Lymphocytes are specialised white blood cells that drive the adaptive immune response through targeted pathogen recognition and long-term immune memory. Understanding how B cells and T cells function is essential for interpreting immune responses, vaccination effects, immunodeficiency states, and immune-mediated disease in clinical practice.

Fever is a regulated immune response in which the body raises its core temperature through coordinated actions of cytokines and the hypothalamus. Understanding fever physiology is important for interpreting infection and inflammation, recognising when fever is protective versus harmful, and guiding appropriate clinical management.

Phagocytosis is a core process of the innate immune system in which specialised cells recognise, engulf and destroy pathogens and cellular debris. Understanding phagocytosis is important for explaining early immune responses, inflammation, and how innate immunity initiates and shapes adaptive immune defence.

Natural killer (NK) cells are innate lymphocytes that rapidly identify and eliminate virally infected or malignant cells without prior antigen sensitisation. Understanding NK cell function is important for explaining early antiviral defence, tumour surveillance, and how the immune system targets abnormal cells that evade adaptive immunity.

Granulocytes are innate immune cells—neutrophils, eosinophils and basophils—defined by cytoplasmic granules that enable rapid responses to infection, inflammation and immune triggers. Understanding their distinct functions is essential for interpreting infection patterns, allergic reactions, parasitic disease, inflammatory conditions and changes seen in full blood counts.

Pattern recognition receptors (PRRs) are innate immune receptors that detect pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), allowing the immune system to rapidly identify infection or tissue injury. Understanding PRRs is essential because this early danger detection triggers inflammation, directs immune cell recruitment and shapes the strength and nature of the subsequent adaptive immune response.

Dendritic cells are specialised immune cells that detect pathogens, capture antigens and present them to naïve T cells to initiate adaptive immune responses. Understanding dendritic cells is essential because they form the critical link between innate immune detection and the activation, direction and strength of adaptive immunity.

Antigen presentation is the process by which cells display antigen fragments on major histocompatibility complex (MHC) molecules to enable recognition by T lymphocytes. Understanding the MHC (HLA) system is essential because it underpins adaptive immune specificity, immune tolerance, transplant compatibility and the development of autoimmune disease.

Humoral immunity is the arm of the adaptive immune system in which B lymphocytes produce antibodies that specifically recognise and neutralise extracellular pathogens. Understanding B-cell activation and antibody structure is essential for explaining vaccination, immune memory, infection defence and antibody-mediated disease.

Cell-mediated immunity is the arm of adaptive immunity in which T lymphocytes detect and respond to infected, malignant or abnormal cells. Understanding T-cell function is essential for explaining antiviral defence, tumour surveillance, transplant rejection and immune conditions that cannot be controlled by antibodies alone.

Cytokines are small signalling proteins that coordinate communication between immune cells and regulate the intensity, direction and duration of immune responses. Understanding cytokine signalling is essential for explaining inflammation, fever, immune activation, resolution of infection and the development of immune-mediated disease.

Inflammation is a coordinated physiological response that detects tissue injury or infection and mobilises immune and vascular defences to contain damage and initiate repair. Understanding inflammation is essential for interpreting clinical signs, guiding treatment decisions, and recognising when a protective response becomes excessive or contributes to chronic disease.

The complement system is a rapid, enzyme-driven cascade of plasma proteins that enhances inflammation, marks pathogens for destruction, and directly lyses microbes. Understanding this system is essential for explaining early immune defence, susceptibility to infection, and the mechanisms behind inflammatory and immune-mediated diseases.

Immune memory is the ability of the adaptive immune system to recognise previously encountered pathogens and respond more rapidly and effectively on re-exposure. Understanding immune memory is essential for explaining vaccination, long-term protection after infection, and why secondary immune responses are faster, stronger, and often prevent clinical illness.

Immune tolerance is the set of mechanisms that teach the immune system to recognise the body’s own tissues as “self” and avoid attacking them. Understanding immune tolerance is essential for explaining autoimmunity, transplant rejection, and how the immune system maintains balance between effective defence and self-protection.

The thymus and other lymphoid organs form the structural framework that supports immune cell development, activation and coordination. Understanding how these organs function is essential for explaining T-cell maturation, immune surveillance, and the body’s ability to mount effective and self-tolerant immune responses.

Mucosal immunity is the specialised immune system that protects the body’s mucosal surfaces, including the gastrointestinal, respiratory, urinary and reproductive tracts. Understanding mucosal immunity is essential for explaining how the body defends against pathogens while tolerating harmless microbes, food antigens and environmental exposures without triggering damaging inflammation.

The microbiome refers to the community of microorganisms that inhabit the body, particularly the gastrointestinal tract, and interact closely with host physiology. Understanding microbiome–immune interactions is essential for explaining immune development, tolerance, protection against pathogens and how microbial imbalance contributes to inflammation, infection and chronic disease.

The immune system is a distributed defence network that protects the body by detecting and eliminating pathogens, recognising foreign substances and maintaining internal balance. Understanding immune function is essential for interpreting infection, inflammation, autoimmunity, vaccination, wound healing and a wide range of conditions where altered immunity affects patient vulnerability and recovery.

Innate immunity is the body’s rapid, non-specific defence system that provides immediate protection against pathogens through barriers, immune cells, inflammatory responses and antimicrobial mechanisms. Understanding innate immunity is essential for recognising early immune responses such as inflammation and fever, interpreting infection risk, and understanding how the adaptive immune system is activated.