Immune System

🎯 Key Points

• B cells mature in Bone marrow (humoral immunity, make antibodies); T cells mature in Thymus (cell-mediated immunity)
• T helper (CD4+) cells coordinate the immune response and are the target of HIV; Cytotoxic T cells (CD8+) directly kill infected cells
• 5 antibody classes: IgG (most abundant, crosses placenta), IgM (FIRST response), IgA (secretions), IgE (allergy), IgD (B-cell activation)
• Primary response = slow, weak (first exposure); Secondary response = fast, strong (memory cells, basis of vaccination)
• Active immunity = body makes its OWN antibodies (long-lasting); Passive immunity = ready-made antibodies given (immediate but short-lived)

Lines of Defence

• 1st line: physical and chemical barriers; skin, mucus, cilia, saliva, tears (lysozyme), stomach acid
• 2nd line: non-specific (innate) internal defences; phagocytes, NK cells, fever, inflammation, interferons, complement system
• 3rd line: specific (adaptive) immunity; lymphocytes (B cells and T cells)

Innate Immunity (Non-Specific)

• Neutrophils and macrophages: phagocytosis (engulf pathogens)
• Natural killer (NK) cells: destroy infected and cancer cells
• Inflammation: redness, swelling, heat, pain; caused by histamine, prostaglandins
• Interferons: antiviral proteins; signal neighbouring cells to prepare defences
• Complement system: proteins that lyse bacteria

Adaptive Immunity (Specific)

• B lymphocytes: mature in Bone marrow; produce antibodies; humoral immunity
• T lymphocytes: mature in Thymus; cell-mediated immunity
• T helper cells (CD4+): activate B cells and cytotoxic T cells; targeted by HIV
• Cytotoxic T cells (CD8+): kill infected cells directly
• Memory cells: long-lived; rapid response on re-exposure (basis of vaccination)

Antibodies (Immunoglobulins)

• Y-shaped proteins; 4 polypeptide chains (2 heavy + 2 light)
• 5 classes: IgG (most abundant, crosses placenta), IgM (first response), IgA (secretions - saliva, breast milk), IgE (allergy), IgD (B cell activation)
• Antigen-antibody binding: neutralization, agglutination, opsonization

Vaccines and Immunity

• Active immunity: body makes own antibodies (vaccination, infection)
• Passive immunity: ready-made antibodies given (maternal antibodies, antiserum)
• Vaccines: attenuated (live weakened), killed, subunit, toxoid, mRNA (COVID)

Disorders

• HIV/AIDS: destroys CD4+ T cells; transmitted via blood, sexual contact, breast milk
• Autoimmune diseases: immune system attacks self (rheumatoid arthritis, lupus, type 1 diabetes)
• Allergy: IgE-mediated hypersensitivity; histamine release (hay fever, asthma)
• SCID: severe combined immunodeficiency; no functional B or T cells

Innate vs Acquired Immunity

• Innate immunity: non-specific, present from birth, provides immediate but generalised defence; includes physical barriers, physiological barriers (acid in stomach, lysozyme in saliva/tears), cellular barriers (phagocytes, NK cells), and cytokine barriers (interferons)
• Acquired (adaptive) immunity: specific to a particular pathogen, develops after exposure to antigens, characterised by specificity and memory; responsible for the stronger and faster secondary response upon re-exposure to the same pathogen
• Primary response: first encounter with an antigen, relatively weak and slow; secondary (anamnestic) response: subsequent encounter with the same antigen, much faster and of higher intensity due to memory cells

Types of Immunity (Active/Passive, Natural/Artificial)

• Natural active immunity: acquired by actually getting infected with a pathogen and producing one's own antibodies
• Artificial active immunity: acquired through vaccination, where antigens are introduced to stimulate the body's own antibody production
• Natural passive immunity: antibodies passed from mother to fetus through the placenta, or to an infant through colostrum/breast milk (provides early immunity to a newborn)
• Artificial passive immunity: ready-made antibodies (antiserum) directly injected into the body, e.g. anti-snake venom serum, used for immediate but short-lived protection

Structure of an Antibody

• Each antibody (immunoglobulin) monomer is made of four polypeptide chains: two identical heavy chains and two identical light chains, joined by disulfide bonds into a Y-shape
• Each chain has a variable region (forms the antigen-binding site, differs between antibodies) and a constant region
• The two arms of the Y bind antigen (Fab regions); the stem (Fc region) interacts with immune cells and complement proteins

Lymphoid Organs (Primary and Secondary)

• Primary lymphoid organs: the sites where immature lymphocytes differentiate into antigen-sensitive mature lymphocytes — the bone marrow (where all blood cells, including lymphocytes, are produced and where B cells mature) and the thymus (where T cells mature)
• Secondary lymphoid organs: the sites where mature lymphocytes meet antigens, proliferate and become effector cells — the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix
• The spleen is a large bean-shaped organ that filters the blood by trapping blood-borne microorganisms and also acts as a reservoir of lymphocytes and erythrocytes
• MALT (mucosa-associated lymphoid tissue) lines the respiratory, digestive and urogenital tracts and makes up about 50% of the lymphoid tissue in the human body

Humoral vs Cell-Mediated Immunity

• Humoral (antibody-mediated) immunity: carried out by B lymphocytes, which on activation produce antibodies that circulate in the blood and lymph (the body's "humors"); it is effective mainly against free pathogens and toxins in body fluids
• Cell-mediated immunity (CMI): carried out by T lymphocytes, which act directly on the target rather than through antibodies; it is effective against intracellular pathogens (viruses, some bacteria), cancer cells and grafted tissue
• It is the cell-mediated (T-cell) arm that is responsible for graft rejection, which is why the body's ability to distinguish self from non-self is central to transplantation

Organ Transplantation and Graft Rejection

• When a tissue or organ is grafted onto a genetically different individual, the recipient's immune system recognises the graft's surface antigens (mainly the MHC/HLA proteins) as foreign and mounts a graft rejection response, driven chiefly by cell-mediated immunity (cytotoxic T cells)
• To minimise rejection, tissue typing and blood-group matching between donor and recipient are done as closely as possible, and the recipient is placed on immuno-suppressant drugs, usually for life
• An autograft (tissue moved from one part of a person's own body to another, e.g. a skin graft) is never rejected, whereas an allograft (from another human) is prone to rejection

🚀 NEET Advanced Edge

Why IgM dominates the primary response but IgG dominates the secondary response: IgM is produced first (pentameric, very effective at agglutination early on) but is NOT retained long-term; class switching during the immune response shifts antibody production toward IgG (monomeric, longer-lived, crosses placenta) for the more durable secondary/memory response — exam questions often ask which antibody class indicates a RECENT infection (IgM) vs past/long-term immunity (IgG).

Why allergy and autoimmune disease both involve immune "errors" but differently: Allergy = immune system overreacts to a HARMLESS foreign antigen (IgE-mediated, e.g. pollen); Autoimmune disease = immune system mistakenly attacks the body's OWN antigens (self vs non-self recognition failure) — different category of immune malfunction, frequently confused in exam options.

Why HIV specifically cripples adaptive immunity: By destroying CD4+ T helper cells, HIV removes the cell that ACTIVATES both B cells (antibody production) and cytotoxic T cells — a single point of failure that collapses the entire adaptive immune coordination, which is why AIDS patients succumb to opportunistic infections that a healthy immune system would easily handle.