Introduction to Immunology

Immune System Basics

• Immunology is the study of physiological mechanisms used to defend the body against invasion by other organisms.
• The immune system is crucial to human survival, and its absence can lead to fatal infections.
• An immunogen is a molecule that stimulates the immune system to produce a response, which is then recognized by immune effector cells or soluble antibodies as an antigen.
• An epitope is the part of the antigen that reacts with immune effector cells or soluble antibodies.

Pathogens

• A pathogen is any organism with the potential to cause a disease.
• There are four main types of pathogens: bacteria, viruses, fungi, and parasites (worms).

Immune System Components

• The immune system consists of cells and tissues that work together to defend the body.
• The immune system in the blood includes red blood cells, plasma, white blood cells (leukocytes), and platelets.
• Haematopoiesis is the process of blood cell formation.
• Myeloid cells can be classified into three categories: granulocytes (neutrophils, eosinophils, basophils), antigen-presenting cells (monocytes, macrophages, dendritic cells), and mast cells.
• Lymphoid cells can be classified into two categories: small lymphocytes (B cells, T cells) and large lymphocytes (NK cells).

Lymphoid Organs

• Primary lymphoid organs include bone marrow and the thymus, where lymphocytes are produced and matured.
• Secondary lymphoid organs include the spleen, adenoids, tonsils, appendix, and lymph nodes, where mature lymphocytes are activated to respond to invading pathogens.

Primary Lymphoid Tissues

• Bone marrow is a soft, spongy tissue that fills the internal cavity of bones, where B cells originate and mature, and T cells originate but leave at an immature stage.
• The thymus is located above the heart, where immature T cells migrate to complete their maturation, progressing from the cortex to the medulla.

Secondary Lymphoid Tissues

• Lymph nodes are sites where mature lymphocytes are activated to respond to invading pathogens, and where they can meet and interact with pathogens.
• Lymph nodes are connected by a network of lymphatic vessels, which collect plasma fluid that leaks out from blood vessels and return it to the blood vessels via the thoracic duct.
• Circulating lymphocytes can move from blood capillaries to lymph nodes, where they can encounter pathogens.

Innate Immunity

• Innate immunity provides immediate, non-specific defense against infection, with a rapid response (hours) and no memory.
• Cells of innate immunity include macrophages, NK cells, and dendritic cells.
• Phases of the immune response include immediate innate immune response, induced innate immune response, and adaptive immunity.

Immediate Innate Immune Response

• Barriers (physical and chemical) prevent entry of pathogens.
• Antimicrobial peptides are produced in response to infection.
• The complement system is a group of nearly 30 serum and membrane proteins that act in an orderly sequence to provide defense against infection.

Induced Innate Immune Response

• Phagocytosis is the process of engulfment and removal of pathogens by cells such as neutrophils and macrophages.
• Cytokines production and inflammation occur in response to infection.
• Toll-like receptors are activated in response to pathogens.
• NK cells are involved in the induced innate immune response.

Phagocytosis

• Phagocytosis is the process of removal of offending agents through recognition and attachment, engulfment, and killing and degradation.
• Opsonin receptors enhance the efficiency of phagocytosis.
• Lysosomal enzymes are involved in the killing and degradation of pathogens.

Cytokines

• Cytokines are low-molecular-weight proteins that stimulate or inhibit the activity, proliferation, or differentiation of other cells.
• There are around 20 different cytokines, including interferons, lymphokines, interleukins, and chemokines.
• Cytokines are produced by cells such as macrophages and T lymphocytes in response to infection or specific recognition of foreign antigens.