Immunity and Immune System: Study Notes for Anatomy & Physiology

Immunity and the Immune System

Types of Pathogens

Humans are susceptible to a variety of pathogens, which are organisms that can cause disease. These include:

• Bacteria: Single-celled organisms that can cause infections such as strep throat and tuberculosis.

• Viruses: Non-living infectious agents that require host cells to replicate, e.g., influenza, HIV.

• Fungi: Eukaryotic organisms causing infections like athlete's foot and candidiasis.

• Parasites: Organisms such as protozoa and worms (helminths) that live in or on a host.

B Cells and Non-Specific Immunity

B cells are a type of lymphocyte involved in the immune response. In non-specific (innate) immunity, certain B cells (such as B-1 cells) can produce natural antibodies that recognize common pathogen-associated molecular patterns (PAMPs) without prior exposure.

• Leukocytes are white blood cells, including B cells, T cells, neutrophils, monocytes, and others.

Innate (Non-Specific) Immunity

Innate immunity is the body's first line of defense against pathogens. It is present from birth and responds quickly to infections.

• Function: Provides immediate defense against infection.

• Components: Physical barriers (skin, mucous membranes), chemical barriers (stomach acid, enzymes), cellular defenses (phagocytes, natural killer cells).

• Activation: Begins working as soon as a pathogen breaches physical barriers.

Inflammation: Steps and Chemicals

Inflammation is a key process in innate immunity, aiming to eliminate pathogens and initiate tissue repair.

1. Recognition: Pathogen invasion is detected by immune cells.

2. Release of Mediators: Chemicals such as histamine, cytokines, and prostaglandins are released.

3. Vasodilation: Blood vessels widen, increasing blood flow to the affected area.

4. Recruitment: Immune cells migrate to the site of infection.

5. Elimination: Pathogens are destroyed by phagocytosis and other mechanisms.

6. Resolution: Tissue repair and return to homeostasis.

Important chemicals: Histamine, cytokines (e.g., interleukins), prostaglandins.

Control of Viral Infections

Viral infections are primarily controlled by both innate and adaptive immune responses.

• Innate immunity: Natural killer (NK) cells destroy infected cells; interferons inhibit viral replication.

• Adaptive immunity: Cytotoxic T cells recognize and kill virus-infected cells; antibodies neutralize viruses.

Both specific and non-specific mechanisms are involved.

Natural Killer Cells

Natural killer (NK) cells are lymphocytes that target and destroy cells infected by viruses or transformed by cancer.

• They recognize cells lacking normal MHC I molecules.

• They kill by releasing cytotoxic granules.

The Complement System

The complement system is a group of proteins that enhance immune responses. There are two main pathways to activate the complement system:

• Classical pathway: Triggered by antibodies bound to antigens.

• Alternative pathway: Triggered directly by pathogen surfaces.

Key components include C1, C2, and C5 proteins. Activation leads to the formation of the membrane attack complex (MAC), which lyses pathogens.

Adaptive Immunity vs. Innate Immunity

Adaptive immunity is specific and develops after exposure to antigens. It involves lymphocytes (B and T cells) and has memory, allowing for a stronger response upon re-exposure.

• Innate immunity: Non-specific, immediate, no memory.

• Adaptive immunity: Specific, delayed, memory present.

Processes in Adaptive Immunity

Two main processes:

• Humoral immunity: Mediated by B cells and antibodies.

• Cell-mediated immunity: Mediated by T cells (cytotoxic and helper T cells).

Helper T Cells and B Cells

Helper T cells (CD4+) interact with B cells to stimulate antibody production. They release cytokines that activate B cells and other immune cells.

Antibody Structure and Subclasses

Antibodies (immunoglobulins) are proteins produced by B cells. They have several subclasses:

• IgG: Most abundant, crosses placenta.

• IgM: First produced in response to infection.

• IgA: Found in mucosal areas.

• IgE: Involved in allergic responses.

• IgD: Functions mainly as a B cell receptor.

BCR and TCR

B cell receptor (BCR) and T cell receptor (TCR) are molecules on B and T cells, respectively, that recognize specific antigens.

Antigens

An antigen is any substance that can induce an immune response, typically a protein or polysaccharide on the surface of pathogens.

Major Histocompatibility Complexes (MHC)

MHC molecules present antigens to T cells. In humans, they are called human leukocyte antigens (HLA).

• MHC I: Present on all nucleated cells; present endogenous antigens to cytotoxic T cells.

• MHC II: Present on antigen-presenting cells; present exogenous antigens to helper T cells.

Class I vs. Class II MHC Proteins

Feature	Class I MHC	Class II MHC
Location	All nucleated cells	Antigen-presenting cells
Antigen Type	Endogenous (from inside cell)	Exogenous (from outside cell)
Recognized by	Cytotoxic T cells (CD8+)	Helper T cells (CD4+)

Cytotoxic T Cells

Cytotoxic T cells (CD8+) destroy infected or abnormal cells by inducing apoptosis.

Helper T Cells

Helper T cells (CD4+) coordinate immune responses by activating other immune cells.

Destruction of Body Cells

Destruction of body cells is mainly associated with cytotoxic T cells (CD8+), which kill infected or cancerous cells. Helper T cells (CD4+) do not directly kill cells but activate other immune cells.

Immunity and Public Health

Understanding immunity is crucial for addressing public health issues such as vaccination, herd immunity, and the control of infectious diseases.

Additional info: Some details, such as the specific complement proteins and the role of B-1 cells in innate immunity, were inferred for completeness.