Shigella dysenteriae causes bacillary dysentery characterized by frequent bloody and mucoid stools, abdominal cramps and fever. It is an invasive enteric pathogen causing mucosal ulceration.
Exo-erythrocytic schizogony (pre-erythrocytic stage) occurs in hepatocytes of the liver where sporozoites multiply into merozoites before they invade RBCs.
In the mosquito midgut, zygotes develop into oocysts on the gut wall; each oocyst produces many sporozoites that migrate to the salivary glands.
Amphetamines stimulate the central nervous system; barbiturates are central nervous system depressants producing sedation and respiratory depression.
Amphetamines are central nervous system stimulants. LSD is a hallucinogen (not a narcotic); heroin is an opioid narcotic; benzodiazepines are anxiolytics/sedatives (not primary pain killers).
Athlete's foot (tinea pedis) is a superficial fungal infection of the skin caused by dermatophytes (fungi).
Chronic excessive alcohol consumption leads to liver injury, fibrosis and ultimately cirrhosis. Other substances may damage the liver but alcohol is the classic cause.
Sporozoites migrate to the salivary glands of the infected female Anopheles mosquito and are injected into humans during a bite. In the human they then invade hepatocytes.
Correct associations: Leishmania donovani → Kala-azar (visceral leishmaniasis); Wuchereria bancrofti → Filariasis; Trypanosoma gambiense → Sleeping sickness; Entamoeba histolytica → Amoebiasis.
Paratope is the antigen-binding site located on the variable region of an antibody (formed by variable domains of heavy and light chains) that specifically recognizes the epitope.
Type I hypersensitivity (allergic reactions) involves IgE antibodies bound to mast cells and basophils; on re-exposure crosslinking of IgE triggers histamine release.
Metastasis is the process by which cancer cells invade, enter circulation or lymphatics and establish secondary tumors at distant sites.
HIV is a retrovirus that carries two copies of single-stranded positive-sense RNA and uses reverse transcriptase to make DNA.
Activated B cells differentiate into plasma cells which are antibody-secreting factories producing large quantities of immunoglobulin.
Primary lymphoid organ — Thymus: site of T-lymphocyte maturation and differentiation (selection of self-tolerant T cells). Secondary lymphoid organ — Tonsils: lymphoid tissue in the pharynx that traps inhaled/ingested pathogens, contains B and T cells, initiates adaptive immune responses and produces antibodies locally.
From the given organs, thymus is a primary lymphoid organ and tonsils are a secondary lymphoid organ. The thymus is a primary lymphoid organ located in the chest behind the breastbone where T lymphocytes (T cells) develop and mature. In the thymus, immature T cells undergo selection processes that ensure they can recognize self-antigens appropriately while eliminating those that would attack the body's own cells, thus achieving immunocompetency. The thymus is particularly active during childhood and gradually decreases in size and function with age. Tonsils are secondary lymphoid organs located in the pharynx (throat region) that help defend against pathogens entering through the mouth and respiratory tract. Tonsils contain lymphoid tissue and are populated with B and T lymphocytes that work together to fight viral and bacterial infections. When pathogens are encountered, tonsils produce antibodies and activate immune responses to neutralize and eliminate these invading microorganisms, providing a first line of defense against respiratory and gastrointestinal infections.
Macrophages are part of the internal innate immune barriers (cellular barriers). They perform phagocytosis of pathogens, secrete cytokines (e.g., IL-1, TNF) to recruit other immune cells, present antigen via MHC II to helper T cells (linking innate and adaptive immunity), and kill microbes using reactive oxygen/nitrogen species and lysosomal enzymes.
