Acquired Immunity — Revision Notes

⚡ 30-Second Revision

Acquired Immunity: — Specific, memory, diversity, self/non-self discrimination.
Types: — Humoral (B cells, antibodies), Cell-mediated (T cells).
B cells: — Mature in bone marrow, produce antibodies (plasma cells), memory B cells.
T cells: — Mature in thymus.

- **Helper T ($CD4^+$): Coordinate, activate other cells, recognize antigen on MHC II. - Cytotoxic T ($CD8^+$):** Kill infected/cancer cells, recognize antigen on MHC I.

Antibody Classes:

- IgG: Most abundant, crosses placenta, long-term immunity. - IgA: Secretions (mucus, breast milk), mucosal immunity. - IgM: First response, pentamer. - IgE: Allergy, anti-parasitic. - IgD: B cell receptor.

Active Immunity: — Body produces own antibodies (natural: infection; artificial: vaccination).
Passive Immunity: — Receives pre-formed antibodies (natural: maternal; artificial: antitoxin).
MHC I: — On all nucleated cells, presents endogenous antigens to $CD8^+$ T cells.
MHC II: — On APCs, presents exogenous antigens to $CD4^+$ T cells.

2-Minute Revision

Acquired immunity is your body's 'learned' defense system, characterized by its ability to specifically target pathogens, remember past encounters, and mount diverse responses. It's mediated by lymphocytes: B cells and T cells.

B cells are the 'antibody factories,' responsible for humoral immunity, where antibodies circulate in body fluids to neutralize extracellular threats. T cells drive cell-mediated immunity; cytotoxic T cells directly kill infected cells, while helper T cells act as 'commanders,' orchestrating the entire immune response.

This system develops immunological memory, meaning a second exposure to the same pathogen triggers a much faster and stronger defense, which is the basis of vaccination. Immunity can be active, where your body makes its own antibodies (e.

g., after infection or vaccination), or passive, where you receive pre-made antibodies (e.g., from mother to baby, or via antivenom). Key antibody classes like IgG (placental transfer) and IgA (secretions) have distinct roles.

Understanding these concepts is vital for NEET.

5-Minute Revision

Acquired immunity, also known as adaptive or specific immunity, is a sophisticated defense mechanism that develops over an individual's lifetime. Its defining characteristics are specificity (targets precise antigens), diversity (recognizes millions of different antigens), memory (remembers past encounters for a faster, stronger secondary response), and self/non-self discrimination. This immunity is primarily mediated by lymphocytes.

B lymphocytes (B cells) are central to humoral immunity. Upon activation, often with the help of helper T cells, they differentiate into plasma cells that secrete antibodies (immunoglobulins).

Antibodies circulate in blood and lymph, neutralizing toxins, blocking pathogen entry, and marking pathogens for destruction. The five main classes are: IgG (most abundant, crosses placenta, long-term), IgA (in secretions like breast milk, saliva, tears), IgM (first antibody in primary response, pentamer), IgE (allergies, anti-parasitic), and IgD (B cell receptor).

T lymphocytes (T cells) are responsible for cell-mediated immunity. They mature in the thymus and recognize antigens presented on MHC (Major Histocompatibility Complex) molecules. **Helper T cells ($CD4^+$) recognize antigens on MHC Class II** (found on APCs like macrophages, dendritic cells, B cells) and orchestrate the immune response by releasing cytokines.

**Cytotoxic T lymphocytes (CTLs or $CD8^+$) recognize antigens on MHC Class I** (found on almost all nucleated cells) and directly kill infected or cancerous cells.

Immunological memory is crucial; after a primary exposure, memory B and T cells persist, ensuring a rapid and potent secondary immune response upon re-exposure. This principle underpins vaccination, which is a form of artificial active immunity.

Immunity can be classified as:

Active Immunity: — Body produces its own antibodies and memory cells.

* Natural Active: After natural infection (e.g., chickenpox). * Artificial Active: After vaccination (e.g., polio vaccine).

Passive Immunity: — Receives pre-formed antibodies; no memory generated.

* Natural Passive: Maternal antibodies (IgG across placenta, IgA in breast milk). * Artificial Passive: Administration of antitoxins or antivenom (e.g., tetanus antitoxin).

Mini-Example: A child gets vaccinated against measles. This is artificial active immunity because the vaccine introduces attenuated measles antigens, stimulating the child's immune system to produce its own antibodies and memory cells, providing long-term protection. If the child is later exposed to the measles virus, the memory cells will quickly launch a strong secondary response, preventing illness.

Prelims Revision Notes

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Acquired Immunity Characteristics: — Specificity, Memory, Diversity, Self/Non-self discrimination. It is a 'learned' and delayed response.
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Types of Acquired Immunity:

* Humoral Immunity: Mediated by B cells and antibodies. Targets extracellular pathogens. * Cell-Mediated Immunity (CMI): Mediated by T cells. Targets intracellular pathogens (infected cells), cancer cells.

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Key Cells:

* B Lymphocytes: Mature in bone marrow. Differentiate into plasma cells (antibody producers) and memory B cells. Have B cell receptors (BCRs). * T Lymphocytes: Mature in thymus. Have T cell receptors (TCRs).

* **Helper T cells ($CD4^+$):** Recognize antigens on MHC Class II. Secrete cytokines to activate B cells, CTLs, macrophages. 'Commanders'. * **Cytotoxic T lymphocytes (CTLs or $CD8^+$):** Recognize antigens on MHC Class I.

Directly kill infected/cancer cells. 'Killers'. * Memory T cells: Provide long-term memory. * Antigen-Presenting Cells (APCs): Dendritic cells, macrophages, B cells. Process and present antigens to T cells using MHC molecules.

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MHC Molecules:

* MHC Class I: On all nucleated cells. Presents endogenous antigens to $CD8^+$ T cells. * MHC Class II: On APCs. Presents exogenous antigens to $CD4^+$ T cells.

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Antibody (Immunoglobulin) Classes:

* IgG: Most abundant (75-80%), crosses placenta (natural passive immunity to fetus), long-term immunity. * IgA: In secretions (saliva, tears, mucus, breast milk - natural passive immunity to infant), mucosal immunity. * IgM: First antibody produced in primary response, pentameric structure. * IgE: Involved in allergic reactions and defense against parasites. * IgD: Primarily acts as a B cell receptor.

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Active vs. Passive Immunity:

* Active Immunity: Body's own immune system produces antibodies and memory cells. Long-lasting. * Natural Active: Infection (e.g., measles). * Artificial Active: Vaccination (e.g., polio vaccine). * Passive Immunity: Receives pre-formed antibodies. Immediate but temporary (no memory). * Natural Passive: Maternal antibodies (placenta, breast milk). * Artificial Passive: Antitoxins (e.g., tetanus antitoxin), antivenom.

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Immunological Memory: — Basis of vaccination. Secondary response is faster, stronger, and more prolonged than primary response due to memory cells.

Vyyuha Quick Recall

All Cats Do Sleep Mostly (for Acquired Immunity characteristics):

Adaptive (Acquired)
Constant Diversity
Specificity
Memory