MCAT - Organ Systems:: Immune / Lymphatic System

The innate immune system differs from the adaptive immune system primarily in that innate immunity:

- **A)** Provides a rapid, nonspecific response using pattern-recognition receptors that detect broad classes of pathogens, without generating lasting immunological memory
- **B)** Is slower to respond but highly specific, generating immunological memory
- **C)** Consists exclusively of B lymphocytes
- **D)** Only functions in invertebrates, being absent in humans

Neutrophils, the most abundant circulating white blood cell, function primarily as:

- **A)** Antibody-producing cells
- **B)** Rapid-responding phagocytes that are typically the first innate immune cells recruited to acute sites of bacterial infection or tissue injury
- **C)** Cells that only function in chronic, long-standing infections
- **D)** The primary cell type mediating allergic reactions

Macrophages, derived from circulating monocytes that migrate into tissues, serve which combination of roles?

- **A)** Only antibody production, with no phagocytic function
- **B)** Only structural support, with no immune function
- **C)** Exclusively short-lived, single-use phagocytes identical to neutrophils in every respect
- **D)** Long-lived tissue-resident phagocytes that engulf pathogens/debris, present processed antigen to T cells (functioning as antigen-presenting cells), and secrete cytokines that help orchestrate both innate and adaptive immune responses

Natural killer (NK) cells are notable among lymphocytes because they:

- **A)** Require prior antigen exposure and clonal expansion before becoming functional, like T and B cells
- **B)** Only recognize and kill cells coated with a specific antibody, with no other mechanism
- **C)** Are part of the innate immune system, capable of recognizing and killing virally infected or tumor cells WITHOUT prior sensitization, notably by detecting the ABSENCE (or downregulation) of normal MHC class I expression ('missing self')
- **D)** Function exclusively by producing antibodies

The complement system contributes to innate immunity through which combination of mechanisms?

- **A)** Opsonization (coating pathogens to enhance phagocytosis), direct pathogen lysis via the membrane attack complex, and recruitment of immune cells via anaphylatoxins (e.g., C3a, C5a)
- **B)** Only antibody neutralization, with no other function
- **C)** Exclusively producing new B cells
- **D)** Only functioning within the adaptive immune system, unrelated to innate immunity

Toll-like receptors (TLRs), a key class of pattern-recognition receptors in innate immunity, function by:

- **A)** Directly producing antibodies against specific pathogens
- **B)** Recognizing broadly conserved pathogen-associated molecular patterns (PAMPs), such as bacterial lipopolysaccharide (LPS) or viral double-stranded RNA, triggering intracellular signaling cascades that activate innate immune cells and inflammatory gene transcription
- **C)** Only recognizing self-antigens, triggering autoimmune destruction
- **D)** Functioning exclusively within B lymphocytes

B lymphocytes mature primarily in the bone marrow, while T lymphocytes complete their maturation primarily in the:

- **A)** Spleen
- **B)** Lymph nodes
- **C)** Thymus, where developing T cells undergo positive and negative selection to eliminate self-reactive or non-functional clones
- **D)** Liver

Helper T cells (CD4+) primarily recognize antigen presented on which type of MHC molecule, typically displayed by professional antigen-presenting cells?

- **A)** MHC class II, which presents peptides derived from extracellular/phagocytosed antigens
- **B)** MHC class I, found on essentially all nucleated cells
- **C)** Neither MHC class I nor II; helper T cells recognize free antigen directly without MHC restriction
- **D)** Only antibody-antigen complexes, never MHC-presented peptide