The type of barrier that involves macrophages is called the cellular or phagocytic innate immune barrier. Macrophages are large specialized cells derived from monocytes that function as professional phagocytes, capable of engulfing and destroying pathogens, dead cells, and cellular debris. These cellular barriers operate as part of the innate immune system, providing non-specific defense against a wide range of microorganisms and foreign substances. When pathogens breach the physical and chemical barriers of the body, macrophages recognize and engulf them through a process called phagocytosis, where the pathogen is surrounded and internalized into a vesicle. Once inside, the pathogen is exposed to destructive enzymes and reactive oxygen species that kill and break down the invader. Macrophages are strategically distributed throughout the body in various tissues, including the lungs (alveolar macrophages), liver (Kupffer cells), brain (microglia), and connective tissues (histiocytes), allowing them to mount immediate responses to infection. Beyond direct pathogen destruction, macrophages also process and present antigens to lymphocytes, bridging innate and adaptive immunity. This cellular barrier is crucial for controlling infections before they become established and for initiating broader immune responses.
Interferons (IFNs) are signaling proteins: Type I (IFN-α, IFN-β) are produced by virus-infected cells and induce an antiviral state in neighboring cells, upregulate MHC I expression and activate NK cells; Type II (IFN-γ) is produced by T cells and NK cells and activates macrophages, enhances antigen presentation and shapes adaptive immunity.
Interferons are a group of cytokines, which are small signaling proteins produced by virus-infected cells and certain immune cells such as lymphocytes and macrophages. These molecules play crucial roles in antiviral defense and immune regulation. When a cell is infected by a virus, it produces interferons that are secreted and bind to receptors on neighboring uninfected cells, triggering the production of antiviral proteins that inhibit viral replication and protein synthesis. This creates an antiviral state in neighboring cells, preventing the spread of infection. Interferons also enhance the activity of natural killer cells and macrophages, boosting the innate immune response against infected cells. Additionally, interferons have immunomodulatory roles, meaning they regulate and coordinate various aspects of the immune response. They increase the expression of major histocompatibility complex (MHC) molecules on cell surfaces, enhancing the ability of the immune system to recognize infected cells. Interferons also promote the differentiation and activation of lymphocytes, strengthening the adaptive immune response. Different types of interferons, including alpha, beta, and gamma interferons, have overlapping but distinct functions in combating viral infections and regulating immune responses. Due to their potent antiviral properties, interferons have been developed as therapeutic agents for treating certain viral infections and cancers.
Key chemical mediators released during inflammation include: histamine (from mast cells; increases vascular permeability), bradykinin (pain, vasodilation), prostaglandins and leukotrienes (derived from arachidonic acid; vasodilation, chemotaxis), cytokines such as IL-1 and TNF-α (fever, leukocyte activation), chemokines (leukocyte recruitment), complement fragments C3a/C5a (anaphylatoxins, chemotaxis) and nitric oxide (vasodilation, antimicrobial).
Histamine, bradykinin, prostaglandins, leukotrienes, cytokines (IL-1, TNF-α), chemokines, complement fragments (C3a, C5a), nitric oxide.
After entry, retrovirus (e.g., HIV) binds receptors (CD4 plus coreceptor), fuses with the host cell and releases RNA. Reverse transcriptase converts single-stranded viral RNA into double-stranded viral DNA. Integrase inserts this viral DNA (provirus) into the host genome. Host RNA polymerase II transcribes viral mRNA and genomic RNA. Viral proteins are translated; structural proteins and genomic RNA assemble at the plasma membrane. Virions bud off acquiring an envelope; viral protease cleaves polyproteins to produce mature infectious particles.
Retroviral replication follows a characteristic sequence of steps after the virus gains entry into the human body. First, the retrovirus attaches to specific receptors on the host cell surface through its envelope proteins, followed by fusion of the viral envelope with the host cell membrane, allowing the viral core to enter the cytoplasm. Once inside, the retrovirus undergoes reverse transcription, a process unique to retroviruses, where the viral enzyme reverse transcriptase synthesizes a complementary DNA (cDNA) strand from the viral single-stranded RNA genome, creating an RNA-DNA hybrid. The RNA template is then degraded and replaced with a second DNA strand, producing double-stranded DNA (dsDNA). This dsDNA is transported into the nucleus where it integrates into the host chromosome through the action of the viral enzyme integrase, forming a provirus. Once integrated, the proviral DNA is transcribed into viral RNA and messenger RNA by the host cell's transcription machinery, and these RNAs are translated into viral proteins by host ribosomes. The newly synthesized viral RNA and proteins are assembled into immature viral particles at the cell membrane through a process called budding, where the nascent virus acquires a lipid envelope derived from the host cell membrane. Finally, the viral protease cleaves the polyproteins into mature, functional viral proteins, completing the maturation process and producing infectious viral particles capable of infecting other cells. This replication cycle allows retroviruses to persist in the host by integrating into the genome, making them difficult to eliminate.
