1. Innate Immunity

•    Key components:

o    Neutrophils: first responders; phagocytosis and microbial killing.

o    Macrophages: phagocytosis, antigen presentation, cytokine release.

o    Natural Killer (NK) cells: recognize and kill virus-infected and tumour cells; 

                                                                                        do not require antigen presentation.

o    Complement system:

    Classical pathway (antibody-mediated), alternative, lectin pathways.

    Functions: opsonisation (C3b), chemotaxis (C5a), cell lysis (MAC – C5b-9).

•    Features:

o    Rapid, non-specific response.

o    No immunological memory.

2. Adaptive Immunity

•    B cells (Humoral immunity)

o    Produce antibodies.

o    Differentiate into plasma cells and memory B cells.

•    T cells (Cell-mediated immunity)

o    CD4+ (helper T cells): recognize MHC II; activate B cells, macrophages.

o    CD8+ (cytotoxic T cells): recognize MHC I; kill infected cells directly.

•    MHC (Major Histocompatibility Complex)

o    Class I: all nucleated cells; present to CD8+ T cells.

o    Class II: antigen-presenting cells (APCs); present to CD4+ T cells.

3. Immunoglobulin Classes

Class        Key Features

IgG            Most abundant, crosses placenta, opsonisation, complement activation

IgA            Mucosal surfaces (gut, saliva, tears, breast milk)

IgM            First produced (primary response), pentameric, good at complement activation

IgE            Allergic reactions, parasite defence

IgD            Mainly on B cell surface (B cell receptor role)

4. Hypersensitivity Reactions

Type           Mechanism                               Examples

I                   IgE-mediated (immediate)         Anaphylaxis, urticaria, asthma

II                  Antibody-mediated cytotoxic     Goodpasture’s, autoimmune haemolytic anaemia

III                 Immune complex-mediated       SLE, post-streptococcal GN

IV                 T cell-mediated (delayed)         Contact dermatitis, TB Mantoux test

5. Self-Tolerance and Autoimmunity

•    Central tolerance: deletion of autoreactive T and B cells in thymus and bone marrow.

•    Peripheral tolerance: anergy, suppression (regulatory T cells), ignorance.

•    Autoimmunity: breakdown of tolerance → self-reactive immune responses.

o    Genetic predisposition (e.g., HLA associations).

o    Environmental triggers (e.g., infections, smoking).

6. Basic Transplant Immunology

•    Rejection types:

o    Hyperacute: minutes to hours; preformed anti-donor antibodies 

                                            (e.g., ABO incompatibility); complement-mediated.

o    Acute: days to weeks; T cell-mediated response against donor MHC.

o    Chronic: months to years; chronic low-grade immune injury, vascular changes, fibrosis.

•    Graft-versus-host disease (GVHD):

o    Donor T cells attack recipient tissues.

o    Seen in bone marrow transplantation.

Extra Revision Pearls

•    HLA-B27: associated with seronegative spondyloarthropathies.

•    HLA-DR3/DR4: linked to type 1 diabetes, autoimmune thyroid disease.

•    C1 esterase inhibitor deficiency → hereditary angioedema (non-itchy swelling, no urticaria).

•    Complement deficiencies: predispose to Neisseria infections.