⚕️ Educational content only. This article provides a scientific overview of the immune system for biomedical science students. It is not medical advice.
The immune system is the body’s defence against infection, foreign substances, and malignant cells. It comprises a complex network of cells, proteins, and organs that work together to recognise and eliminate pathogens while tolerating the body’s own tissues. Understanding immunology is fundamental to biomedical science, underpinning disciplines from haematology to microbiology to clinical biochemistry.
Key Takeaways
- The innate immune system provides rapid, non-specific defence; the adaptive immune system provides slower, antigen-specific, memory-forming responses.
- Key innate cells: neutrophils, macrophages, dendritic cells, NK cells, mast cells. Key adaptive cells: T lymphocytes and B lymphocytes.
- Antibodies (immunoglobulins) are produced by plasma cells (differentiated B cells) and mediate humoral immunity.
- Laboratory tests assess immune function via FBC and differential, immunoglobulin levels, lymphocyte subset analysis, and complement testing.
Innate Immunity: The First Line of Defence
The innate immune system provides immediate, non-specific protection. Physical barriers (skin, mucous membranes, cilia) prevent pathogen entry. When breached, pattern recognition receptors (PRRs) — including Toll-like receptors (TLRs) and NOD-like receptors — recognise conserved microbial structures called pathogen-associated molecular patterns (PAMPs). This triggers inflammation, phagocytosis, and the release of cytokines and chemokines. Key innate cells include: neutrophils (first responders, phagocytic, short-lived); macrophages (phagocytic, antigen-presenting, long-lived tissue residents); dendritic cells (bridge between innate and adaptive immunity via antigen presentation); natural killer (NK) cells (kill virally infected and tumour cells without prior sensitisation); and mast cells and basophils (mediators of allergy and parasitic defence via IgE-mediated degranulation).
Adaptive Immunity: Specificity and Memory
The adaptive immune system activates 4–7 days after initial antigen exposure and generates immunological memory. It involves two major branches: humoral immunity (B cells → plasma cells → antibody production) and cell-mediated immunity (T cells → cytotoxic T cells, helper T cells). Antigen-presenting cells (APCs) display antigen fragments on MHC molecules: MHC class I (on all nucleated cells) presents intracellular peptides to CD8+ cytotoxic T cells; MHC class II (on APCs) presents extracellular antigens to CD4+ helper T cells. CD4+ T helper cells coordinate the immune response through cytokine secretion, activating B cells (Th2), macrophages (Th1), and supporting CD8+ T cell responses.
Immunoglobulins (Antibodies)
Antibodies are glycoproteins produced by plasma cells. They have a Y-shaped structure with two heavy chains and two light chains, with variable regions forming the antigen-binding site (Fab) and a constant region (Fc) that mediates effector functions. The five main isotypes are IgG (most abundant in serum; crosses placenta), IgA (found in secretions; mucosal defence), IgM (first produced in primary response; pentameric), IgE (mediates allergy and anti-parasitic responses), and IgD (B cell receptor; low serum concentration). Serum immunoglobulin levels (IgG, IgA, IgM) are measured clinically to detect immunodeficiency or overproduction (as in myeloma).
Complement System
Complement is a cascade of serum proteins that amplify immune responses. Three pathways (classical, lectin, and alternative) converge at C3 cleavage, producing C3b (opsonisation), C3a and C5a (anaphylatoxins), and the membrane attack complex (MAC, C5b-9) which lyses bacteria. Complement deficiencies increase susceptibility to encapsulated organisms (C3 deficiency) or Neisseria species (terminal complement deficiency). CH50 and AP50 assays assess classical and alternative pathway function respectively; individual component levels (C3, C4) are measured in clinical immunology.
References
- Janeway CA Jr, et al. Immunobiology: The Immune System in Health and Disease. 9th ed. Garland Science; 2022.
- NIH National Institute of Allergy and Infectious Diseases. Overview of the Immune System. niaid.nih.gov
- Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2022.