Structure features to label/textually depict: two heavy (H) chains and two light (L) chains linked by disulfide bonds; each chain has variable (V) and constant (C) regions (VH, VL, CH, CL). The two antigen-binding sites are at the N-terminal variable regions (Fab regions) — contain paratopes that bind epitopes. The stem is the Fc region (constant heavy-chain domains) responsible for complement activation and binding to Fc receptors. A hinge region provides flexibility. Approximate molecular mass ~150 kDa. (If drawing, show Y-shaped molecule, label VH, VL at tips, CH/CL in arms and Fc in stem, indicate disulfide bonds and antigen-binding sites.)
Immunoglobulin (Ig) is a Y-shaped molecule of two identical heavy chains and two identical light chains with variable and constant regions forming Fab (antigen-binding) and Fc regions (effector functions).
Key cells of the innate immune system: neutrophils (rapid phagocytosis of microbes), macrophages/monocytes (phagocytosis, antigen presentation), dendritic cells (antigen capture and presentation to adaptive system), natural killer (NK) cells (kill virus-infected and tumor cells), mast cells and basophils (release histamine, mediate inflammation), eosinophils (combat helminths), and barrier epithelial cells (produce antimicrobial peptides). These act non–specifically and provide the first line of defence.
The innate immune system comprises several types of cells that provide the first line of defense against pathogens. Neutrophils are the most abundant white blood cells and are the first responders to infection, engulfing and destroying bacteria through phagocytosis. Macrophages, derived from monocytes, are larger phagocytic cells that patrol tissues and present antigens to activate adaptive immunity. Dendritic cells act as professional antigen-presenting cells, bridging innate and adaptive immunity by processing and presenting pathogenic antigens. Natural killer (NK) cells are lymphocytes that recognize and kill virus-infected cells and tumor cells without prior sensitization. Mast cells and basophils release chemical mediators like histamine during inflammatory responses. Eosinophils are particularly effective against parasitic infections and contribute to allergic responses. Epithelial cells lining the skin, mucous membranes, and respiratory tract form physical barriers and produce antimicrobial substances. Together, these cells work through phagocytosis, cytotoxicity, inflammation, and secretion of antimicrobial compounds to provide rapid, non-specific protection against diverse pathogens.
Definition: A vaccine contains whole organisms or parts (or genetic material) that elicit an immune response and immunological memory without causing full disease. Major types: 1) Live attenuated vaccines (weakened pathogen; e.g., measles vaccine) — strong, long-lasting immunity. 2) Killed/inactivated vaccines (e.g., killed polio) — safer but may need boosters. 3) Subunit vaccines (protein or polysaccharide antigens) and conjugate vaccines (polysaccharide linked to protein; e.g., Hib) — focus on specific antigens. 4) Toxoid vaccines (inactivated toxins; e.g., tetanus, diphtheria). 5) Recombinant protein vaccines (antigen produced by genetic engineering). 6) Viral vector vaccines (gene for antigen delivered by harmless virus). 7) DNA and mRNA vaccines (deliver genetic instructions for antigen production). Choice depends on pathogen, safety and type of immune response required.
