Immune system overview

The immune system identifies and eliminates foreign pathogens (bacteria, viruses, and other microorganisms) by continuously monitoring body fluids. The lymphatic system supports immunity by returning excess fluid to the bloodstream; many of its structures also function as key immune organs.

Major immune organs and tissues

  • Lymph nodes: Filter interstitial fluid and remove pathogens via macrophages.

  • Red bone marrow: Produces lymphocytes; site of B-cell maturation.

  • Spleen: Filters blood and removes pathogens via macrophages.

  • Thymus: Site of T-lymphocyte maturation.

Lymph nodes are bean-shaped filtering stations along lymphatic vessels. Lymph enters, moves through chambers (cortex with B cells; medulla with other immune cells), and is screened by macrophages and other cells so pathogens can be removed and immune responses initiated.

Major immune cell types

  • Macrophages, dendritic cells, neutrophils: Destroy pathogens by phagocytosis (engulfing and digesting them); part of the nonspecific/innate response.

  • Natural killer cells: Non-specifically destroy infected cells.

  • B lymphocytes: Produce antibodies targeting specific antigens in body fluids.

  • T lymphocytes: Target cells infected by specific pathogens.

Together these form both the nonspecific (innate) and specific (adaptive) branches of immunity. A video demonstration shows a macrophage phagocytosing a sheep red blood cell.

Self vs. non-self recognition

Every nucleated cell carries surface markers that act as identification tags. Major histocompatibility complex (MHC) glycoproteins are central to this process:

  • MHC class I: Present on every nucleated cell; the general “self” marker. High genetic variation in MHC alleles (unique except in identical twins) is a major reason organ transplants can be rejected.

  • MHC class II: Found only on antigen-presenting cells (macrophages, dendritic cells, natural killer cells, and B cells). These markers link the innate and adaptive responses by displaying pathogen information to other immune cells.

Antigens and antigen presentation

An antigen is a protein or large carbohydrate unique to a pathogen. When the immune system detects an antigen, it mounts a specific response. Macrophages and dendritic cells ingest pathogens, digest them, and display fragments of their antigens on their surface (antigen presentation). This alerts T cells and enables a targeted adaptive response.