Immune System: Acquired Immunity

Role of the Immune System

• The immune system's role is to detect, attack, and remove pathogens like viruses and bacteria, as well as abnormal body cells.
• It defends against disease and helps maintain homeostasis.

Immunity

• The body's ability to resist and eliminate invading pathogens or abnormal cells.
• This can be innate (non-specific) or acquired (specific).

Antigen

• A foreign substance triggers an immune response.
• Lymphocytes recognize specific regions on antigens called antigenic determinants.
• Antigens are typically proteins on the surface of pathogens, toxins, or infected cells.

Innate Immunity

• Present at birth.
• Non-specific, acting against all invaders similarly.
• Includes barriers and inflammation.
• Includes the first and second lines of defense.

Examples of innate immunity

• Skin
• Mucous membranes
• Phagocytic cells (neutrophils, macrophages)
• Inflammation
• Natural killer (NK) cells

Physical Barriers

• Skin is a physical barrier and secretes antimicrobial substances.
• Mucous membranes trap pathogens in respiratory/digestive tracts.
• Cilia help sweep out mucus and trapped pathogens.
• Tears/saliva contain enzymes like lysozyme
• Stomach acid destroys ingested pathogens.

Functions of Immune Cells and Chemicals

• B lymphocytes produce antibodies (humoral immunity) and create plasma and memory cells.
• T lymphocytes destroy infected cells and regulate the immune response.
• Helper T cells release interleukins (to activate B cells and cytotoxic T cells).
• Cytotoxic T cells kill virus-infected or cancerous cells using perforins.
• Dendritic cells & macrophages act as APCs (Antigen-Presenting Cells) and activate T cells.
• Cytokines help coordinate immune responses.
• Antibodies bind specific antigens and neutralize or mark them for destruction.

Inflammation

• A non-specific response to injury or infection that is part of innate immunity.
• Attracts immune cells to the site.
• Aims to contain pathogens, remove damaged cells, and start repair.
• Signs: redness, heat, swelling, pain, and loss of function.

Hallmarks of the Adaptive/Specific Immune Response

• Antigen Specific - Recognizes and targets specific antigens.
• Systemic - Not limited to infection site; acts throughout the body.
• Memory - Remembers previously encountered antigens for faster future responses.
• Adaptive immunity involves lymphocytes (T & B cells) and develops after exposure.

B Lymphocytes

• Originates in Bone marrow
• Function: Produce antibodies (humoral immunity)
• Activation Requires antigen + help from Helper T cells
• Types: Plasma cells (make antibodies), Memory B cells
• Mode of Action: Secrete antibodies into body fluids

T Lymphocytes

• Origin: Bone marrow and matures in the thymus.
• Function: Kills infected cells or assists other immune cells.
• Activated by antigens presented by APCs
• Types: Helper T cells, Cytotoxic (Killer) T cells, Memory T
• Mode of Action: Acts by direct contact with infected/cancerous cells.