A vaccine is a preparation containing weakened or inactivated pathogens, pathogenic antigens, or genetic material that stimulates protective immunity against a specific disease without causing the disease itself. Vaccines work by priming the adaptive immune system to recognize and respond rapidly to the actual pathogen upon future exposure. Several types of vaccines are used clinically. Live attenuated vaccines contain weakened forms of the pathogen that can still replicate but cause little or no disease, providing strong immunity. Killed or inactivated vaccines contain pathogens that have been chemically or physically inactivated, making them safe but sometimes requiring booster doses. Subunit vaccines contain only specific antigenic components of the pathogen, including toxoid vaccines (inactivated bacterial toxins) and conjugate vaccines (polysaccharide antigens linked to carrier proteins). Recombinant vaccines are produced using genetic engineering to express specific antigens in host cells. DNA and mRNA vaccines deliver genetic instructions for cells to produce the antigen themselves, representing newer technology. Viral vector vaccines use harmless viruses to deliver pathogenic genes into cells. Each vaccine type has advantages regarding safety, efficacy, storage, and immunogenicity depending on the target disease.
Diagnosis pathway: 1) Initial screening: HIV antibody/antigen tests (rapid assays, ELISA detecting anti-HIV antibodies and p24 antigen). 2) Confirmatory test: Western blot or nucleic acid test (HIV RNA PCR) to detect virus. 3) Staging and AIDS diagnosis: measure CD4+ T lymphocyte count and viral load. Clinical diagnosis of AIDS is made when CD4 count <200 cells/µl (or <14% CD4) or when the patient develops AIDS-defining opportunistic infections (e.g., Pneumocystis pneumonia, Kaposi's sarcoma).
Diagnosis of AIDS in an HIV-infected person involves a systematic approach combining serological, molecular, and clinical assessments. Initial screening is performed using HIV antibody/antigen tests such as rapid tests or enzyme-linked immunosorbent assay (ELISA), which detect HIV-specific antibodies or antigens in blood or saliva. Positive screening results must be confirmed using more specific tests such as Western blot, which detects HIV proteins, or HIV RNA polymerase chain reaction (PCR), which directly detects viral genetic material and is the gold standard for confirmation. Once HIV infection is confirmed, the progression to AIDS is determined by measuring CD4+ T cell count, which reflects immune system damage. AIDS is diagnosed when the CD4+ T cell count falls below 200 cells per microliter of blood (or below 14 percent of total lymphocytes), indicating severe immunosuppression. Additionally, AIDS diagnosis is made when specific opportunistic infections occur, such as Pneumocystis jirovecii pneumonia, tuberculosis, toxoplasmosis, cytomegalovirus infection, or cryptococcal meningitis, or when clinical criteria such as wasting syndrome or HIV-associated dementia are present. Regular monitoring of CD4 count and viral load helps assess disease progression and guide antiretroviral therapy.
Justification: The immune system normally distinguishes self from non‑self by central and peripheral tolerance mechanisms. Failure of tolerance (due to genetic predisposition, molecular mimicry, infection, or breakdown of regulatory T cell function) leads to immune recognition of self-antigens. This generates autoantibodies and autoreactive T cells that attack host tissues, causing chronic inflammation and organ damage. Clinical examples include rheumatoid arthritis (autoantibodies against joint components), type I diabetes (destruction of pancreatic β-cells) and systemic lupus erythematosus (wide spectrum of autoantibodies). Hence autoimmunity is a misdirected immune response against self.
Autoimmunity represents a fundamental failure of immune tolerance, wherein the immune system misdirects its protective mechanisms against the body's own tissues and cells. Normally, self-tolerance is maintained through central tolerance in the thymus and bone marrow, where self-reactive lymphocytes are eliminated, and peripheral tolerance, where regulatory T cells and other mechanisms suppress autoreactive cells. When self-tolerance fails, autoreactive B cells produce autoantibodies against self-antigens, and autoreactive T cells directly attack self-tissues, causing chronic inflammation and tissue damage. This misdirected response occurs due to genetic predisposition, environmental triggers, infections, or breakdown of regulatory mechanisms. Rheumatoid arthritis exemplifies autoimmunity where antibodies attack joint tissues causing inflammation and destruction. Type 1 diabetes results from T cell destruction of insulin-producing pancreatic beta cells. Systemic lupus erythematosus involves widespread autoantibodies against nuclear antigens affecting multiple organs. Other examples include multiple sclerosis, where immune cells attack myelin in the nervous system, and Graves' disease, where antibodies stimulate thyroid hormone production. The chronic nature of autoimmune diseases reflects the persistent failure of immune regulation, making them difficult to cure and requiring long-term immunosuppressive management.
