing long-term protection against future infections.

In summary, innate immunity provides immediate, nonspecific defense against a wide range of pathogens; passive immunity involves the transfer of pre-formed antibodies; and acquired immunity develops after exposure to specific pathogens and involves the immune system's memory response for long-term protection.