Immune System — Innate & Adaptive Immunity

Innate immunity (non-specific, fast):

Physical barriers: skin, mucus, cilia

Phagocytes: neutrophils, macrophages, dendritic cells

Natural killer cells: destroy virus-infected and cancer cells

Complement system: proteins that lyse pathogens

Inflammation: redness, heat, swelling, pain

Adaptive immunity (specific, slow, memory):

B cells — humoral immunity; produce antibodies (immunoglobulins)

T cells — cell-mediated immunity

Helper T cells (CD4⁺) — activate B cells and cytotoxic T cells

Cytotoxic T cells (CD8⁺) — kill infected cells directly

Regulatory T cells — suppress immune response

Memory cells — enable faster response upon re-exposure

MHC (Major Histocompatibility Complex):

MHC I — on all nucleated cells; presents intracellular antigens to CD8⁺ T cells

MHC II — on antigen-presenting cells; presents extracellular antigens to CD4⁺ T cells

HIV destroys CD4⁺ Helper T cells → immune collapse → AIDS

Reference:

Wikipedia: Immune System

https://en.wikipedia.org/wiki/Immune_system