Diphtheria: causative agent Corynebacterium diphtheriae. Spread by respiratory droplets and close contact. Characteristic signs include sore throat, low-grade fever, formation of a greyish pseudomembrane over tonsils/pharynx that can obstruct airways, cervical lymphadenopathy (‘‘bull neck’’), and complications like myocarditis and peripheral neuropathies from diphtheria toxin. Typhoid: causative agent Salmonella enterica serovar Typhi (Salmonella typhi). Transmitted by faecal–oral route (contaminated food and water). Presents with sustained high fever, headache, malaise, abdominal pain, constipation or diarrhoea, ‘rose spots’ on the trunk, hepatosplenomegaly; severe cases may develop intestinal bleeding or perforation.
Diphtheria is caused by the bacterium Corynebacterium diphtheriae and is transmitted through respiratory droplets from infected individuals or carriers. Symptoms include a sore throat, fever, and formation of a characteristic pseudomembrane (false membrane) in the throat that appears grayish-white and can obstruct the airway. Lymphadenopathy (swollen lymph nodes in the neck) creates a characteristic bull-neck appearance. The bacterium produces a potent exotoxin that can cause myocarditis (inflammation of the heart muscle) leading to arrhythmias and heart failure, and nerve palsies affecting cranial nerves, particularly the vagus nerve, causing paralysis of the soft palate and respiratory muscles. Typhoid fever is caused by the bacterium Salmonella typhi and is transmitted through the faecal-oral route via contaminated food or water. Symptoms develop in stages: the first week presents with prolonged high fever that rises in a step-like pattern, severe headache, and general malaise. The second and third weeks show abdominal pain, constipation or diarrhoea, rose spots (faint pink rash on the trunk), and hepatosplenomegaly (enlargement of liver and spleen). Complications include intestinal haemorrhage and perforation, which can be fatal if untreated. Both diseases are preventable by vaccination and treatable with appropriate antibiotics if diagnosed early.
The presence of merozoites (intraerythrocytic forms released from schizonts) in peripheral blood indicates malaria caused by Plasmodium spp. Diagnosis: blood smear microscopy showing Plasmodium trophozoites/merozoites or rapid antigen tests. Clinical features include fever with chills, periodicity depending on species.
The diagnosis is malaria, an infectious disease caused by Plasmodium species parasites. The presence of merozoites in the blood is a characteristic finding that confirms malaria infection. Merozoites are the asexual stage of the Plasmodium parasite that are released from infected red blood cells and invade new erythrocytes, causing their rupture and the characteristic fever and chills associated with malaria. The fever and chills pattern corresponds to the synchronized rupture of infected red blood cells at regular intervals, typically every 48 or 72 hours depending on the Plasmodium species involved. Microscopic examination of blood smears showing merozoites, along with clinical symptoms of fever and chills, is a standard diagnostic method for malaria confirmation.
(i) Principal causative species: Wuchereria bancrofti (others include Brugia malayi, Brugia timori). (ii) Clinical features: acute episodes of fever and lymphangitis; chronic lymphatic obstruction leading to lymphedema and massive enlargement of limbs or genitalia (elephantiasis), scrotal hydrocele, and secondary infections. (iii) Mode of transmission: vector-borne — female mosquitoes (species vary by region: Culex, Anopheles, Aedes) transmit infective larvae; microfilariae show nocturnal periodicity and are taken up by mosquitoes and later transmitted as infective larvae during subsequent bites.
