Phagocytosis and White Blood Cells

Phagocytosis is a crucial process in the non-specific immune response, forming an important part of the body's defense against pathogens. This section delves into the types of white blood cells and the detailed steps of phagocytosis.

There are two main types of white blood cells:

1. Phagocytes: These cells ingest and destroy pathogens through phagocytosis before they can cause harm.
2. Lymphocytes: These are involved in immune responses.

Definition: Phagocytosis is the process by which cells engulf large particles, including pathogens.

The process of phagocytosis involves several steps:

1. Attraction: Chemical products of the pathogen attract the phagocyte along a concentration gradient.
2. Attachment: Receptors on the phagocyte's cell-surface membrane attach to chemicals on the pathogen's surface.
3. Engulfment: The phagocyte engulfs the pathogen, forming a phagosome.
4. Digestion: Lysosomes within the phagocyte release lysozymes into the phagosome, hydrolyzing the pathogen.
5. Absorption and Disposal: The hydrolysis products of the pathogen are absorbed by the phagocyte.

Highlight: Phagocytes can move out of blood vessels and into other tissues, allowing them to respond to infections throughout the body.

The final step in this process is crucial for the specific immune response:

6. Antigen Presentation: The phagocyte attaches the antigen to its surface in a complex called MHC (Major Histocompatibility Complex).

Vocabulary: MHC (Major Histocompatibility Complex) is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules.

This detailed understanding of phagocytosis is essential for A level Biology immunity notes and forms a foundation for comprehending more complex immune processes.

Recognition of Self and Non-Self

The ability of the immune system to distinguish between 'self' and 'non-self' is fundamental to its function. This section explores how the body identifies foreign entities and why it doesn't attack its own cells.

Definition: An antigen is a molecule that induces an immune response in the body. Antigens can be proteins or glycoproteins.

Key points about self and non-self recognition:

1. All cell types, viruses, toxins, and organic materials have proteins or marker molecules on their surface that identify them.

2. The specific immune system can target different antigens and respond to them specifically.

3. Lymphocytes use the shape of molecules to identify non-self matter that needs to be destroyed.

4. The body produces millions of different lymphocytes capable of identifying various infections early in development. These remain in the body throughout life.

Highlight: The immune system's ability to recognize 'self' prevents it from attacking the body's own cells.

Protein markers, such as MHC proteins, play a crucial role in distinguishing native cells $self$ from foreign cells $non-self$. This mechanism ensures that the immune system only targets potential threats while leaving healthy body cells unharmed.

Vocabulary: MHC (Major Histocompatibility Complex) proteins are a group of genes that code for proteins found on the surface of cells that help the immune system recognize foreign substances.

Understanding this concept of self and non-self recognition is essential for A level Biology immunity Notes and forms the basis for comprehending more complex immune processes, including autoimmune disorders and transplant rejection.

Specific Immune Response

When the first two lines of defense - physical and chemical barriers and the non-specific immune response - are insufficient to prevent infection, the specific immune response is triggered. This response is more targeted and effective against specific pathogens.

The specific immune response has two distinct parts:

1. B-lymphocytes: Responsible for humoral responses
2. T-lymphocytes: Responsible for cell-mediated responses

Definition: The cell-mediated response is specific to the antigen and involves T-lymphocytes responding to antigens presented on other cells, rather than antigens within body fluids.

Cells that can present antigens on their surface (Antigen Presenting Cells or APCs) include:

• Transplant cells
• Cancer cells
• Phagocytes that have engulfed and hydrolyzed a pathogen
• Virally infected body cells

Highlight: The ability of T-lymphocytes to recognize antigens presented on other cells is crucial for defending against intracellular pathogens like viruses.

There are two main types of T-lymphocytes:

1. Helper T-cells
2. Cytotoxic T-cells $killer T-cells$

Each of these T-cell types plays a specific role in the immune response. Helper T-cells assist in activating other immune cells, while cytotoxic T-cells directly kill infected or abnormal cells.

