The spleen filters blood and removes old or damaged red blood cells. It also plays a role in initiating adaptive immune responses and houses lymphocytes and other immune cells.

The two main components are the innate immune system and the adaptive immune system.

The innate immune system provides a general, non-specific defense against pathogens. It is the first line of defense against infections.

The major components are physical barriers (e.g., skin, mucous membranes), chemical barriers (e.g., enzymes, gastric acid), inflammatory response, phagocytic cells (e.g., neutrophils, macrophages), and natural killer cells.

The adaptive immune system provides a specific, targeted response against pathogens. It can recognize and remember specific pathogens to mount a stronger response on future exposure.

The two main types are humoral immunity (antibody-mediated) and cell-mediated immunity.

The key cells are B lymphocytes (B cells), which produce antibodies that can bind to and neutralize pathogens or mark them for destruction.

The key cells are T lymphocytes (T cells), including cytotoxic T cells that can directly kill infected cells, and helper T cells that activate other immune cells.

APCs, such as dendritic cells and macrophages, process and present antigens from pathogens to T cells, initiating the adaptive immune response.

The MHC is a set of genes that code for proteins found on the surfaces of cells, which are responsible for displaying antigens for recognition by T cells.

The primary immune response is the initial response to a pathogen, which is slower but leads to immunological memory. The secondary response is faster and stronger due to the presence of memory cells from the primary response.

Cytokines are signaling proteins produced by various immune cells that regulate and coordinate the immune response, promoting inflammation, cell recruitment, and other immune functions.

An autoimmune disease is a condition where the body's immune system mistakenly attacks and destroys its own healthy cells and tissues.

Immunodeficiency is a state in which the immune system is impaired or defective, leading to an increased susceptibility to infections and other diseases.

Active immunity is acquired through the body's own immune response, either by exposure to a pathogen or through vaccination. Passive immunity is acquired through the transfer of antibodies from another source, such as from a mother to a fetus or through an injection of antibodies.

Complement proteins are part of the innate immune system and work to opsonize pathogens (mark them for destruction), create pores in their membranes, and promote inflammation.

Humoral immunity involves the production of antibodies by B cells to neutralize pathogens or mark them for destruction. Cell-mediated immunity involves the destruction of infected cells by cytotoxic T cells and the activation of other immune cells by helper T cells.

Mast cells are involved in the inflammatory response and can release histamine and other inflammatory mediators upon encounter with pathogens or allergens.

Antibodies are proteins produced by B cells that bind to specific antigens on pathogens. Antibody-mediated immunity (humoral immunity) involves the neutralization or destruction of pathogens by antibodies, as well as the activation of other immune mechanisms.

The thymus is an organ responsible for the maturation and selection of T cells, which are essential for cell-mediated immunity.

Primary lymphoid organs (bone marrow and thymus) are where lymphocytes (B and T cells) are produced and mature. Secondary lymphoid organs (lymph nodes, spleen, etc.) are where adaptive immune responses are initiated and coordinated.

Innate immunity is a non-specific, general defense mechanism present from birth, while adaptive immunity is a specific, acquired response that develops after exposure to a pathogen and provides immunological memory.

Lymph nodes are secondary lymphoid organs that filter lymph fluid and trap pathogens, allowing for the initiation of adaptive immune responses by concentrating antigens and immune cells in one location.

B cells are responsible for humoral immunity and produce antibodies, while T cells are responsible for cell-mediated immunity and can either directly kill infected cells (cytotoxic T cells) or activate other immune cells (helper T cells).

Basophils and eosinophils are granulocytes involved in the inflammatory response and allergic reactions. Basophils release histamine and other inflammatory mediators, while eosinophils combat parasitic infections and regulate inflammatory responses.

Memory cells are long-lived B and T cells that remain after an initial exposure to a pathogen. They allow for a faster and stronger immune response upon re-exposure to the same pathogen.

