Types of Immunity > Acquired Immunity

Acquired immunity is the specific defence developed after exposure to a pathogen, characterised by memory and a stronger response on re-exposure.

The immunity developed when the body produces its own antibodies in response to exposure to antigens through infection or vaccination is called as active immunity.

The immunity obtained by the direct transfer of ready-made antibodies from another individual is called as passive immunity.

Acquired immunity is the resistance that the body develops during life after exposure to specific foreign substances called antigens. It is also called adaptive immunity, or specific immunity, because the immune response is directed against a particular pathogen and becomes stronger with repeated exposure.

• It explains how the body remembers infections and responds faster the next time.
• It forms the biological basis of vaccination, immunisation, and antiserum therapy.

1. Specificity: The immune system recognises a particular antigen and produces a response specifically against it.
2. Diversity: The body can respond to a very large number of different antigens because it can generate many types of antigen receptors.
3. Self and Non-self Recognition: The immune system usually distinguishes the body’s own cells from foreign substances. Failure of this recognition may lead to autoimmune problems.
4. Memory: After the first exposure, the immune system forms memory cells; therefore, the second response is faster and stronger.

Immune response:

• The first exposure to a pathogen produces a primary response.
• The primary response is of low intensity.
• A later exposure to the same pathogen produces a secondary or anamnestic response.
• The secondary response is highly intensified because the body retains memory of the first encounter.

Lymphocytes involved:

• The primary and secondary immune responses are carried out by two special types of lymphocytes present in the blood.
• These are B-lymphocytes and T-lymphocytes.
• B-lymphocytes produce proteins in response to pathogens.
• These proteins are called antibodies.
• T-cells do not secrete antibodies themselves, but they help B-cells to produce them.

Antibodies:

• Each antibody molecule has four peptide chains.
• Two of these are small light chains.
• Two are longer heavy chains.
• Therefore, an antibody is represented as H₂L₂.
• Different types of antibodies produced in the body include IgA, IgM, IgE, and IgG.

Structure of an antibody molecule

Types of immune response:

• When antibodies are found in the blood, the response is called the humoral immune response.
• This is one type of acquired immune response.
• The second type is cell-mediated immune response or cell-mediated immunity (CMI).
• Cell-mediated immunity is mediated by T-lymphocytes.

Graft rejection:

• The body is able to differentiate between self and non-self.
• Cell-mediated immune response is responsible for graft rejection.
• Before any graft or transplant, tissue matching and blood group matching are essential.
• Even after this, the patient may have to take immunosuppressants throughout life.

Acquired immunity is broadly divided into active immunity and passive immunity.

1. Active Immunity:

Active immunity develops when a person’s own immune system produces antibodies and memory cells after exposure to an antigen. It develops slowly, but it is usually long-lasting because memory cells are formed.

a. Natural Active Immunity:

Natural active immunity develops after natural infection by a pathogen.

Examples:

• Immunity after recovering from measles.
• Immunity after chickenpox infection.

b. Artificial Active Immunity:

Artificial active immunity develops after vaccination.

Examples:

• Polio vaccine.
• BCG vaccine.
• Tetanus toxoid booster.

2. Passive Immunity:

Passive immunity is produced when ready-made antibodies are given to a person instead of being produced by the person’s own immune system. It provides immediate protection but is short-lived because memory cells are not formed.

a. Natural Passive Immunity:

Natural passive immunity occurs when antibodies pass from mother to child.

Examples:

• Antibodies pass through the placenta to the foetus.
• Antibodies present in colostrum given to the newborn after birth.

b. Artificial Passive Immunity:

Artificial passive immunity occurs when antibodies from another source are injected into the body.

Examples:

• Anti-rabies serum.
• Tetanus antitoxin.
• Snakebite antivenom.

• Acquired immunity is a pathogen-specific defence system developed over a lifetime that remembers infections and forms the basis of vaccination.
• Its main features include specificity against distinct pathogens, diversity in recognising various antigens, self/non-self recognition, and immunological memory.
• Initial pathogen exposure causes a low-intensity primary response, while later exposures trigger a much faster and stronger secondary response due to memory cells.
• B-lymphocytes produce protective proteins called antibodies (structured as H₂L₂), and T-lymphocytes help B-cells while mediating cellular responses.
• The immune response is classified as either humoral (antibody-mediated in the blood) or cell-mediated (T-cell mediated, responsible for graft rejection).
• Immunity can be active (the body produces its own antibodies, providing long-term protection) or passive (the body receives ready-made antibodies for immediate, short-term protection).