(i) The scientific name of the filarial worm that causes filariasis is Wuchereria bancrofti, which is responsible for the majority of filariasis cases worldwide. Brugia malayi and Brugia timori are also important causative agents in certain regions, particularly in Southeast Asia. (ii) Symptoms of filariasis result from lymphatic obstruction caused by adult worms living in lymphatic vessels and nodes. These include lymphedema and swelling of limbs and genitalia, a condition called elephantiasis where affected areas become grossly enlarged and thickened due to chronic inflammation and fibrosis. Patients experience recurrent fever, lymphangitis (inflammation of lymphatic vessels), and in males, hydrocele (accumulation of fluid in the scrotum). Chronic infection leads to disability and social stigma. (iii) Filariasis is transmitted by infected female mosquitoes, particularly Culex, Anopheles, and Aedes species, which take blood meals from infected humans. During the blood meal, mosquitoes ingest microfilariae (larval stage) from the infected person's blood. The microfilariae develop into infective larvae within the mosquito over 10-14 days, and these infective larvae are transmitted to another human during the next blood meal, establishing the infection cycle.
Abrupt cessation of alcohol or drugs in dependent individuals produces a constellation of withdrawal symptoms. Mild-to-moderate symptoms include anxiety, restlessness, irritability, tremors, sweating, nausea, vomiting, headache, insomnia, muscle cramps, and strong drug/alcohol craving. Severe withdrawal (particularly alcohol, benzodiazepines, or barbiturates) can produce autonomic hyperactivity, hallucinations, delirium tremens (confusion, severe agitation, visual/tactile hallucinations), and seizures — medical emergency.
Common withdrawal symptoms of drugs and alcohol abuse reflect the body's dependence on these substances and the neurochemical imbalances that occur upon cessation. Psychological symptoms include anxiety, irritability, depression, and intense cravings for the substance. Neurological symptoms include tremors, headaches, insomnia or sleep disturbances, and in severe cases, hallucinations or delirium. Gastrointestinal symptoms include nausea, vomiting, and loss of appetite. Cardiovascular symptoms include increased heart rate and elevated blood pressure. Musculoskeletal symptoms include muscle aches and pain. Autonomic symptoms include sweating and chills. Severe alcohol withdrawal can progress to delirium tremens, characterized by confusion, hallucinations, seizures, and life-threatening cardiovascular instability. The severity and duration of withdrawal symptoms depend on the substance used, duration of abuse, and individual factors. Withdrawal from alcohol and benzodiazepines is particularly dangerous and can be life-threatening, requiring medical supervision. Understanding these symptoms is important for managing addiction and providing appropriate medical support during the withdrawal process.
The term ‘common cold’ covers illnesses caused by numerous viral pathogens (numerous rhinovirus serotypes, multiple coronaviruses, adenoviruses, etc.). High antigenic diversity and frequent antigenic variation across many serotypes mean a vaccine would need to target a very large number of distinct antigens. In addition, the protective mucosal (IgA) immunity is often short-lived and sterilising immunity is difficult to achieve. Because disease is usually mild and self‑limiting, the cost–benefit of developing a universal vaccine is low. These factors make a single effective vaccine against all causes of the common cold impractical.
It is not possible to produce an effective vaccine against the common cold due to several interconnected factors related to viral diversity and immune response characteristics. The common cold is caused by more than 100 different virus serotypes, primarily rhinoviruses, but also including coronaviruses, adenoviruses, enteroviruses, and parainfluenza viruses. This extreme antigenic diversity means that immunity to one serotype does not provide protection against others, making a single vaccine impractical. Additionally, these viruses undergo frequent antigenic variation and mutation, allowing them to evade previously acquired immunity. The common cold viruses primarily infect the upper respiratory tract, where mucosal immunity (mediated by secretory IgA antibodies) is the primary defense mechanism. However, mucosal immunity is short-lived and wanes relatively quickly, providing only temporary protection. Furthermore, the economic burden of developing and maintaining vaccines against so many different serotypes would be prohibitive given that the common cold is generally self-limiting and causes minimal mortality. The combination of high viral diversity, rapid antigenic variation, short-lived mucosal immunity, and the mild nature of the disease makes vaccine development against the common cold impractical and economically unfeasible.