Self-recognition is the ability of the immune system to identify and tolerate the body's own cells and tissues (self). Non-self recognition is the ability to identify and mount an immune response against foreign or abnormal cells and molecules (non-self).

The bone marrow is the primary lymphoid organ where B cells and other immune cells are produced and mature.

The skin acts as a physical barrier and is part of the innate immune system, providing the first line of defense against pathogens and preventing their entry into the body.

Interferons are cytokines produced by various cells in response to viral infections. They have antiviral properties and can activate other immune cells, promoting an antiviral state in nearby cells.

Active immunity is acquired through the body's own immune response, either by exposure to a pathogen or through vaccination. Passive immunity is acquired through the transfer of antibodies from another source, such as from a mother to a fetus or through an injection of antibodies.

Mucous membranes, such as those lining the respiratory and digestive tracts, act as physical barriers and produce mucus containing enzymes and other substances that can trap and eliminate pathogens, providing innate immune protection.

Natural killer cells are part of the innate immune system and can recognize and kill virus-infected cells and tumor cells through the release of cytotoxic granules.

The primary immune response is the initial response to a pathogen, which is slower but leads to immunological memory. The secondary response is faster and stronger due to the presence of memory cells from the primary response.

The complement system is a group of proteins that work together to opsonize pathogens (mark them for destruction), create pores in their membranes, and promote inflammation as part of the innate immune response.

Regulatory T cells (Tregs) are a subtype of T cells that help to maintain self-tolerance and prevent autoimmune reactions by suppressing the activity of other T cells.

Antibodies, produced by B cells, can neutralize pathogens by binding to them, mark them for destruction by other immune cells (opsonization), or activate the complement system.

PRRs are receptors found on various immune cells that can recognize and bind to conserved molecular patterns (PAMPs) found on pathogens, triggering an innate immune response.

Dendritic cells are a type of antigen-presenting cell that can capture and process antigens from pathogens, then present them to T cells, initiating the adaptive immune response.

Macrophages are phagocytic cells that can engulf and destroy pathogens, as well as present antigens to T cells to initiate the adaptive immune response. They also produce cytokines to regulate immune responses.

Neutrophils are the most abundant type of white blood cell and are among the first responders to sites of infection, where they can phagocytize and kill pathogens through the release of antimicrobial substances.

The lymphatic system, which includes lymph vessels, lymph nodes, and lymphoid organs, is essential for the circulation and trafficking of lymphocytes (B and T cells) and other immune cells throughout the body, allowing for the coordination of immune responses.

The major histocompatibility complex (MHC) is a set of genes that code for proteins found on the surfaces of cells, which are responsible for displaying self and non-self antigens for recognition by T cells, allowing for the distinction between healthy and infected cells.

Cytokines are signaling proteins produced by various immune cells that regulate and coordinate the immune response, promoting inflammation, cell recruitment, and other immune functions.

The blood-brain barrier is a highly selective barrier that separates the blood from the brain, helping to protect the central nervous system from pathogens and immune cells, while also allowing for controlled immune surveillance and response.

Antibodies can exist in different isotypes (IgG, IgM, IgA, IgE, IgD), each with different functions, such as neutralization, opsonization, complement activation, or protection at mucosal surfaces.

Hypersensitivity reactions, such as allergies and autoimmune diseases, occur when the immune system overreacts or responds inappropriately to harmless substances or self-antigens, leading to tissue damage and inflammation.

Toll-like receptors (TLRs) are a type of pattern recognition receptor found on various immune cells that can recognize and bind to conserved molecular patterns (PAMPs) found on pathogens, triggering an innate immune response and initiating the adaptive immune response.

The thymus is an organ responsible for the maturation and selection of T cells, which are essential for cell-mediated immunity. T cells undergo positive and negative selection in the thymus to ensure they can recognize self and non-self antigens.

In birds, the bursa of Fabricius is the organ responsible for the maturation and development of B cells, which are essential for humoral immunity and antibody production.