Vocabulary: Cytotoxic T-cells, also known as killer T-cells, are lymphocytes that kill cells that are infected with viruses or are otherwise damaged or dysfunctional.

Understanding the specific immune response, particularly the cell-mediated response A level biology, is crucial for comprehending how the body defends against complex pathogens and maintains long-term immunity.

The Role of T and B Lymphocytes

T and B lymphocytes play crucial roles in the specific immune response. This section explores their functions and how they contribute to immunity aqa A level Biology.

B-lymphocytes:

• Produce antibodies, which are protein molecules specific to an antigen
• When antibodies bind to specific antigens on a pathogen, they neutralize it
• Antibodies are present in plasma, so B-lymphocytes are involved in 'humoral immunity'

Definition: Humoral immunity refers to antibody-mediated immune responses.

T-lymphocytes:

• Have specific receptors on their plasma membranes, which are specific to antigens present on body cells
• Involved in cell-mediated immunity

Highlight: The specificity of T and B lymphocytes allows for targeted immune responses against particular pathogens.

Cellular response of lymphocytes:

• There are millions of lymphocytes with different receptors for different antigens
• This diversity allows the immune system to respond to a wide range of pathogens

Vocabulary: Antigens are substances (usually proteins) on the surface of cells, viruses, fungi, or bacteria that the immune system recognizes as foreign.

Understanding the roles of T and B lymphocytes is essential for grasping concepts related to humoral response A level Biology AQA and cell-mediated response A level biology. This knowledge forms the foundation for comprehending more complex immunological processes, including vaccination and autoimmune disorders.

Page 5: T and B Lymphocyte Roles

This page details the specific functions of T and B lymphocytes in the immune response.

Definition: B-lymphocytes produce antibodies specific to antigens, while T-lymphocytes have specific membrane receptors.

Highlight: Antibodies in plasma are key to humoral immunity, neutralizing pathogens when binding occurs.

Page 6: Lymphocyte Response and T-Cell Activation

This page explains the cellular response of lymphocytes and the process of T-cell activation.

Vocabulary: Clonal expansion refers to the mitotic division that increases the number of specific lymphocytes.

Example: Antigen-presenting cells $APCs$ bind to T-helper cells with complementary CD4 receptors.

Page 7: Humoral Immune Response

This page focuses on B-lymphocytes and humoral immunity, explaining how antibodies function in the immune response.

Definition: Humoral immunity involves antibodies that are soluble in blood and tissue fluid.

Highlight: The process requires T-helper cells to stimulate B-lymphocyte mitosis for clonal selection.

Overview of Cell Recognition and the Immune System

The immune system is a complex network of defenses that protects the body against pathogens and other harmful substances. This section of AQA A Level Biology focuses on how the body recognizes and responds to threats, both through non-specific and specific mechanisms.

Definition: Pathogens are organisms that can cause disease.

The immune response is divided into two main categories:

1. Non-specific response: This is immediate and the same for all pathogens. It includes physical and chemical barriers, as well as cellular defenses like phagocytosis.

2. Specific response: This is slower but tailored to each pathogen. It involves cell-mediated responses (T lymphocytes) and humoral responses (B lymphocytes).

Highlight: The ability to distinguish between 'self' and 'non-self' is crucial for the immune system to function properly.

Physical barriers form the first line of defense against pathogens. These include:

• Skin: Keratinized and waterproof, preventing entry of most pathogens
• Mucus membranes: Lining the airway and gut, trapping pathogens
• Cilia in the nose: Filtering out pathogenic organisms

Chemical barriers also play a vital role:

• Stomach acid: Gastric juices at pH 2 kill most pathogenic organisms
• Lysozyme in tears: Breaks down cell walls of pathogens and viruses
• Skin secretions: Create unfavorable conditions for pathogen growth

Vocabulary: Lysozyme is an enzyme that can break down bacterial cell walls.

This overview sets the stage for a deeper exploration of the immune system's components and mechanisms in the following pages.