Understanding Immunity

Immunity

• Immunity is the state of being resistant to a noxious agent, pathogen, or infectious disease.
• Immunity can occur naturally through prior exposure or immunization.
• It's the body's natural or induced response to potentially infectious exposure.

Considerations

• Immunocompetent patients
• Hypersensitivity
• Autoimmune disorders
• Immunodeficiency

Functions of the Immune System

• Protect the body from foreign antigens.
• Identify and destroy mutated, potentially harmful "self" cells.
• Remove cellular debris like dead or damaged cells.

Organs of the Immune System

• Tonsils and adenoids
• Thymus
• Lymph nodes
• Appendix
• Bone marrow
• Spleen
• Peyer's patches
• Lymphatic vessels

Leukocytes (WBCs)

• They are the primary cells involved in specific and non-specific immune responses.
• They are derived from stem cells in bone marrow.
• They use the circulatory system to transport to the site of inflammatory response.
• A normal count is 4,500-10,000 cells/mm3.
• Leukocytosis means there is an infection and the count is > 10,000 WBCs.
• Leukopenia means the count is < 4500 WBCs.

Types of Leukocytes

• Granulocytes consist of 60 – 80% of leukocytes.
• Granulocytes are derived from myeloid stem cells in the bone marrow.
• Granulocytes are instrumental in inflammatory responses.
• Granulocytes protect the body from microorganisms.
• They include neutrophils, eosinophils, and basophils.
• Bone marrow-derived lymphocytes (B Cells) are involved in humoral immune responses and produce antibodies or immunoglobulins.
• The thymus-derived lymphocytes (T Cell) are participants in cellular-mediated immune responses with T-cell activation occurring when macrophages present the T cell with a phagocytized antigen.
• Natural killer cells target virus-infected and tumor cells.

Antigens

• They are chemical markers that identify cells.
• Human cells have "self" antigens called human leukocyte antigens.
• Foreign cell antigens will not match.
• They are recognized as foreign and destroyed.

Antibodies

• They are immunoglobulins (Ig) or Gamma Globulins.
• They are proteins produced by plasma cells in response to foreign antigens.
• They are specific for only one antigen.
• They label antigens for destruction.
• There are five classes of human antibodies: IgG, IgA, IgM, IgD, IgE.

Two Mechanisms of Immunity

• Cell-mediated immunity (T-cells) acts directly to destroy infection-causing pathogens without using antibodies, activating phagocytes and T cells.
• Humoral immunity (B-Cells) protects the body by circulating antibodies to fight against pathogens, producing specialized WBCs (leukocytes) to seek out and destroy invaders.

Immune Process

• The process starts when the body is exposed to a particular pathogen with infecting microbes invading cells.
• Antigens are proteins on the outer surface of pathogens, that evoke an immune response.
• WBC phagocytes engulf and swallow the pathogen.
• After the pathogen is destroyed, the phagocyte displays pieces of itself on the antigens, known as an antigen-presenting cell (APC).
• Memory T cells will bind to the APC to fight similar pathogens in the future.
• Helper T cells fight against the infecting agent, activating T cells.

B Cells and Immune Response

• Activated B cells produce Y-shaped antibodies that bind to the pathogen's attachment site (antigen) and interfere with its ability to infect other cells.
• This process, called neutralization, does not destroy pathogens; it makes them ineffective.
• Antibodies cause pathogens to clump together (agglutination), reducing their activity.
• This increases the likelihood that the clump will be detected and phagocytized by leukocytes.
• Antibodies signal leukocytes (macrophages and neutrophils) to come in and engulf the pathogen.
• Antibodies fight infection by triggering inflammatory chemicals to destroy the pathogen.

Antibody Responses

• First exposure to a foreign antigen stimulates antibody production in small amounts.
• Second exposure to the antigen causes memory cells to make large amounts of antibody.
• The principle of secondary antibody response is the mechanism by which vaccines work.
• Allergic responses occur when antibodies respond to foreign but harmless antigens.

Four Types of Immunity

• Natural Active Immunity
• After a person becomes ill with an infection, the body produces its own antibodies to fight it and protect from future infections.
• Artificial Active Immunity
• The body is exposed to weakened or dead pathogens via a vaccine, making T cells or antibodies to prevent illness.
• Natural Passive Immunity
• Natural antibodies are passed from one body to another, from mother to baby through the placenta or breastfeeding.
• Artificial Passive Immunity
• Serum from another person or animal with antibodies against a pathogen is received, protecting from infection.